ABSTRACT
Geological Field Report on
Stream- and Road-cut Sections
of Jaflong – Tamabil –
Jaintiapur
Area,
Sylhet,
Bangladesh
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Sylhet is a North-Eastern district of Bangladesh, bordered by
Meghalaya, Assam, and Tripura. It is a hilly area with
numerous rivers, with most having their source in India. The
region is located in the Sylhet Basin, a portion of the Bengal
Basin, and is significant from a geological perspective. The
area is home to the oldest exposed deposit in Bangladesh,
Sylhet limestone, and other strata. The study area, located
between 25°04’ and 25°11’ North Latitudes and 92°00’ and
92°12’ East longitudes, aims to characterize the area using
physiography, geomorphology, structure, stratigraphy,
petrography, correlation with standard geologic succession,
economic geology, and paleo-environment of depositional
history. Sylhet is crucial to Bangladesh's economic
development and serves as Bangladesh's main hydrocarbon
reservoir. The Sylhet Basin also offers high-quality building
supplies like sand, gravel, and other materials.
Shah Ahmed Sayemur Rahman Sayem
GHF 318 : Geological Field Study
Geological Field Report on Stream- and Road-cut Sections
of Jaflong-Tamabil-Jaintiapur Area, Sylhet, Bangladesh
Report submitted in requirement of partial fulfilment of the
syllabus for the 3rd year B.S. (Honours)
____ Submitted by ____
Name : Shah Ahmed Sayemur Rahman Sayem
Group : Group 4
Class Roll No : FH-070-037
Exam Roll No : 118622
Session : 2019 – 20
Submission Date : 30th July 2023
Department of Geology
Faculty of Earth and Environmental Sciences, University of Dhaka
February 2023
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Abstract
Sylhet is a North-Eastern district of Bangladesh. Meghalaya, Assam, and Tripura, three Indian
states, border it on the north, east, and south, respectively. Chittagong, a division of Bangladesh,
borders it on the southwest, while Dhaka and Mymensingh, a division of Bangladesh, border it
on the west. A hilly area, Sylhet is home to numerous rivers such as the Shari, Hari, and Goyain.
Most of the rivers have their source in India.
Geographically, the region is situated in the Sylhet Basin, which is a portion of the Bengal Basin
in the north. Because this basin contains all of Bangladesh's stratigraphic units from the Eocene
to the Recent Holocene, it is significant from a geological perspective. Sylhet limestone is the
oldest exposed deposit in this area. The Kopili Shale, Barail Group, Surma Group, Tipam
Sandstone, and Dupi Tila Formation are all strata that are exposed in younger orientations from
this formation.
We, the students of 3rd year, Department of Geology, University of Dhaka went on a field work
in this district. Our field work was held in the Jaflong – Tamabil - Jaintiapur sections and the
report is an overview of the field work. The study area lies between 25°04’ and 25°11’ North
Latitudes and between 92°00’ and 92°12’ East longitudes. This report aims at geologically
characterizing the area, which includes the physiography, geomorphology, structure,
stratigraphy, petrography and its interpretation, correlation with standard geologic succession,
economic geology along with the interpretation of the paleo-environment of depositional history.
Tectonic activity is present in this region, and it is continuing to subduct beneath the Shillong
plateau. This explains why there are several structures present, including monoclines, various
faults, numerous sets of joints, unconformity, dragging, etc. In addition to all of this, there are
more sedimentary formations visible, such as flame structures, convoluted bedding, ripple marks,
cross-bedding, trough cross-bedding, and tidal sequence.
Sylhet is crucial to the nation's economic development. It serves as Bangladesh's main
hydrocarbon reservoir. The Sylhet Basin is home to several of the country's natural gas fields. In
addition to hydrocarbons, it also offers high-quality building supplies like sand, gravel, and other
materials. Additionally, the presence of fossiliferous and non-fossiliferous limestone reveals the
age of the rock.
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Acknowledgement
First of all, I want to express my gratitude to Almighty Allah for providing me with the chance
of a lifetime to do such a field study. I would especially want to thank Dr. Subrota Kumar Saha,
Chairman of the Department of Geology at the University of Dhaka, for allowing me the
opportunity to conduct fieldwork in Sylhet's "Jaflong – Tamabil - Jaintiapur Area."
My sincere gratitude is extended to our team's leader, Dr. Md. Anwar Hossain Bhuiyan,
Professor of the Department of Geology at the University of Dhaka, for his unwavering
leadership and heartfelt cooperation throughout the fieldwork as well as for his inspiration,
counsel, and assistance in comprehending numerous technical issues. He made every point so
clear that we had no trouble understanding it. He improved each of our working capacities. In
fact, I've been debating him for his outstanding assistance with the fieldwork for this field report.
He was a reliable source of insightful viewpoints, thoughts, and opinions.
I also want to express my sincere gratitude to my esteemed professors Dr. Md. Mostafizur
Rahman (Associate professor), Mahmud Al Noor Tushar (Lecturer) and Tamanna Meheran
Shemu (Lecturer), all of the Department of Geology, University of Dhaka, for their supportive
guidance, suggestions, and enthusiastic participation throughout the field survey.
My appreciation also extends to Goainghat Upzilla Parishad for allowing our faculty members
and students to stay in their Dak Banglo. I want to express my gratitude to the locals who
assisted us in organizing transportation and other services.
We are grateful for the hard work put in by the volunteers on the transportation, food, and
medical committees.
I want to thank my group members: Gobinda Basak, Mst. Tasmiya Akther Toma, Nazifa Anjum
and Nusratul Zannat Nupur who helped and supported me a lot throughout the field to complete
my fieldwork. Their cooperation and collaboration helped us to make a great team and make our
fieldwork effective.
Lastly, I want to thank the laboratory and office assistants for their contribution in completing
the field report.
In conclusion, I can confidently state that without the invaluable assistance of the individuals
indicated above, this report would never have been successfully finished.
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Figure A : Group photo in front of Curzon Hall, University of Dhaka
Figure B : Group photo at Base Camp
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Table of Contents
Contents
Abstract
Acknowledgement
Table of Contents
List of Tables
List of Logs
List of Figures
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Chapter 1 : Introduction
1.1 Purpose and Scope
1.2 Location, Extent and Accessibility
1.3 Topography, Relief and Drainage Pattern
1.3.1 Topography and Relief
1.3.2 Drainage Pattern
1.4 Previous Works
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Chapter 2 : Fieldwork Methods
2.1 Field Methods
2.2 Report Organization
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Chapter 3 : Structure
3.1 Major Structures
3.1.1 Fold/Homocline/Monocline
3.1.2 Faults
3.1.3 Unconformity
3.1.4 Joints
3.2 Minor Structures
3.2.1 Nodules
3.2.2 Ball and Pillow
3.2.3 Burrows
3.2.4 Cross Stratification
3.2.5 Cross bedding and cross lamination
3.2.6 Trough Cross Bedding
3.2.7 Clay Galls, Clay Balls and Clay Palettes
3.2.8 Heterolithic Bedding
3.2.9 Load Cast
3.2.10 FLANE Structure
3.2.11 Turbidite Sequence
3.2.12 Leaching and Liesegang
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Chapter 4 : Regional Geology
4.1 Tectonic Setting
4.2 Stratigraphy
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Chapter 5 : Sedimentology
5.1 Grain size Analysis
5.2 Heavy Mineral Analysis
5.3 Thin-section Petrography
5.4 Facies Analysis and Depositional Environments
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Chapter 6: Sequence Stratigraphy
6.1 Sequence Boundary
6.2 Stacking Patterns and System Tracts
6.3 Sequence Model
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Chapter 7: Economic Geology
7.1 Petroleum System Analysis
7.2 Economic Mineral Deposits
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Chapter 8: Environmental Issues
8.1 Blowout Events in Sylhet Field
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Chapter 9: Summary and Conclusions
9.1 Summary and Conclusions
9.2 References
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Chapter 10 : Annex
10.1 Geological Map of The Study Area
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List of Table
Title
Table 01 : Stratigraphic Succession of the investigation Area
Table 02 : Stacking patterns and System Tracts
Table 03 : Stratigraphic Sequence Model
Table 04 : Blowout in Sylhet Gas Field
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List of Logs
Title
Log 1 : Sylhet Limestone and Kopili Shale
Log 2 : Renji Sandstone and Laterite
Log 3 : Lower Bhuban Formation
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List of Figures
Title
Figure A : Group photo in front of Curzon Hall, University of Dhaka
Figure B : Group photo at Base Camp
Figure 1.1 : Location Map of Jaintiapur Upazila
Figure 1.2 : Satellite Image of Jaintiapur Upazila
Figure 1.3 : Contour Map of Bangladesh
Figure 1.4 : Topographical Map of Jaintiapur, Sylhet
Figure 1.5 : Drainage Pattern of Jaintiapur, Sylhet
Figure 2.1 : Clinometer
Figure 2.2 : Hammer
Figure 2.3 : Sample Bag
Figure 2.4 : Measuring Tape
Figure 2.5 : Pocket Lens
Figure 2.6 : Diagonal Scale
Figure 2.7 : Brush
Figure 2.8 : Field Notebook
Figure 2.9 : Base Map
Figure 3.1 : Micro Fold
Figure 3.2 : Drag Fold
Figure 3.3 : Dauki Fault in Nayagang Stream Cut Section
Figure 3.4 : Local Fault
Figure 3.5 : Joints
Figure 3.6 : Nodules
Figure 3.7 : Ball and Pillow
Figure 3.8 : Cross Stratification
Figure 3.9 : Trough and Cross Bedding
Figure 3.10 :Clay Galls and Balls
Figure 3.11 : Heterolithic Bedding
Figure 3.12 : Load Cast
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Figure 3.13 : Flame Structure
Figure 3.14 : Turbidite Sequence
Figure 3.15 : Leaching and Liesegang
Figure 4.1 : Tectonic setting of Bengal Basin
Figure 4.2 : Tectonic Map of Bangladesh
Figure 4.3 : Geological Exposed Formation of Bangladesh
Figure 4.4 : Stratigraphic Succession of Sylhet Trough
Figure 5.1 : Histogram of Renji Sandstone
Figure 5.2 : Cumulative Curve of Renji Sandstone
Figure 5.3 : Biotite under plane polarizer (40X)
Figure 5.4 : Biotite under cross polars (100X)
Figure 5.5 : Kyanite under plane polarizer (40X)
Figure 5.6 : Kyanite under cross polars (100X)
Figure 5.7 : Rutile under plane polarizer (40X)
Figure 5.8 : Rutile under cross polars (100X)
Figure 5.9 : Zircon under plane polarizer (40X)
Figure 5.10 : Zircon under cross polars (100X)
Figure 5.11 : Sylhet Limestone under plane polarizer (40X)
Figure 5.12 : Calcite Mineral with Fossils under cross polars (100X)
Figure 5.13 : Discocyclina in Limestone under plane polarizer (40X)
Figure 5.14 : Alveolina in Limestone under plane polarizer (40X)
Figure 5.15 : Nummilites in Limestone under plane polarizer (40X)
Figure 5.16 : Fossiliferous Limestone Facies
Figure 5.17 : Laminated Kopili Shale Facies
Figure 5.18 : Jenum Shale Facies
Figure 5.19 : Renji Sandstone Facies
Figure 5.20 : Trough bedded sandstone facies
Figure 5.21 : Parallel laminated sandstone facies
Figure 5.22 : Ripple Cross Laminated Sandstone Facies
Figure 5.23 : Nodular Shale Facies
Figure 5.24 : Bouma Sequence Facies
Figure 5.25 : Heterolithic Bedding Facies
Figure 5.26 : Dihing Formation
Figure 7.1 : Oil and Gas Fields in Bangladesh and Assam Region
Figure 7.2 : Gas Migration Paths
Figure 7.3 : Hardrocks at Jaflong, Sylhet
Figure 7.4 : Gravels and Boulders in Jaflong Section
Figure 7.5 : Sylhet Limestone (Limestone with Shale)
Figure 7.6 : Laterite
Figure 8.1 : Maguchara Blowout
Figure 8.2 : Tengratila Blowout
Figure 10.1 : Base Map of the Investigated Area
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Chapter 1 : Introduction
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General Information :
Geological fieldwork, also known as geologic survey, is the methodical
exploration of any location on Earth to gather geological data. The study and
interpretation of rocks, topographic features, etc. are typically part of geological
fieldwork. The Department of Geology requires students to participate in geological
fieldwork, which adds to their practical expertise. Due to this, the Department of
Geology at the University of Dhaka organized a scheduled field trip to Jaintiapur, Sylhet
this year. 51 third-year students and their esteemed instructors travelled to a geological
field site from 19th February to 23rd February, 2023, as part of the BS (Hons.)
curriculum.
1.1 Purpose and Scope
In order to fully comprehend theoretical information, practical experience is
essential in the field of geology. To build a better understanding of geological
happenings, fieldwork and laboratory study is required for a thorough grasp of the many
geological aspects.
Finding a link between theoretical and practical knowledge is the goal of doing a field
study, which will then be used to solve a variety of problems in real life. The main
purposes of this fieldwork include the following things
1. Converting a base map into a geological map using the data obtained from the
fieldwork
2. Studying the lithology of the exposed outcrops found in the local area and
establishing their depositional environments
3. Study various tectonic and sedimentary structures to depict the tectonic history and
various processes associated with the formation of the sedimentary structures
4. Constructing a sedimentary stratigraphic column based on the gathered data
5. Laboratory analysis of the thin sections of the samples collected systematically from
the suitable outcrops.
6. Interpretation of the provenance, paleocurrent direction
7. Comment on the economic importance of the geological area.
Our study area is in the NE part of Bangladesh, the northern part of the Surma Basin,
which holds a high potential for hydrocarbon generation.
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The study of the area with correlation can scope to
1. Recording sedimentary succession
2. Recording structural data
3. Recording geological data on maps
4. Building a bigger picture of the collected data
5. Recognize the character of sedimentary rocks
1.2 Location, Extent, and Accessibility
The studied area Jaintapur is located 45 km in the north-eastern part of
Sylhet district and at the foot of the Jaintia Hills near the India-Bangladesh border that
lies between 25◦05’N to 25◦11’N latitude and 92◦00’E to 92◦11’E longitude. The area is
surrounded in the north by Khashi – Jaintia hill range of Meghalaya, in the south by the
Shari River, in the west by the Dauki river, and in the east by the Lalakhal area.
Jaintapur Upazila covers an area of 280.30 sq. km. The road distance is approximately
240 km and air distance is approximately 199 km from Dhaka. Our base camp was
located adjacent to the Dauki river. The field was orchestrated through the Dauki river
and Tetulghat section on day 1, Sharighat and Noyagong stream section on day 2, and
Shari River and Dupigao section on day 3.
The transport system and communication of Sylhet is well connected with Dhaka city.
The journey to the study area was made by bus which took approximately 7 to 7:30
hours. Some of the sections were far from the base camp so a bus was arranged to
travel to the destinations. Exposures were mostly road-cut so it was easily accessible by
bus. Most of the sections were studied by traversing and Shaighat river also studied by
walking on day 3. Most of the sections were well exposed, so examining them did not
prove to be too difficult.
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Figure 1.1 : Location Map of Jaintiapur Upazila (Source : google.com)
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Figure 1.2 : Satellite Image of Jaintiapur Upazila (Source : Google Earth)
Figure 1.3 : Contour Map of Bangladesh (Source : www.maps.lib.utexas.edu/maps)
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1.3 Topography, Relief and Drainage Pattern :
The explored region of Jaintaipur and surrounding area is bounded on the
northeast by the sudden scarp of the 4000 to 6000 meter high Shillong Plateau, and on
the east by the Khasi-Jaintia Hill Range [Khan, 1978].
1.3.1 Topography and Relief :
The study area, which includes the Khashi-Jaintia hill range's slopes, is
characterized by high and low attitudes to the north and south, respectively. In Indian
Territory, the hill range extends to an elevation of more than 2 km and several hundred
meters. On the other hand, the region within Bangladesh is represented by minor
hillocks in a narrow strip trending east-west and depressions further to the south,
popularly known as "Haors". The north-eastern region of Sylhet (Jaintiapur) is
distinguished by low rounded hillocks scattered with cliffs and scarps. The primary
mountain ranges are Jaintiapur (up to 54 meters), Hari (Dupi Tila 91.2 meters), and
Lalakhal (Kesara Pahar 153m).
Figure 1.4 : Topographical Map of Jaintiapur, Sylhet (Source : www.enzm.topographic-map.com)
Holocene rock beds are found within the gravelly alluvial fans within the Jaintiapur and
its connecting ranges, in Dauki River and in Sonatila (Rashid 1977). The source range
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of these rock may be Shillong Massif, Jaintia Slope, Khasi Slope. These rocks are
composed of transformative rocks (quartzite, gneiss, and schist), molten rocks (stone,
basalt, gabbro) and sedimentary rocks(mainly sandstone). The measure changes from
stone to boulders and adjusted to sub-rounded in shape.
1.3.2 Drainage Pattern :
There are a number of streams and streamlets within the region. Various
streams and streamlets have been set up a dendritic design of seepage organize. It
recommends that the zone was dissolved unevenly. The generally major streams are
less in number and are of perpetual sort, that's they stream indeed within the dry
season, but amid blustery season they stream with their full quality and gotten to be
able to carry expansive boulders too far off places while the minor streams are huge in
number and of intermitted sort, that's they are regular in their stream, and water ceases
to stream amid the dry spell. The Dauki and Hari (walled in area) are the noticeable
waterways of the explored range.
The Dauki Waterway begun from Khasi-Jaintia Slope Ranges (Khan, 1978) and
streaming southward and enters Bangladesh close Dauki Bazar, India. It streams with
huge current with carried an expansive sum of boulders of volcanic and changeable root
from Shillong massif.
The Shari Stream begun from Khasi-Jaintia Slopes close Jowai, India and Bangchara
(Paul, 1988). It takes a winding course and joins the Surma close kalaruka. The two
fundamental tributaries of the Hari Stream, to be specific the Rangapani and Nayagang
are the other two critical waterways of the examined range which keeps up the seepage
framework of the central parcel of the range. Among these two, the Rangapani streams
into Bangladesh close Sripur and streams southeast ward for a few distances and turn
towards southwest for flowing down to the swamps. The Nayagang enters Bangladesh
close Puranassampara, streams southwest and at long last meet the swamps (Paul,
1988). It is to note that the Nayagang could be a winding waterway and the Rangapani
is a braided river.
Water supply gets to be rare within the sloping zones where streamlets and the
profound wells are the only source of water. But within the dry season, that's , within the
time of Walk or April the water table gets to be drop. As a result, most of the tube wells
ended up dry. The people of the region confront a water emergency in that time. Few
electrical pumps are utilized to supply water but this is often exceptionally expensive.
The water of lakes is utilized for family purposes as well as drinking water within the
plain arrive. This shortage proceeds till rain.
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Figure 1.5 : Drainage Pattern of Jaintiapur, Sylhet
1.4 Previous Works :
The Sylhet area beside the investigated region has been said to be profoundly imminent
of oil and gas from numerous a long time. Expansive number of works has been done
on the considered zone as well as the shole area of Sylhet.
For this reason, in Sylhet, huge volume of investigation work-geological, geophysical
and penetrating exercises carried out since 1923 by diverse organizations and Barma
oil company (BOC) had been the pioneer. Three more oil company to be specific
Pakistan Petroleum Ltd. (PPL), Pakistan Shell Oil Company (PSOC) and Stanvac Oil
Company (SVOC) joined in afterward (Dr. Guha, 1975).
Numerous geologists had been worked on Sylhet through. The work information back to
early fifties of the century, when Evans P.(1932) to begin with distributed the
stratigraphy of the Tertiary progression in Assam which is considered as the Book of
scriptures to the stratigraphy of the locale till nowadays. Among the geologists Holtrop
J.F. and Keizer J. distributed a relationship chart “Chart of Surma Basin Wells” in 1966.
They pushed and ineffectively uncovered within the frame of “Upper Marine Shale” in
the Surma Bowl for relationship inside the bowl.
Maroof Khan, M.A. (1978) distributed a report and a reconnaissance geologic outline
within the scale1 inch to 1 mile of the eastern and north-eastern Surma Basin. The
outline grasped the entire Tertiary succession of the range but the Sylhet Limestone
which shapes inliers within the east bank of the Dauki Stream.
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Palynostratigraphic examination of Oligocene outcrop test was done by Wallid K.M.
(1982), Reiman, K.V. (1983).Wallid palynological examination and Reiman's (1983)on
the sub-crop have clearly uncovered the nearness of Oligocene shapes.
Haque, M. (1982) examined the improvement of Surma Bowl and its relations to
hydrocarbon amassing. He created a plot of palynological Zonation of the Cenozoic
progression within the Surma Bowl. He moreover looked into the uncovered and
subsurface Stratigraphic of Surma Bowl.
Hiller and Elahi (1984) published the auxiliary improvement and hydrocarbon capture
within the Surma Bowl. They concluded that the Surma Bowl could be a demonstrated
Miocene Gas area and was Basically stamped by the contemporaneous interface of the
major structural developments.
Khan et, al (1988) uncovers that the gasses found within the Bowl are hereditarily
essentially to each other and are produced likely from terrestrial kerogen at different
levels of development identical to roughly 0.6 to 1.5% vitrinite re reflectance oil from
patharia and Sylhet-7 have comparative characteristics and may have sourced from the
Oligocene sediments.
Paul, D.D (1988) re-examined the structure and tectonics of the north-eastern part of
the bowl and commented that the east-west trending Auxiliary highlight (blame) were
created by the strengths, coming about from the beneath pushing of the Indian plate
towards NNS course where it collided with the Eurasian plate.
A comprehensive seismic framework and basic stock individually was built up for the
primary time within the Surma Bowl with German Specialized Assistance performed in
1979-1982 (Elahi and Hiller, 1984).
The Surma Bowl was moreover considered by M.A. Maroof Khan of Petro Bangla,
Monwar Ahmed of BAPEX. This range was also examined in points of interest by D. K.
Guha of petroleum Institute.
The zone was moreover examined as of late by a number of students administered with
their instructors of the Department of Geography of University of Jahangirnagar,
University of Dhaka and University of Rajshahi.
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Chapter 2 : Fieldwork Methods
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2.1 Field Methods :
Method of the Investigation :
The traversing & spot location methods were utilized for the study within
the field which was carried out along the areas where the rocks are well uncovered. The
separate was measured by traversing framework which is known as steeping. The
attitude of the beds were measured by Clinometer. The data was plotted on the base
map to induce a clear view of the examined region & outcrop. All the data like bearing &
separate of another area, points of lithologic contact & sample, attitude of beds, other
structures, lithology, physical highlight etc were noted down on a field note book. Photos
were taken in each area & other appropriate geologic components.
Equipment’s utilized in the field :
Geographical field works requests high degree of earnestness and sufficient labour. A
few equipment is fundamental to complete the field work. The following supplies were
used within the field to assist for continuing on this examination :
➢ Base map : Scale 1 : 25,000 a geological base outline for Jaintiapur and
connecting ranges of Sylhet area to find the particular regions & to plot the
plunge & strike perusing.
➢ Clinometers : It is for degree the dip, strike and plunge sum of the bed.
➢ Hammer : To gather samples and to discover out the right beds and other
sedimentary structures.
➢ Sample bag & elastic band : It is utilized to gather rock test of individual stations
with appropriate labelling and to avoid the test from air.
➢ Sample identification slip : To demonstrate specific sample in specific area.
➢ Acid bottle : It contains weaken HCl, which is utilized to decide the nearness of
calcareous constituents inside the rock bodies.
➢ Field Notebook : To note the collected information.
➢ Measuring tape : It is utilized to measuring the distance.
➢ Pocket lens : To watch the surface (shape and measure) of the rocks.
➢ Camera : To take photos of the geological feature of the area.
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➢ Global Positioning System (GPS) : It is utilized to decide longitude, scope,
elevation of diverse segment.
➢ Wooden Pencils, Coloured Pencils, Diagonal scale, Pocket Knives etc.
Figure 2.1 : Clinometer
Figure 2.2 : Hammer
Figure 2.3 : Sample Bag
Figure 2.4 : Measuring Tape
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Figure 2.5 : Pocket Lens
Figure 2.6 : Diagonal Scale
Figure 2.7 : Brush
Figure 2.8 : Field Notebook
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Figure 2.9 : Base Map
2.2 Report Organization :
The report is structured on such a way that viewers will have an improved
comprehension of the subject content. It begins by providing a summary and then
moves on to an outline of what is in the report. Each chapter develops on the prior to
one, resulting in a logical evolution of knowledge.
The abstract acts as a brief introduction, providing viewers with a swift overview of the
complete report's content. The appreciation statement delivers gratitude toward
individuals or institutions who contributed to the investigation.
The table of contents serves as an outline, allowing viewers to move forward to the
areas that particularly fascinate them. It allows viewers to discover specific details
without experiencing to read the whole report.
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The opening chapter delivers the framework by outlining the investigation's goal,
geographical location, and geological features. It offers context by referring earlier
studies in the same or related fields.
The chapter on essential field methodology illustrates the strategies and tools utilized
during the fieldwork, making sure the investigation's transparency and accuracy.
The examination of structures in various tectonic and sedimentary regions enable a
more emphasized investigation of the geological characteristics uncovered during the
field visit. This section helps viewers in understanding all aspects of the
investigation field.
The chapters on stratigraphy and depositional environment provide crucial insights into
the region's geological past. A master log is included in the fieldwork to provide a
through record of the numerous rock strata encountered.
The sedimentology chapter digs into laboratory tests and results in order to give light on
the features and composition of the stones assembled throughout the course of the
study.
Sequence stratigraphy investigates the sequential placing an order of geological
classes, assisting viewers in understanding the studied area's temporal evolution.
The economic portion evaluates the area's potential commercial deposits, which may
include rich minerals or materials, and emphasizes the relevance of the discoveries
beyond academic curiosity.
The summary and conclusion tie all the pieces together, summarising the main
outcomes and offering a comprehensive understanding of the geological aspects of the
studied area. This final section often outlines potential avenues for future research and
exploration.
Overall, the well-organised and structured format of geological study reports ensures
that readers can grasp the complexities of the subject matter while appreciating the
significance of the research conducted in the field.
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Chapter 3 : Structures
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3.1 Major Structures :
The structure of the area that has been investigated is an approximated
anticline by the field crew. It is a large homoclinal fold that is composed of a syncline
and a relatively minor anticline. The right lateral movement of the Dauki fault was
responsible for the development of the refolded structure.
It is obvious that the structure of the Surma Basin and the places that are neighbouring
to it are more active on a tectonic stage, as shown by the fact that the Surma Basin has
subsided by around 30 to 40 feet over the period of the last several hundred years. At
the present time, the middle part of the Surma Basin is subsiding at a rate of 21
millimetres per year, meanwhile the northern part is sinking at a rate of 1.5 to 2.5
millimetres per year.
The underthrusting of the Indian plate in a north-northeasterly direction is responsible
for the evolution of the structure of the region, which was caused by the forces
responsible for the formation of the area.
3.1.1 Fold :
Anticlinal Fold :
The region under investigation is an important F1 type anticlinal fold that exhibits an
uneven E-W to NW-SE trend. The axis complies with the Dauki fault to the east, where
it is truncated on its way from Dauki to Sripur. An F2-type fold has been superimposed
over the main fold. Superimposed fold has a NE-SW inclination.
As the northern side is not adequately exposed in Bangladesh, we focus on working on
the southern flank, which continues in Meghalaya, India. Part of the northern part is
visible in Bangladesh, although only in Sripur and the Tamabil - Jaflong road cut region.
The southern side dips approximately from 20° to 50° southward; around Dupitila and
the Shari River, almost vertical beds can be observed. The northern face is more
steeply sloping. Evidence of support for the anticlinal fold framework :
1. The fault bed plunges in the opposite direction all the way from Sripur-Tamabil to
Dauki. It is not simply in a few faulted blocks, as several authors assuming
homocline have showed in their studies.
2. The axis of the fold contains a portion of the most ancient rocks. In the scenario
where it is not an anticlinal fold, younger rocks such as Tipam and Dupitila may
often be found at the road cut section that runs between Jaintia and Tamabil.
Page 26 of 90
Figure 3.1 : Micro Fold
3. During the investigation of the Rangapani river area, we observed bed dips in a
variety of directions, including northeast, southeast, and southwest, all of which
followed a distinct phase. Because of this, we are able to deduce that they took
place during a later period of the superposition of folds. The process of deducing
anticlinal folds is facilitated as a consequence.
Drag Fold :
In the Shari river expand which lies close to Affifanagar and Lalakhal Tea properties,
there are several prominent drag folds. The movement of the competent sandstone bed
in the opposing direction with each other is what causes the formation of drag folds in
the silty shale bed.
Figure 3.2 : Drag Fold
Page 27 of 90
3.1.2 Fault
The Dauki fault is the most important regional fault in the investigated area. In addition,
a few more local faults were seen in a wide range of rock formations. In terms of fact,
the region that we investigated is a part of a larger area that has been drastically
reshaped as a result of the movement of the Dauki fault system, which is the primary
factor in the region's existing topography. The movement of the various formations is
what made it possible to identify the regional faults.
Dauki Fault :
It is considered the westward elongation of the Naga-Disang thrust system, whereas the
Dauki fault is an arrangement of faults that move in the opposite direction, from east to
west. The up-thrown block is symbolised by the Shillong Massif. Because the fault is not
well exposed, gravity measurements lead one to assume that it is a deeply embedded
fault. The zone of faulting is five km broad and may be distinguished by its significant
fracturing and steep dips. The Dauki fault is visible along the southern boundary of the
Shilong plateau for approximately 170 kilometres, originating at the Jadukata River (lat.
25o 14’ 30’’ N; long. 91o 13’ 00’’ E) in the west and ending at Haflong (lat. 24o 44’ 00’’ N;
long. 93o 02’ 30’’ E), where it enters the Haflong-Disang thrust.
Evidences that support the existence of the fault are as follows :
1. Within a few hundred metres, there were sudden structural changes in the
topography, and there were also significant variances in the relief. Bangladesh is
located at a lower altitude than India, which is located at a higher elevation. The
current elevation of the Shilong plateau is the result of several instances of uplift
that occurred along the Dauki fault system over a significant amount of time. Evans
(1964) states that the total amount of structural relief on both sides of the Dauki
faults can reach up to 13,000 metres in height.
2. In addition, the occurrence of fault breccias and mylonites in the fault zone of the
Sylhet Limestone provides more evidence of faulting in this investigated region.
3. Variations in the positions of the angles at which the beds are arranged.
4. The Dauki River as it flows in its most straight path.
5. Major faulting may be inferred from the existence of terraces along the river bank.
Page 28 of 90
Figure 3.3 : Dauki Fault in Nayagang Stream Cut Section
Local Fault :
The same force that triggered the Dauki fault possibly active some other faults, and it is
possible that tiny blocks of rocks, for example the Sylhet limestone, have been thrown
up to the surface through these other faults. However, this is only a hypothesis at this
point because there is very little proof to backing up it.
Figure 3.4 : Local Fault
3.1.3 Unconformity
An unconformity is a contact between two rock units in which the upper unit is usually
much younger than the lower unit. Unconformities are typically buried erosional
surfaces that can represent a break in the geologic record of hundreds of millions of
years or more. Two types of unconformity have been found in the investigated area.
Page 29 of 90
Disconformity :
In the field of geology, a disconformity is often an erosional contact that runs
perpendicular to the bedding planes of the upper and lower rock layers. When exploring
the fossils in the rock units that are higher and lower in the sequence, researchers
frequently fall across disconformities. This is because disconformities are difficult to spot
in stratified sedimentary rock sequences. A significant disconformity exists between the
Barail group and the Surma group. This specific type of unconformity was discovered by
us in two distinct locations: one was in the Shari river area close to the Indian border,
and the other was close to the eastern portion of the Gaurishankar river at a latitude of
25 o 08’ 24’’ N and a longitude of 92 o 07’ 18’’ E. A narrow strip of lateritic conglomerate,
which is distinctive of the Barail group, serves as a representation of it. Pebbles,
cobbles, and granules make up the band of laterite, which ranges in colour from red to a
dark brownish colour and is formed of granules. The nature of the unconformity is nondepositional, which means that the laterite may have evolved as a result of the
consolidation of the results of weathering of the Barail group of rocks (hematite
cemented sandstone) after prolonged exposure prior to the deposition of the Bhuban
deposits.
Angular Unconformity :
Most of the mountain ranges are covered in gravel beds. This gravel bed generates a
sedimentary unconformity with the Barail and Surma group. Recent gravel beds have
been arranged over the inclined underlying beds in a horizontal pattern, indicating an
angular unconformity. Along the Rangapani river section, where the gravel beds cover
the Barail sandstone, such a type of unconformity is seen in the field. Gravel beds have
been found to create an angular unconformity within the Surma Group in the village of
Uzaninagar, which is adjacent to Jaintiapur.
3.1.4 Joints
As joints are an indicative related with the structure of faults, the joints identified in our
studied area can be interpreted as the consequence of Dauki fault movement.
Additionally, an investigation of the regional environment of the area suggests that
multidirectional impacts of varying intensities have been responsible for the formation of
the researched area. As a result, joints of varying orientation and extension occur over
the studied area. Joints are common characteristics developed in more resistant
sections of sandstone, siltstone, and silty shale of various formations.
Page 30 of 90
Some prominent joints location are mentioned below :
1. Massive scale vertical to sub-vertical joints have been witnessed in the Sylhet
limestone near the Dauki River.
2. Highly jointed Barail sandstone is shown in a road-cut section at Naljhari.
3. The Surma Group in Tetulghat has several inclined joints with a low dip.
4. It appears that very closely spaced parallel joints Strike joints may be observed in
Dupi Tila sandstone near Sharighat.
5. Numerous subvertical joints were found in Surma Group shale in Rangapani
section.
Figure 3.5 : Joints
Page 31 of 90
3.2 Minor Structures :
3.2.1 Nodules
A nodular structure refers to nodules in rocks or silt which vary in composition or
structure from the neighbouring material. The Tetulghat section of the investigated area
contains nodular features.
Figure 3.6 : Nodules
3.2.2 Ball and Pillow
Ball-and-pillow formations are typically associated with soft-sediment deformation,
which occurs when sediment that has not yet been completely compacted and lithified
into rock undergoes distortion. These formations are typically encountered in areas
where sedimentary layers have been exposed to tectonic forces or other pressures,
resulting in the sediment's unique deformation. These constructions may be found in the
Nayagang sector of the study area.
Figure 3.7 : Ball and Pillow
Page 32 of 90
3.2.3 Burrows
The structures or marks that animals that burrow through sedimentary rocks depart
behind are called burrows. Burrows, as compared to body fossils, which are the genuine
remains of organisms, are a type of sedimentary structure. Burrow constructions have
been seen in the Tetulghat area of the researched area.
3.2.4 Cross Stratification
It is a type of internal sedimentary structure seen in many sedimentary rocks that
consists of an angle to the main bedding. It is quite common in the research area.
Figure 3.8 : Cross Stratification
3.2.5 Cross Bedding and Cross Lamination
Cross lamination can produce a single set or several sets from a single bed.
Stratification is classified as cross lamination or cross bedding based on the established
height, which is less than or larger than 6cm. Trough cross stratum is curved crested,
whereas tabular cross stratum is straight crested.
3.2.6 Trough cross Bedding
Trough cross-beds feature bottom surfaces that are curved or scoop-shaped and
abbreviate the underlying beds. The foreset beds are likewise curved and join
tangentially with the bottom surface. They are related to sand dune migration.
Page 33 of 90
Figure 3.9 : Trough and Cross Bedding
3.2.7 Clay Galls, Clay Balls and Clay Palettes
A dry, curled clay shaving formed from dried, cracked mud that has been imbedded and
flattened in as and layer. This is the Fluvial Environment (Meandering River)'s signature.
Figure 3.10 : Clay Galls and Balls
3.2.8 Heterolithic Bedding
Heterolithic bedding is a type of geological formation composed of interbedded sand
and mud layers. It forms mostly in tidal flats habitats. Heterolithic bedding comes in
three varieties: flaser bedding, lenticular bedding, and wavy bedding.
Page 34 of 90
Figure 3.11 : Heterolithic Bedding
Flaser bedding is sand with mud streaks that is commonly found in troughs.
Wavy bedding is distinguished by interbedded rippling sand and mud layers with equal
mud and sand composition.
Lenticular bedding is generated when mud suspended in water is deposited on the
surface of tiny sand formations after the water's velocity reaches zero.
3.2.9 Load Cast
Load cast is produced by sinking one bed into another. Load casts are prevalent on the
soles of sandstone strata overlaying mud casts, exhibiting as bulbous structure and
maybe on the way to becoming ball and pillow structure seen in the Surma group's
Shari river area.
Figure 3.12 : Load Cast
Page 35 of 90
3.2.10 Flame Structure
Flame structures are usually generated in sands, muds, and marls. The formations
range in size from 5 to 30 cm and are formed by mudstones that are injected into
underlying sandstones. The injection is caused by substantial changes in dynamic
viscosity within sediment layers. As a result, fine-grained sediments act as diapiric
intrusions.
Figure 3.13 : Flame Structure
3.2.11 Turbidite Sequence
A turbidity current is a particular combination of fluidal and sediment gravity flow that is
in charge of dispersing enormous volumes of clastic material into the deep ocean or
shelf environment, and turbidite is the geologic deposit of a turbidity current. Other
sedimentary features including solemarks, ripple lamination, and flame structures are
frequently seen alongside turbidites.
Figure 3.14 : Turbidite Sequence
Page 36 of 90
3.2.12 Leaching and Liesegang
Leaching is the loss of colloids and soluble materials from the top soil layer due to
percolating precipitation. Eluvated, or dragged downhill, the lost materials are often
redeposited in a lower stratum. In sedimentary rocks, colourful cement bands called
"Liesegang rings" can be seen cutting through the bedding. Mineral bands that are
placed in these secondary sedimentary formations in a predictable, recurring pattern.
Figure 3.15 : Leaching and Liesegang
Page 37 of 90
Chapter 4 : Regional Geology
Page 38 of 90
4.1 Tectonic Setting :
Plate tectonics allows for a comprehensive reorganization of the structural evolution that
took place in this region, and it can serve as the foundation for such a reconstruction.
According to Alam (1989), the Bengal basin is situated on the eastern side of the Indian
subcontinent and encompasses the greater part of Bangladesh, West Bengal, India, as
well as a section of the Bay of Bengal. The region under investigation may be found in
the Bengal Geosyncline's Surma Basin, which is also referred to as the Sylhet Trough
due to its recognition as a basinal depression. Within the Sylhet Trough can be found
the Sylhet Anticline, a fold structure that trends from northeast to southwest and has
dimensions ranging from 13 kilometers in length and 3 kilometers in width.
The Surma Basin and the provinces that surround it have had a falling of around 30 to
40 feet over the period of the previous several hundred years. This is evidence that
indicates the structure of the Surma Basin and the regions that surround it is
experiencing an increase in tectonic activity. The elevation of the northern part of the
Surma Basin is decreasing at a rate of 2.5 millimeters per year at the present time. The
driving force behind the construction of the regional structure, which was brought
approximately as a consequence of the Indian plate being forced downward in a NorthNortheasterly direction.
Figure 4.1 : Tectonic setting of Bengal Basin
Page 39 of 90
Figure 4.2 : Tectonic Map of Bangladesh
Page 40 of 90
4.2 Stratigraphy :
The region that was studied comprises largely of detrital sedimentary rocks, and the
majority of the area exhibits tertiary rock formations. Approximately 17000 metres is the
as a whole thickness of all of the rock units. Because of the absence of sufficient trace
fossils as well as the sudden and frequent changes in facies, interactions between
different rock groups can be exceedingly difficult to discern. However, with the
exception of the Sylhet Limestone, the rock units are separated into subgroups based
on the lithologic characteristics of the rock.
The geological structure of the region has been broken down into a few distinct
formations for a better understanding. In the standard sequence, the Sylhet limestone
deposit is discovered to be the oldest of the rock types. According to the law of
superposition, the following description provides the standard sequence in which the
formation under study occurred.
Alluvium
Dihing formation
Dupi Tila formation
Girujan clay
Tipam sandstone
Surma group
Barail sandstone
Kopili shale
Sylhet limestone
Evans (1932) was the first scientist who provided names to the numerous formations
which make up the tertiary successions in Assam. Despite the fact that it might be
problematic to associate formations that are separated by hundreds of kilometres
without the assistance of paleontological evidence and also because to the frequent
facies alterations that occur.
Page 41 of 90
Figure 4.3 : Geological Exposed Formation of Bangladesh
Numerous streams and their tributaries were visible. The tilting-induced accumulation of
erosion and depositional processes has continued to contribute to the emergence of the
investigated region's display physiography. The Eocene was a time of steadily declining
mainland rack conditions within the Bangladesh region; nonetheless, it was not sharply
impacted by the late Palaeocene mainland collision among India and Asia. The whole
area was submerged in the water during the middle to late Eocene, when the range was
restrained by a large marine incursion brought on by apparent bowl ward subsidence.
The Sylhet limestone was kept in an atmosphere that was open and warmly marine.
Extremely fossiliferous limestone serves as evidence of a shallow marine environment.
Page 42 of 90
The stratigraphic succession of the investigated area is tabulated below :
Age
Group
Recent
Formation
Lithology
Thickness
(m)
Alluvium
Unconsolidated Sand, silt and clay
Pleistocene
Dihing
Well rounded,
boulder sized
sphericity
Pliocene
Dupi Tila
Sandstone
Coarse grained, yellowish sandstone
with subordinate claystone containing
quartz pebbles
300
Girujan Clay
Whitish
colour
massive
sticky
claystone
containing
ferruginous
specks sparsely
350
Tipam
Sandstone
Yellowish brown, Medium to coarse
grained cross bedded sandstone
1000
Alteration of gray coloured, moderately
hard, fine to very fine grained
sandstone and bluish gray laminated
shale
1250
Renji
Pink coloured, medium to coarse
grained, well sorted sandstone with
subordinate laminated shale
950
Kopili Shale
Black, fissile, splintery shale with high
clay content
100
Sylhet
Limestone
Light coloured, very hard and compact,
massive fossiliferous Limestone
30 +
Tipam
Mio-Pliocene
Miocene
Surma
Oligocene
Barail
Eocene
smooth cobble to
gravel with high
200 +
Jaintia
[Paul, 1988 and field investigation]
Table 1 : Stratigraphic Succession of the investigation Area
Page 43 of 90
Figure 4.4 : Stratigraphic Succession of Sylhet Trough
This type of deposition was followed by the accumulation of Kopili shale in an extremely
small thickness, which is indicative of a transition from a shallow marine environment to
a bowl that accepts clay. Such a natural change occurred when the range began to be
replaced by the collision occasion. Diverse regions of the bowl were subject to marine
relapse throughout the Oligocene era. The Himalaya's pace of ascent accelerated. As a
result, streaming of several streams began at the extremely beginning of this session.
These rivers transported enormous quantities of silt and stored it, which improved the
way a delta was set up. Rocks in the Barail group have lithological properties that
suggest a delta to the statement's near-shore environment. Ocean was retracted from
the investigated range following the Barail's declaration, as shown by a territorial conflict
among the Barail with the lower Surma. Under the humid, tropical to subtropical climate,
iron-rich laterite may have formed throughout the protracted appearance of Barail.
The greatest orogenic upliftment of the Himalaya occurred during the Miocene era. The
mega delta was improved by the sand, debris, and clay particles transported and stored
by many streams. As the shoreline retreated, the delta moved steadily to the south. A
number of pebbles were positioned beneath such Surma environmental testimonies.
The grain's size and shape reveal the moo's vitality level over extended journey. The
southerly growth of the delta continued during the late Miocene and early Pliocene
periods. In this way, the environment gained over it. Tipam dregs were preserved in
high vigour in a fluvial habitat on the mainland. Massive bedding and the lack of direct
bedding suggest a speedy declaration.
Following placement of Tipam Girujan clay, a locally produced lake within the fluvial
system was retained beneath a lacustrine environment. Dupitila was retained beneath a
fluvial environment on the continent during the Pliocene. Quartz granules being nearby
suggests that Dupitila was disallowed from handling maritime transgression and
relapse. Following the Dupitila's testimony, the zone underwent a significant structural
change. The whole tertiary layers collapsed due to displacement along the Dauki fault.
Page 44 of 90
Because of the proximity of the conglomeratic layers, it is evident that the range
underwent upliftment up to the Pleistocene.
The rocks were placed uniformly over the slope after being transported by Pleistocene
streams. Secondary development as bed stack storage. Between the declaration of the
Dupitila layout and the later alluvium, there is a temporal gap indicated by the rocks.
The alluvium deposits suggest that the studied area was again buried below the
sedimentation as well as fluvial framework throughout the recent past and subsequent
periods.
Page 45 of 90
Chapter 5 : Sedimentology
Page 46 of 90
5.1 Grain size Analysis
Petrography is the examination of rocks in lean areas employing a petrographic
magnifying instrument. Estimate examination, lean area examination, and the presence
of microfossils are utilized within the petrographic think about. Grain measure
examination is fundamental for categorization, depositional history examination, and so
on. Lean area examination is advantageous for overwhelming mineral investigation,
compositional and textural examination, and microstructural examination. The consider
of microfossil event has importance for building up whether dregs have been set by
transportation or in situ.
Sample No : 01
Name of the Sample : Renji Sandstone
Time of Sieving : 25 mins
Amount of Sample : 100 gm
Diameter
(phi)
Diameter
(mm)
Weight
retain (gm)
Weight
percent (gm)
Cumulative
weight percent
(gm)
1
1.50
2
2.50
3
3.50
4
>4
0.495
0.351
0.250
0.177
0.124
0.088
0.063
<0.063
0.11
0.08
1.24
6.60
20.51
14.29
6.88
3.73
0.21
0.15
2.31
12.30
38.23
26.64
12.82
6.95
0.21
0.36
2.67
14.97
53.20
79.84
92.66
99.61
Total Amount of Sample
Sieve loss
= 100 gm
= (100 – 99.61) gm
= 0.39 gm
Total Coarse Sand (wt%)
Total Medium Sand (wt%)
Total Fine Sand (wt%)
= 0.21
= 2.46
= 89.99
Page 47 of 90
Wentworth
size class
Coarse Sand
Medium Sand
Fine Sand
Very Fine Sand
Silt + Clay
Figure 5.1 : Histogram of Renji Sandstone
Description of the Histogram :
The histogram was generated using analytical data on grain size. A regular
arithmetic graph paper is utilized in this case. A histogram is created by comparing the
weight percent retention in the vertical scale to the diameter of grain in mm in the
horizontal axis. The histogram illustrates a unimodal grain size distribution with modal
classes ranging from 1.00 mm to 0.495 mm and 0.25mm to 0.124 mm, which are
referred to as principal maxima and secondary maxima, respectively.
Unimodal distribution of grain size may result due to any of the following reasons :
➢ Lack or abundance of certain grain size in the source materials.
➢ Different modes of deposition.
➢ Abnormal variation in depositional energy.
➢ Mixing of materials from two or more sources.
➢ Improper sampling
Page 48 of 90
Figure 5.2 : Cumulative Curve of Renji Sandstone
Description of the Cumulative Curve :
The cumulative curve was constructed by taking cumulative weight % in the vertical
scale and grain size in mm in the horizontal scale. The picture displays a "S" shaped
curve from which quartile and percentile values were obtained to compute the Trask
Method grain size parameter.
25 percentile (25p) or 1st quartile (Q1)
= 0.161 mm
50 percentile (50p) or 2nd quartile (Q2)
= 0.129 mm
75 percentile (75p) or 3rd quartile (Q3)
= 0.095 mm
Median (Md) = Q2
= 0.129 mm
Co-efficient of sorting, S0
=
Co-efficient of skewness, Sk
=
Q1
Q2
=
𝑄1 𝑄3
0.161
0.129
=
𝑀𝑑2
Page 49 of 90
= 1.248
0.161 × 0.095
0.1292
= 0.919
From Trask Method,
1. Median is 0.129 mm indicate that the velocity of transporting media was weak, which
could transport fine sand sized particles, probably by saltation and suspension.
2. Sorting is 1.248 which indicates moderately sorted. It means that depositional media
get enough time to well sorting.
3. Skewness is 0.919, where positive mean scam admixture and the distribution of
grains are termed as finely skewed.
Page 50 of 90
5.2 Heavy Mineral Analysis
Two distinct types of minerals compose the majority of rocks. There are two types of
minerals: heavy minerals and light minerals. The economy gets an advantage from
heavy minerals. On how accurately samples are taken, a heavy mineral analysis's value
is greatly dependent. The process of intensive sampling must be carefully planned.
After sampling, they proceed to a few phases of finalization before producing a slide.
Here are some heavy minerals found in Renji Sandstone :
Biotite : It is identified by its brown colour, high relief, strong pleochroism, subhedral
form under plane polarized light and parallel extinction, II order red maximum
interference colour under crossed polarized light.
Figure 5.3 : Biotite under plane polarizer (40X)
Figure 5.4 : Biotite under cross polars (100X)
Page 51 of 90
Kyanite : It is Identified by its colourless appearance, high relief, two directional
cleavage under plane polarized light and I order red maximum interference colour,
inclined extinction under cross polarized light.
Figure 5.5 : Kyanite under plane polarizer (40X)
Figure 5.6 : Kyanite under cross polars (100X)
Page 52 of 90
Rutile : It is identified by its subhedral form, brownish red colour, very high relief under
plane polarized light and parallel extinction, higher order maximum interference colour
under crossed polarized light.
Figure 5.7 : Rutile under plane polarizer (40X)
Figure 5.8 : Rutile under cross polars (100X)
Page 53 of 90
Zircon : It is identified by its subrounded form, high relief, colourless appearance under
plane polarized light and parallel extinction, higher order maximum interference colour
under crossed polarized light.
Figure 5.9 : Zircon under plane polarizer (40X)
Figure 5.10 : Zircon under cross polars (100X)
Page 54 of 90
5.3 Thin-section Petrography
The petrographic properties of the rock samples are examined in the thin section. The
rock samples were extracted during the fieldwork, analysed and assigned to slides for
petrographic investigation under the microscope. Rocks are composed of mostly of four
materials which are :
•
•
•
•
Framework grains,
Matrix,
Cement and
Pore spaces
These elements are most effectively studied in thin sections of rocks utilising a
petrographic microscope. As a consequence, thin section investigation is essential in
petrology.
Slide 01 : Fossiliferous Limestone (Sylhet Limestone)
Thin Section of Sylhet Limestone :
Texture : The grains of the observed rock are sub-rounded, moderately sorted, no
orientation and loosely binding grains.
Grains Framework :
Bioclasts : 35%
Ooids : 5%
Peloids : 30%
Calcite : 10%
Intraclast : 5%
Micrite : 15% (Peloidal, Microsparitic)
Cavity Structure : Geopetal Type
Fossil Contents : The fossils are Nummulites, Foraminifera and Alveolina are in large
amount.
Cement : Fibrous Calcite type cement found.
Evidence of compaction : Broken grains, Porosity intergranular and dissolution and
porosity reduce due to Calcitization and Compaction.
Name of the rock : The observed rock is Packstone (Fossiliferous).
Page 55 of 90
Depositional Environment : The depositional environment was warm, calm and
shallow sub-tidal with high energy condition.
Figure 5.11 : Sylhet Limestone under plane polarizer (40X)
Calcite
Figure 5.12 : Calcite Mineral with Fossils under cross polars (100X)
Page 56 of 90
Discocyclina
Figure 5.13 : Discocyclina in Limestone under plane polarizer (40X)
Alveolin
a
Figure 5.14 : Alveolina in Limestone under plane polarizer (40X)
Page 57 of 90
Figure 5.15 : Nummilites in Limestone under plane polarizer (40X)
Page 58 of 90
5.4 Facies Analysis and Depositional Environments
The term "rock facies" refers to a particular kind of rock unit. A single bed or a group of
beds might be present. It ought to ideally be a distinct rock that formed under particular
sedimentation circumstances, suggesting a particular procedure, set of circumstances,
and depositional environment. Following this, the facies are divided into lithofacies
associations, which may be used to understand depositional settings using
combinations of the physical, chemical, and biological processes discovered during the
facies study. In light of this, we may evaluate the depositional environment by
examining the facies. Every sedimentary formation we've discovered has a particular
sort of facies, which reveals the depositional environment. The analysis of facies
relationships will now be done, and each observable formation's depositional
environment will be interpreted separately.
5.4.1 Sylhet Limestone : In the Sylhet Limestone Formation study region, two different
facies have been identified. These are :
a. Crystalline Limestone Facies (Lc) and b. Fossiliferous Limestone Facies (Lf)
But some sub-facies are also observed. All the facies are tabulated below :
Facies
Texture
Limestone
with Very fine grained, dark grey
interbedded shale
coloured.
Light grey coloured, hard and
Crystalline Limestone
indurated
Fossiliferous
Dark grey coloured
Limestone
Limestone
with Yellowish coloured, very fine
terrigenous sediment sand to mud, micritic
Structure/Fossil
Thinly laminated
No specific structure
Nummulites,
Asselina
Alveolina,
Massive to thinly laminated
Depositional Environment : According to its facies association, the limestone from
Sylhet was deposited in a shallow marine environment, in a peaceful setting with
relatively little sediment supply. From the shelf edge, where mudstone has been laid
down, to the shelf habitats, where wackestone was deposited first, then micrite, its
depositional context changes. The depositional environment gradually moved towards
the continent as a result. The fact that fossils may be found everywhere suggests that
this limestone's origins were biochemical in nature.
Figure 5.16 : Fossiliferous Limestone Facies
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5.4.2 Kopili Shale : There are two different types of facies in Kopili Shale.
Facies
Texture
Dark grey coloured, silt and
clay sized
Laminated shale
Shale with interbedded
limestone
Dark grey coloured.
Structure
Thinly laminated
Laminated and thin layered
crystalline limestone.
Depositional Environment : For kopili shale, a shallow marine environment also
served as the depositional setting. Shale is calcareous and highly fracturable, which
imply significant levels of organic matter and give it a dark gray to black appearance.
The presence of organic materials indicates shelf settings and denotes a low energy
situation.
Figure 5.17 : Laminated Kopili Shale Facies
Log 1 : Sylhet Limestone and Kopili Shale
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5.4.3 Jenum Shale : We have found two different facies in Jenum Shale.
Facies
Texture
Parallel laminated shale
Dark grey coloured, silt
sized
Shale with coal lenses
Light grey coloured and
black coal lenses, silt and
clay sized
Structure
Paper Laminated with silty
streak and silt lenses.
Fissile
Lenses of coal with
lamination
Depositional Environment : The relationship between the facies and the depositional
environment of the prodelta with little current. Given that the higher section has some
coal lenses, a swampy environment was also present.
Figure 5.18 : Jenum Shale Facies
5.4.4 Renji Sandstone : The facies we found on Renji Sandstone are tabulated below :
Facies
Tabular cross bedded
sandstone
Trough cross bedded
sandstone
Parallel laminated
sandstone
Laminated sandstone with
interbedded silt
Texture
Very fine grained sand,
reddish colour
Very fine to fine grained
sand, yellowish to brownish
coloured
Very fine grained sand,
yellowish coloured
Very fine grained sand with
silt, grey to bluish grey
coloured
Structure
Tabular cross bedding,
clay galls
Large scale Trough cross
bedding
Parallel lamination
Parallel lamination of sand
and silt
Depositional Environment : Renji sandstone's facies associations imply that it was
deposited mostly in a river setting. The presence of large-scale trough cross bedding
suggests a high energy situation. Given that the outcrop is shaped like a grin, the sand
facies support the channel fill deposit. This deposit was likely subjected to weathering
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and erosion for a considerable amount of time based on the existence of laterite and
conglomerate.
Figure 5.19 : Renji Sandstone Facies
5.4.5 Laterite : The facies observed on Laterite are mentioned below :
Facies
Laterite bed
Texture
dark brown coloured,
conglomerate and quartz
pebble
Structure
Massive bed
Depositional Environment : It was created in an oxidizing environment in a warm,
humid climate as a result of changes in the water table.
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Log 2 : Renji Sandstone and Laterite
5.4.6 Bhuban : Numerous amount of facies are observed in Bhuban formation and the
facies are :
Facies
Texture
Structure
Ripple cross laminated
sandstone
Very fine to fine grained sand,
yellowish colour
Ripple cross lamination
Massive sandstone
Fine sand with little silt
No structure
Ripple cross bedded
sandstone
Fine sand and silt, bluish grey
colour
Ripple cross bedding
Trough cross bedded
sandstone
Grey to yellowish coloured,
fine grained sand
Large trough cross bedding
Parallel laminated sandstone
Yellow to light brown cored,
fine grained sand
Parallel lamination
Sand and mud alteration
Very fine sand with clay and
silt
Lamination, load cast and
flame structure
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Flaser bedding
Very fine sand with silt, bluish
grey coloured
Ripple cross lamination with
thin and discontinuous mud
drapes
Wavy bedding
Fine sand with mud, brownish
coloured
Wavy bedding, continuous
mud drapes with sand ripples
Lenticular bedding
Mud with little sand, dark grey Discontinuous sand ripples in
coloured
mud
Bouma sequence
Sand and silt
Ripple cross laminated sand,
parallel laminated silt and mud
Nodular shale
Silt, bluish grey coloured
Calcareous nodules
Depositional Environment : Its facies relationship suggests that the environment in
which Bhuban was deposited was deep marine to deltaic. Shale predominates in Lower
Bhuban, and the occurrence of the Bouma sequence suggests a deep marine
environment in the submarine fan. A deltaic environment with fluctuating energy
circumstances is indicated by the presence of heterolithic bedding and cross bedding.
Figure 5.20 : Trough bedded sandstone facies
Figure 5.21 : Parallel Laminated Sandstone Facies
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Figure 5.22 : Ripple Cross Laminated Sandstone Facies
Figure 5.23 : Nodular Shale Facies
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Figure 5.24 : Bouma Sequence Facies
Log 3 : Lower Bhuban Formation
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5.4.7 Bokabil : Many types of facies are found on bokabil formation and they are :
Facies
Heterolithic
bed
Texture
Flaser
bedding
Very fine sand with silt,
bluish grey coloured
Wavy
bedding
Fine sand with mud,
brownish coloured
Lenticular
bedding
Mud with little sand, dark
grey coloured
Fined grained sand, grey
coloured
Dark coloured, clay
dominated
Upper gas sand
Upper marine shale
Shoreface deposits
Structure
Ripple cross lamination with
thin and discontinues mud
drapes
Wavy bedding, continuous
mud drapes with sand
ripples
Discontinuous sand ripples
in mud
Large trough cross bedding
Massive
Fine sand dominated with silt Hummocky cross bedding
Depositional Environment : Facies associations suggest that it was deposited in a
fluvial to deltaic environment, most especially in channel-related tidal flats. Indicating a
change from tidal flats to river environments, litho unit thickness rises from north to
south. Trough cross bedding, another sign of the fluvial environment, may be found in
the Upper Gas Sand. Pro delta to open marine environment in the intertidal zone is
supported by younger direction tides and thick units. Following that, a tidal flat
environment once more formed, and over time, as shown by the Upper Marine Shale,
the environment began to deposit more marine material. A stormy environment was
then indicated by the deposition of shoreface sediments.
Figure 5.25 : Heterolithic Bedding Facies
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5.4.8 Dihing Formation : Sandstone and quartzite gravels are present, together with a
sandy to silty matrix. From cobbles to pebbles, gravel comes in all different sizes. The
major support comes from clasts. Shillong massif, which is igneous and metamorphic in
nature, is the source of the clasts. In a fluvial setting, it may have been created by
glacial outwash or river water.
Figure 5.26 : Dihing Formation
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Chapter 6 : Sequence Stratigraphy
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6.1 Sequence Boundary
In stratigraphy, the sequence boundary is a surface that denotes a substantial shift in
sedimentary deposition. It denotes a change in environmental circumstances or a break
in deposition by marking the border between two sets of rock layers (strata).
The region's stratigraphy has been divided into a variety of formations. The Sylhet
limestone formation is discovered to be the earliest in the conventional order. According
to the rule of superposition, the investigated formation typically occurs in the following
order, from oldest to youngest :
1) Alluvium
2) Dihing Formation
3) Dupi Tila formation.
4) Girujan clay.
5) Tipam sandstone.
6) Surma group
7) Barail sandstone
8) Kopili shale
9) Sylhet limestone
In Evans' (1932) description of the Tertiary successions of Assam, the terms of the
formations are established. Despite the fact that frequent facies changes make it
impossible to compare formations that are hundreds of kilometers apart without the aid
of palaeontological evidence.
6.2 Stacking Patterns and System Tracts
The sedimentary rock strata present in the northeastern Bangladeshi area of Sylhet are
referred to as Sylhet stratigraphy. These rock strata are a member of the Paleogene-era
Surma Group, a geological formation that also contains a series of sediments. There
are a number of stacking structures and system sequences that may be found in the
Sylhet strata.
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➢ Stacking Patterns : Sedimentary layers are organized vertically and described as
stacking patterns within a depositional region or a set of strata. Changes in
environmental conditions, the accessibility of sediment, and changes in sea level
are the causes of these phenomena. Examples of common stacking patterns are
:
✓ Aggradational Stacking : A vertical succession of sedimentary strata
generated by sediment deposition under essentially steady environmental
conditions. This might result in a repeating and parallel layering.
✓ Progradational Stacking : Progradational stacking refers to sedimentary
strata that show a general seaward migration or extension of the
depositional region. This pattern commonly develops as sediment supply
increases or sea level rises.
✓ Retrogradation Stacking : Retrogradational stacking refers to the
stratification of sedimentary strata that show a landward movement or
contraction of the depositional region. If the amount of sediment is
dwindling or the sea elevation is rising, this pattern can develop.
Table 02 : Stacking patterns and System Tracts
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➢ System Tracts : System tracts are collections of sedimentary rocks which reflect
particular depositional settings and are enclosed by unconformities or significant
sedimentary surfaces. They are given names based on where they are in the
sequence and the related sedimentary processes. System tracts that are often
used include :
✓ Highstand System Tract (HST) : A deposit of sediments that forms during
periods of comparative sea-level highstands and is defined by
progradation. Due to minimal subsidence or sea-level rise over this time,
the sediment supply to the basin might surpass the available space. The
growth of coastal or shallow marine deposits is a feature of the HST.
✓ Transgressive System Tract (TST) : Represents a stage of sea level rise
that leads to the repositioning of sediment. Shorelines move inland during
transgression, and sediments are laid on top of previously created
deposits. The TST frequently covers deeper or offshore marine facies.
✓ Lowstand System Tract (LST) : Occurs when the water level is quite low
and is defined by aggradational sedimentary deposition. Because of
subsidence or lowering sea levels, the supply of sediment may be greater
than the available space. The LST is frequently linked to progradational
sedimentary deposits, including deltaic or river systems.
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6.3 Sequence Model
A strong tool for analyzing and interpreting the vertical as well as lateral interactions of
sedimentary layers in a particular location is sequence stratigraphy. It aims to explain
the shifting depositional conditions and comparative sea-level changes that throughout
time formed the sedimentary formations. The geological history of a region is
characterized by a variety of system tracts, boundary surfaces, and depositional
sequences, which are identified by sequence stratigraphy.
Table 03 : Stratigraphic Sequence Model
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Chapter 7 : Economic Geology
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7.1 Petroleum System Analysis
“Petroleum system” is a general term that covers everything related to petroleum
geology. It includes the basic elements (source, reservoir, seal, and overburden rock)
and the processes (trap formation, generation-migration-accumulation) that makes up
the petroleum. It also includes all petroleum that came from the same source rock and
that can be found in shows, seeps, or accumulations. Petroleum systems are useful for
resource assessment, exploration, and academic study.
Figure 7.1 : Oil and Gas Fields in Bangladesh and Assam Region
7.1.1 Source Rock
The rock that formed in the jenum formation is thought to be the primary source rock of
the investigated location. Jenum Formation is comprised of sandstone, siltstone, and
silty shale. The sandstone is predominantly pink in colour, covered to light yellow and
grey, and is composed of extremely fine to medium grained, frequently cross-bedded,
and thin to thick-bedded Argillaceous and Ferruginous components. The block joined
sandstone may be found in a variety of environments.
Kopili shale is another source rock. It is a dark grey to black shale that is exceedingly
fissile, densely bedded to paper laminated, and heavily jointed. There is interbedded
sandstone with argillaceous matrix. However, the area of the kopili shale is less than
that of the jenum formation.
The Kopili formation has a TOC of 0.5% to 0.8%, while Jenum has a TOC of 0.3%,
however Jenum shale has a high thickness and extent. As a result, Jenum contains
more petroleum than kopili shale due to its smaller extent and thickness.
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7.1.2 Reservoir Rock
Surma group rock is considered reservoir rock for petroleum. The reservoir rock is
simultaneously permeable and porous. The Surma group is composed of bedded,
laminated siltstone, shale, silty shale, claystone, and sandstone. It is made up of
yellowish grey sandstone, bluish grey shale, sandy shale, and siltstone. Sandstone is
fine to medium grained, sub-angular, and almost sorted, durable, weather resistant, and
creates cliffs. The shale is blue grey in colour, highly laminated, hard, and jointed.
Sandstone is a desirable reservoir rock in this case, but shale is not due to sandstone
has strong porosity and permeability, which is helpful in the production and migration of
water or petroleum-oil-gas, but shale has a lower degree of permeability.
Surma is separated by two formations: Bhuban and Bokabil, and a contact between
Bhuban and Bokabil. Upper bokabil is sometimes referred to as upper gas sand.
A different reservoir rock is Dupitila, which is a water reservoir known as our primary
aquifer. The lithology is dominated by sandstone and siltstone, with claystone interbeds.
The sandstone is yellowish brown in colour, medium to coarse in texture, less compact,
and very porous. It ranges from huge to thickly bedded. The sandstone is dominated by
quartz, with a substantial proportion of mica and dark coloured minerals. It has quartz
granules and clay ball stones and is sometimes distinguished by iron encrustation.
7.1.3 The Trap and Seal
One field team examined Anticline as the structure of the studied geographic area.
According to Khan (1978), it is a large homoclinal fold with a gentle anticline and
syncline. The Dauki fault's right lateral displacement resulted in the formation of a
refolded structure. This anticlinal structure is considered as a petroleum trap in the
examined area. It's a low intensity anticline that's appropriate for petroleum
accumulation.
Upper marine shale is a seal for petroleum that lies above the Surma group and
prevents gas migration. The impermeable layer acts as a seal, preventing further
movement and assisting in the accumulation of petroleum in the trap and reservoir rock.
7.1.4 Timing of Hydrocarbon Migration and Accumulation
After petroleum is produced by thermal maturation, hydrocarbons move to reservoir
rock and collect in the reservoir after traps and seals are established.
The initial main migration for capillary pressure occurs within the borail source rock. The
hydrocarbons then migrate to a surma group reservoir (Eocene formation), which is
thought to represent secondary migration beneath the trap. As a result, it may be
concluded that the hydrocarbons migrated and accumulated after the Eocene era till
present. As the source rock, trap, reservoir rock, seal, and migration period were all
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ideal, we got a hydrocarbon gas reservoir inside the examined region and its adjacent
Sylhet area.
Figure 7.2 : Gas migration paths from Oligocene shale source to Miocene sand
reservoirs (adapted after Hiller and Elahi 1984)
7.2 Economic Mineral Deposits
There are no economically viable deposits in the subject of investigation area. Mineral
resources that are important in terms of the Bangladeshi economy are few. Tertiary rock
in the studied region has no commercially valuable mineral resources.
Sandstone
The sandstones of the Jenam, Tipam, and Dupi Tila formations are comparatively
poorly cemented. Such sandstones are failed to satisfy the basic standard requirements
related to usage as a construction material.
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Figure 7.3 : Hardrocks at Jaflong, Sylhet
Gravel and Boulders
The gravels transmitted by the Dauki and Rangapani rivers are the most commercially
important geological constituent of the area. Boulders that range from rounded to subrounded in shape. Pebbles are mostly made up of gneiss, quartzite, and granite.
Hundreds of tonnes of such hard rock gets transported daily throughout the whole
country by train for use in construction, road construction, multi-storeyed constructions,
railway ballasts, and other purposes. The volume of this hard rock is estimated to be
one million cubic feet (Khan M, 1978). The gravels are usually 3 to 8 feet long and 2 to
4 feet thick. It is about one and a half miles long, fifty feet wide, and four feet thick in the
Dauki river, while it is only about one and a half miles long, fifty feet wide, and four feet
thick in the Rangapani river.
Figure 7.4 : Gravels and Boulders in Jaflong Section
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Sylhet Limestone
In the area being investigated location on the eastern bank of the Dauki River, a very
little faulted portion of Eocene Sylhet limestone is readily apparent. The Chatak
cement factory utilises Sylhet Limestone to make cement that is superior to all other
cement in the world. The company has extracted all of the limestones. Its resource is
now too limited to justify further investigation. It offers lime to the locals as well as other
home applications.
Figure 7.5 : Sylhet Limestone (Limestone with Shale)
Sand
The Dauki and Rangapani rivers supply an enormous quantity of sand, which is widely
used as a construction material across the whole country. The sand known as Sylhet
sand is of superior quality.
Gas and Oil
At the present time, 22 natural gas fields and 1 oil field (well-7 of Sylhet gas field) have
been identified in Bangladesh. The Surma basin was site to the majority of the gas and
oil fields. This gas and oil were discovered in the sandstone reservoirs of the Bhuban
and Boka Bil formations. Surma group is Mio-Pliocene in age and is located at a depth
range from the surface. These gas and oil reserves are located in the folded belt with
moderate anticlinal fold-forming traps (Prof. Badrul Imam, 1984). As a consequence,
there is a possibility of discovering hydrocarbon in the explored location, for which an
extensive geological exploration of the Jaflong-Lalakhal area is extremely required.
We have been travelled to the Kailashtila Gas Field in Sylhet, which is one of 22 natural
gas fields in Bangladesh. According to the Bangladesh Atomic Energy, a minor amount
of Uranium (Ur) may be discovered in the investigated geographic area.
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Laterites
Laterites can be found mostly between the Barail and Surma groups of rocks and cover
a considerable area in Jaintiapur, are locally utilized for modest uses. The identified
laterite is porous and yellow. It has a tough covering ferruginous accumulation on the
accessible surface. Because of its massive hardness, it is often used as slabs in pool
staircases. Small pool culverts are also constructed on a small scale.
Figure 7.6 : Laterite
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Chapter 8 : Environmental Issues
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8.1 Blowout Events in Sylhet Field
Blowout is sudden and uncontrolled flow of fluids from the underground,
when the fluid is gas then it is called gas blow out. This can happen when the drill string
reaches an overpressure zone of oil, gas or water at the underground, which can cause
the fluid (gas, oil or water) to enter the drill string with a lot of force. These fluids (gas, oil
or water) can then rise to the surface and hit the rig floor violently and create blowout.
Blowout can be classified into three categories and they are surface, subsea and
underground blowout.
A various amount of blowout events occurred in Sylhet Gas Field and some of the major
events are discussed here :
Blowout in Sylhet :
The Sylhet-1 well was drilled upto a depth of 2377 meters and then found
gas, after they installed a casing, the blowout became uncontrollable, was set on fire
and the entire rig was ruined. A large depression was created, where the rig
submerged. The Sylhet-4 well had a similar blowout when it was drilled to only 314
meters beneath the surface.
Effect of the blowout : A crater was produced and overflowed with water as a
consequence of the blowout at the Sylhet-1 well, producing an enormous pond which is
still there today and expelling gas from the subsurface into the year. The blowout at
Sylhet-4 is particularly dangerous due to the well was abandoned at the moment, and
gas is endlessly leaking out from cracks in the well site and neighbouring hillside, which
frequently causes fire.
Table 04 : Blowout in Sylhet Gas Field
Blowout in Magurchara :
The aim of this well was to drill to a depth of around 3400 meters beneath
the ground and to drill through a layer of weak sand called Tipam Sandstone Formation
near the surface and then through a solid layer of shale called Upper Marine Shale
(UMS) without placing a casing in the fragile Tipam Sand layer above the UMS. The
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well came across a gas zone at a depth of about 800 meters from the surface. After
drilling 840 meters vertically, the drillers halted the drilling and began to remove the drill
pipes to get ready for changing the direction of the well from its vertical path, but during
this activity, the gas from the gas zone below, entered the drill hole and pushed its way
up, which ultimately caused the well to burst.
Effect of the blowout : Drilling by a US company led to a big gas blast in Moulvibazar,
Bangladesh, ruining a lot of property and nature. The victims were unhappy with the
government’s inaction in paying them for the damages. They wondered why the
government was silent for so long on their compensation demands. The fire damaged
the plants and animals of the Lawachara forest near the well. It burnt a teak plantation,
bamboo huts, and other vegetation from different years. Almost 100 acres of Lawachara
forest were totally burnt. Half of the forest resources on 111 acres and a third on 106
acres were also lost. Experts said the loss was impossible to recover. After the fire, wild
animals came to nearby homes looking for food.
The reports said the forest resources lost Tk 9,858 crore, while 29 tea gardens lost Tk
46 crore. The railway lost Tk 21 crore, Jalalabad Gas Tk 43 lakh, the electricity Tk 4
crore. Local Khasia people lost betel leaf farms worth Tk 18 lakh[12]. A group said the
country lost Tk 9000 crore and a lot of gas in the fire, and the environment, ecology and
wildlife were badly hurt.
Figure 8.1 : Maguchara Blowout
Blowout in Tengratila :
Driller began to draw out the drill string after constructing a well through a
loosely consolidated sand layer following the Upper Marine Shale without arranging
casing in the loose sand unit. This generates a cleansing procedure on the gas zone
and drives gas into the drilling hole, leading to its blow out.
The major two factors of the accident were :
i) Failing to put any casing in the loose sand zone (Tipam sand unit) and
ii) Pulling out the drilling rig from the gas zone while the sand above continued to be
uncased and unprotected.
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Effect of the blowout : A gas well in Tengratila exploded on January 8, 2005, causing
ten thousand terrified people to leave their homes. The fire was so big that it could be
seen from 30 kilometers away. The blast burned millions of cubic feet of gas from
Niko’s Tengratila gas field.
Another explosion happened on June 17, 2005, six months after the first one. The gas
field was on fire for several months and the authorities did not take any quick and
effective actions. The Department of Environment (2005) reported that the environment
suffered a lot of damage, especially the soil of the nearby field. The land of the area was
mostly hilly, with some alluvium and basin parts. The gas and sand emissions affected
the hills, valleys, mounds and low lands around the blowout. The fire also damaged the
area of Tengrabazar, the houses, the forest and the fruit trees on the hills. The sand and
clay soil under the ground was mixed with gas from the main field to 2-3 km away from
Tengratila. The soil resources loss was split into four levels: most affected, very
affected, moderately affected and less affected.
Figure 8.2 : Tengratila Blowout
The blowout losses are too big for Bangladesh and hard to measure in
money. The environment is also damaged badly. The effect will hurt the country for a
long time in economy and ecology. If more blowouts happen, the gas reserve and
energy sector will be in danger. We can’t undo the accidents, but we can prevent them
by taking the necessary steps. Automated drilling rig is expensive but safer. If not,
manual drilling rig workers should be careful and stop the kick, which is the first sign of
blowout.
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Chapter 9 : Summary and Conclusions
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9.1 Summary and Conclusions
The field study was conducted in Sylhet, a hilly region of Bangladesh that
has many rivers flowing into it from the Shillong Plateau. These rivers, along with other
water bodies like streams and khal, have a winding course and transport a large amount
of sediment. The area is influenced by the tectonic activity of the Bengal basin, and has
several tectonic features. The two most prominent structures are a faulted anticline,
which is a fold that is broken by faults, and the Dauki fault, which is a major fault that
runs along the northern edge of the region and is believed to be the western
continuation of the Naga-Disang thrust fault system, which is a system of faults that
thrust one rock unit over another. The area also has smaller-scale structures like
monoclines, which are folds that dip in one direction, faults, which are fractures in the
rocks, multidirectional joints, which are cracks in the rocks that have different
orientations, and unconformities, which are gaps in the geological record between
different formations.
The observed sediments and rocks in the study area, which range from the Eocene to
the Recent eras. The basin has different sedimentary rocks, such as sandstone, shale,
mudrock, conglomerate, breccia, limestone, and others. These rocks are divided into
several groups: Jaintia, Barail, Surma, and Tipam. On top of these groups are the Dupi
Tila formation and the alluvium. Only the limestone group of Sylhet has fossils. This
work used the data from the rocks to infer their depositional environment and sequence
stratigraphy. These rocks were mostly formed in shallow sea, sea, river, and delta
settings.
The study area is economically important for the country because it has resources like
hydrocarbons, gravel, and groundwater reserves. Hydrocarbons are organic
compounds that consist mainly of hydrogen and carbon atoms. They are used as fuels
or raw materials for various industries. Gravel is a type of coarse aggregate that is used
for construction or landscaping purposes. Groundwater is water that is stored
underground in pores or cracks in rocks or sediments. It is used for drinking or irrigation
purposes.
To conclude, the study area is a unique region in Bangladesh for its unique nature of
being the only region that represents almost all of the stratigraphy of Bangladesh.
Stratigraphy is the study of the layers or strata of rocks or sediments. It helps to
understand the geological history and evolution of an area. The study area’s tectonic
features and economical value only make it more worthy of being an area of interest for
researchers and explorers.
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9.2 References
➢ Banglapedia.
➢ Encyclopedia Britannica.
➢ Evans, P. (1933): Tertiary succession in Assam, than, geol. Inst. India, v-27.
➢ Evans, p. (1964): The tectonic frame work of Assam, Geol. soc. India, Jour. vol.5,
pp-80-85.
➢ Geological note book wrote during field work.
➢ Haque, M. 1982: Tectonic setup of Bang and its Relation to Hydrocarbon
Accumulation, Phase-1: Centre for policy Research (D.U), and University Field
staff International (UFSI), U.S.A.
➢ Haque, M. N.: Paleontology of the Tertiary limestone and associated sediments,
M. S. Thesis, 1969.
➢ Hari Prasad Paul (1988): Structure and tectonics of north Eastern part of the
Surma Basin. Sylhet, Bang. M.sc Thesis, Geology Dept. Dhaka University.
➢ Hiller, K. and Ellahi, M., 1984: Structural development and hydrocarbon
entrapment in the development in the Surma Basin, Bang. (Northwestern Indo Burman Fold Belt), 5th offshore South East Asia conf. session- 6. logging,
Singapore.
➢ Holtrop, J. F. and Keizer, j. (1970): Some aspect of the Stratigraphy and
correlation of the Surma Basin Wells, East Pakistan, ESCAFE minerals
Resources Development Series,no.6.
➢ Imam, M.B, 2005: Mineral Resources of Bangladesh.
➢ Khan, M.A (1978): Geology of the eastern and north eastern part of sadar
subdivision Sylhet district in Bangladesh. Record of G. S. B. vol.2, part-iv.
➢ Mathur, L. P. and Evans, P: 1964: Oil in India Inter. Geol. Cong. 22nd Session
➢ Monsur M. H.: An Introduction to the Quaternary Geology of Bangladesh. A
complimentary research of IGCP 347. Quaternary Stratigraphic Correlation of the
Ganges-Brahmaputra Sediments (1995)
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Chapter 10 : Annex
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10.1 Geological Map of The Study Area
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Figure 10.1 : Base Map of the Investigated Area
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Shah Ahmed Sayemur Rahman Sayem