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Geological Structure


Geological Structure any physical arrangements of rocks (such as an anticline or reef) that may hold an accumulation of oil or gas. In Bangladesh there are many prospective structures which are as follows:

Titas structure a subsurface anticlinal structure located in brahmanbaria district. It is identified by seismic survey. The structure has no surface geomorphic expression and is covered by Titas-Meghna river floodplain. The tista river runs through the crestal part of the structure.

Titas anticline is a north-south elongated semi-domal structure. This is in contrast to all the neighbouring anticlines, which have narrow elongated forms. It is also structurally higher than the surrounding Rashidpur and Habiganj structures suggesting that Titas may have been influenced by tectonically positive element from the deep subsurface. Titas anticlinal closure is one of the largest (168 km) in Bangladesh. The structure is asymmetric in nature with steeper dip in the eastern flank and gentler dip in the western flank. There has been indication of faulting in the deeper level in the eastern flank, as shown by seismic reflection discontinuities.

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Shell oil company drilled Titas-1 exploratory well in the structure, about 4 km north of Brahmanbaria town and discovered gasfield in 1962. The Titas gasfield has been the largest producer of gas till date in Bangladesh. The gasfield has 11 gas zones in between depth of 2616m to 3,124m below surface, and five of them are considered major. The total gas initially in place (GIIP) in the Titas gasfield is estimated at 4,134 billion cubic feet of which the recoverable reserve is 2,100 billion cubic feet. More than 75% of the recoverable reserve has already been produced by the end of the year 2000. The gas is housed in multilayered Miocene-Pliocene sandstone reservoirs of Bhuban and Boka Bil formations. Drillings in Titas structure have encountered overpressure at depths in excess of about 3,600m. Gas production from the Titas gasfield started in 1968 and at present the production rate is about 280 to 300 million cubic feet of gas per day.

Rashidpur structure an anticlinal structure in the habiganj district. The anticline is north-south trending, 35 km long, 7 km wide and is asymmetric in nature with steeper (22° to 25°) eastern flank and gentler (8° to 12°) western flank. The axis of the anticline is in structural alignment with Teliamara surface anticline of Tripura to the south in India. In the outcropping Dupi Tila Formation in Rashidpur structure, flank reversal can be noticed to indicate extent of axial stretch.

The Rashidpur structure is affected by a major north south trending longitudinal fault in the eastern flank with down thrown block to the east. There are transverse faults as well as a major one passes through the middle part of the structure.

Shell Oil Company drilled exploratory well in Rashidpur structure and discovered gasfield in 1960. This was the first of the five large gasfields Shell Oil discovered during 1960s. The gas initially in place (GIIP) in Rashidpur gasfield is estimated at 2,242 billion cubic feet with a recoverable reserve of 1,309 billion cubic feet. There are two gas zones at depths between 1,380m to 2,787m below surface. The gas reservoirs are sandstones of Miocene-Pliocene age and are considered to have been originated in shallow marine depositional environment. The reservoir porosity-permeability values of the sandstone reservoirs are very good. Drilling in the Rashidpur gasfield has encountered overpressure zone in the deep subsurface level.

Sylhet Gas Field Company Limited, a subsidiary of petrobangla, operates the Rashidpur gasfield. Rashidpur is a producing gasfield and is producing about 90 million cubic feet of gas per day at the beginning of 2001.

Fenchuganj structure lies under fenchuganj upazila of sylhet district and about 40 km south east of Sylhet town. It is bounded by longitude 91°-92° East and latitude 24°30'-24°37' North. Tectonically this structure is located in the transition zone between the central surma basin and the Folded Belt in the east. It is an elongated structure and about 30 km long and 8 km wide. It is a reversibly faulted asymmetrical anticline with NNE- SSW trending axis. The eastern flank has sharp dip than the western flank. The amount of dip in the eastern flank varies from 30°-35°, whereas in the western flank dip varies from 20° to 25°. Fenchuganj anticline is structurally higher in comparison to Jalalabad, Kailash Tila, and Beanibazar anticline with reference to Upper Marine Shale of Miocene age but it is lower than Atgram anticline according to prominent horizon like Upper Marine Shale (UMS). The exposed rock is Dupi Tila of Plio-Pleistocene age.

The structure has both dipped and faults closure. Dip closure decreases and fault closure increases with the increasing depth. The anticline has amplitude of about 200m. Structural pattern of Fenchuganj anticline commenced after deposition of the Uppermost Miocene sediment ie Upper Marine Shale.

Fenchuganj gasfield discovered by Petrobangla in 1988 is a moderate size field, which has the deepest well in Bangladesh (total depth 4977m). There is some non-commercial oil found in this field. The field is not under production now. [Badrul Imam]

Habiganj structure lies in the northeastern part of Bengal Foredeep and in the south-central part of Surma Basin. It is the northern culmination of Baramura anticline of Tripura (India), is separated by a saddle. The structure is almost symmetrical and trending SSE.

Seismic and well data of Habiganj structure have proved that the structure has entirely separate closure. Including Baramura it is a giant structure of about 130 km long. In Habiganj structure the exposed rock is Dupi Tila and the topographical height is about 20m above mean sea level. No fault has been detected on flanks

Habiganj structure has one of the largest producing gasfield of the country. Pakistan Shell Oil Company discovered the field in 1963. The gasfield has a closure 11 km long and 4.5 km wide. It is known for the excellent quality reservoirs with up to 30% porosity and several darcy permeability. So far 10 wells have been drilled in this structure. The Upper Gas sand extends from 1,255m to 1,585m subsea.

The Upper Gas sand is fine grained, clean, unconsolidated and well sorted. Lack of clay matrix and presence of glauconite in Upper Gas sand indicate the deposition in high-energy littoral to sublittoral environment. The sand indicate a beach and barrier bar characteristic. The Upper Marine Shale acts as a cap rock for Upper Gas sand. [SK Aminul Islam]

Beanibazar structure lies in the northeast of Surma Basin where most of the gasfields are located. The drilling of the Beanibazar X-1 well in 1980-81 discovered gas. The structure is asymmetrical anticline having narrow crests and steep flanks, complicated by strike faults. Like other structures Beanibazar structure also contains almost exclusively a classic sequence of deltaic, fluvial, and to a lesser degree of marine sandstones, siltstones, shales and claystones. The thickness of these sediments increases southwestward and westward. Tectonically Beanibazar structure is the result of two major forces, a westward compressional force that has resulted in the formation of the Indo-Burman Range and a northern component associated with movement along the Dauki Fault parallel to the shillong massif. The primary result of this tectonics is a series of north-south oriented asymmetrical anticlines in eastern Bangladesh, India and Burma and the degree of deformation increase eastward.

This Beanibazar structure was first recognised by the Pakistan Shell Oil Co in the early 1960's based on single fold seismic data and later Prakla seismic defined it more precisely by a survey conducted during 1979-80. This structure, which was originally, called the Mama Bhagna anticline, trends north-south with a total closure area of about 35 sq km and closure height of 214m at the Upper Gas Sand horizon. The structure is defined exclusively based on seismic data and drilling results of one well. These data suggest no faulting or other structural complications associated with Beanibazar.

Bibiana structure located on Block-12 in the Surma Basin of Bangladesh. The structural pattern is similar to the Beanibazar structure. The Bibiana structure is trapped in a large four way subsurface structural closure trending north to south approximately 35 km north of Rashidpur anticline. The reservoir section is within the Miocene Surma Group where hydrocarbons are located within six sand packages, each are separated by shale. The sixth sand package located in the Lower Bhuban Formation.

The structural closure at Bibiana is transgressional. The core of the structure is possibly complicated by both longitudinal wrenches faulting along with conjugate shear faulting. The above structural geometry can often compartmentalise hydrocarbon accumulation. Aerial closure of successively deeper zones in the Bibiana structure decreases as the structural core is approached. [M Lutfar Rahman Chowdhury]

Patharia structure the eastern most anticline in the Sylhet Trough, bordering the Indian state of Assam. It forms the Patharia hill range, has surface oil and gas seepages and has been the site of five exploratory wells, although a commercial petroleum pool is yet to be found. The structure is an elongated asymmetric anticline about 50 km in length in a north to northeast direction, with a swing to a more NNE trend taking place north of the central part of the structure. Part of the eastern flank and the northern and southern plunging noses of the anticline lie in Indian Territory.

Drilling at Patharia Structure Sylhet

The anticline has a gentle axial crest about a kilometre wide with bed dips of up to 20°. This contrast with the lateral flanks which are steep to almost vertical. Patharia is the highest structurally uplifted structure in the Surma Trough. It is 3,900m, 2,400m and 1,900m (each 300 30m) higher than the Beanibazar, Fenchuganj and Atgram structures respectively, judging from the occurrence of Middle Bhuban shale at the crest of the Patharia anticline and in the subsurface of the others. A thick sequence of Neogene section is exposed in the Patharia structure. These formations are, from top to bottom - Dupi Tila, Girujan Clay, Tipam, Boka Bil and Bhuban.

The youngest Dupi Tila Formation forms the typical low rolling hills in the far-off flanks of the anticline whereas the Middle Bhuban shale forms the core of the anticline. A condensed geological map of the Patharia anticline is shown in a figure including the locations of the Madhabgang stream section, Nikharigang stream section and BOC road section. All cut across the structure. Patharia-5 well is located at a crestal position on the Patharia anticline, at a place named Jamkandi, about seven km south of the Patharia-1, 2, 3 and 4 wells. The well was spudded in 1989 with a target depth of 5,000 m and a primary objective of testing Oligocene Barail Group. The reason for the termination of Patharia-5 well at 3,438m was the abnormal hole deviation of 34°. The decision for abandonment of the well was taken in April 1992 as the Chinese deviation drilling company finally failed to straighten the hole. Patharia-5 well has encountered overpressure at a relatively shallow depth of about 480m.

A reinterpreted model for the Patharia structure is the recognition that clay movement and diapir development in the core of the anticline has significant implications in the development of the structure. It has been suggested that the squeezing of the mobile Middle Bhuban shale/clay leads to the east heading thrust, clay diapirism in the core of the anticline and developments of back thrust. The bangladesh petroleum institute has proposed a reinterpretation of the geologic model of the Patharia anticline based on satellite imagery, aerial photographs, field data and regional geologic settings. A rather more complex structural pattern has now been suggested in which clay flowage, clay diapirism, plane of decollement and disharmonic foldings have been implicated. [Sifatul Quader Chowdhury]

Hararganj structure situated in the eastern part of maulvi bazar district. Tectonically, it is located in the southern part of Sylhet Trough. The northern pitch lies within Bangladesh territory and the southern pitch lies in Tripura, India. The structure is about 70 km long of which about 30 km lies in Bangladesh. Maximum elevation is 335.89m.

Hararganj is an elongated, doubly plunging fold with narrow crest box like anticline. It is asymmetric with steeper eastern flank than the western flank. The core is squeezed out, rather torn off by the influence of three-strike fault. The central part is seismically blind due to the steepness of the beds. Hararganj range is dissected by innumerable stream. The pattern of these streams is dendritic and trellis.

The oldest rock Upper Bhuban Formation crops out in the core is sandstone dominated and commonly show cross bedding, flaser type of bedding, ripple mark and channel deposit. Shales are generally sandy or silty, deposited under shallow marine environment.

The Boka Bil Formation conformably underlies the Upper Bhuban Formation and is characteristically more argillaceous than the underlying and overlying strata. One oil and a few gas shows have been found in the crestal part of Hararganj. Both the oil and gas seepage is found in Dholi Chhara, a tributary of Sagarnal Chhara.

The oil of Hararganj is heavy oil (API-17°) with strong terpenic smell and is to be biodegraded. The source of the oil is terrestrial. Oil and Natural Gas Corporation had drilled an exploratory well on the south of Bangladesh Border and terminated as dry hole. [SK Aminul Islam]

Kailash Tila structure located about 20 km south east of Sylhet town and lies in golabganj upazila of Sylhet district. This structure lies in the folded flank of the Bengal Foredeep on the eastern marginal part of the Sylhet Trough and surrounded by the Beanibazar structure in the east, Sylhet structure in the north and Fenchuganj structure in the south. A very minor saddle separates the structure from Fenchuganj anticline. Primarily it is exposed on the surface and the outcrops are identified as Dupi Tila Formation. It is an asymmetrical anticline with NNE-SSW trending axis. There is a thrust fault near the northern plunge of the structure that separates the Sylhet structure from Kailashtila structure. It is about 6 km long and 3 km wide. The west flank is steeper than the east flank.

Kailas Tila gasfield is located in this structure and was discovered by Pakistan Shell Oil Company in 1962. It is one of the largest gasfields in Bangladesh. It has GIIP of 3.65 TCF with a recoverable reserve of 2.52 TCF. Kailashtila gasfield produces a high amount of condensate along with the gas. Kailash Tila began production in 1983 and has been in production ever since. Some non-commercial oil zone has been discovered at a deeper level in this gasfield. [Badrul Imam]

Jaintiapur structure lies in between two contrasting structural set ups, the uplifting Shillong massif in the north and the subsiding Surma Basin in the south. It is bounded by the Khasi-Jaintia hills and Shillong massif in the north, Goyain trough in the south, Atgram anticlinal structure in the east and Goyain Trough and foothills of Khasi-Jaintia range in the west. The area forms a narrow east-west elongated strip and is characterised by intermittent swamps between the hills. Tectonically, the whole area has been divided into three zones: zone-i - the area between Dauki river in the west and Naljuri bazar in the east, zone-ii - the area between Naljuri and Assampara in the east, and zone-iii - the area between Jaintiapur in the west and Hari river in the east including Dupigaon.

The exposed rocks of Jaintiapur structure can be classified from older to younger as Sylhet Limestone, Kopili, Barail, Bhuban, Boka Bil, Tipam Sandstone, Girujan Clay, Dupi Tila and Dihing formations.

Jinjira structure the geological structure of Jinjira, or st martin’s island. The structure is simple and is represented by an anticlinal uplift. The island lies on the eastern flank of the anticline. A little of the axis of the anticline is traceable along the west coast of Dakshinpara. The exposed portion of the axis runs NNW to SSE, approximately parallel to the island. There is a fault along the northwestern shoreline with a trend nearly parallel to the axis. The fault seems to be reverse in nature with the eastern side as the upthrown block.

The anticline is slightly asymmetrical with a monoclinal swing on the eastern flank. The bedrock on the eastern flank near the axis dips very gently at an angle of 3° to 5° towards the east, increasing eastwards to 10° to 12°. The monoclinal swing gives a high dip of 30' and above at Siradia. Very little of the western flank is exposed above the sea which records a dip of 6' towards the west. The monoclinal swing gives the anticline a box-like shape.

The birth of the island is related to the regional tectonics of Southeast Asia. As a sequel to the most dynamic Himalayan orogeny during Middle Miocene (around 15 million years before present) along the Tethyan belt, the land enclosed by the tethys was emerging and by Pliocene (about 5 to 2 million years before present), the sea retreated as far south as 10'S latitude. While the Girujan Clay of the Tipam Group were being deposited under the fluviatile continental environment in the north, fossiliferous marine sandstone of the coastal cliff south of Cox's Bazar and the marine sediments of the Dakshinpara Formation on St Martin's Island in the south represent deposition under a near shore environment in a retreating sea in the Late Miocene (around 10 million years before present). The Pliocene sediments might have been deposited but subsequently became eroded prior to the deposition of a Late Pleistocene coralline limestone bed. The eroded Dakshinpara sandstone-conglomerate sequences provided a good rocky platform to the growth of Late Pleistocene corals. [Sifatul Quader Chowdhury]

Dakhin Nhila structure lies under teknaf upazila of cox's bazar district and bounded by latitude 20°52' to 21°07'N and longitude 92°08' to 92°18'E and is situated at extreme south east of Bangladesh territory. Hills and valleys striking N I7'W and S 17'E represent it. The maximum elevation is about 266m above mean sea level. The northern pitch is represented by low relief due to saddle separating from Inani anticline, whereas southern pitch abruptly merges with the plain land.

Dakhin Nhila anticline is an elongated, asymmetric, and box like structure. It is a complicated structure due to the presence of longitudinal and transverse faults. The oldest exposed rock is Upper Bhuban that is about 545m thick. The exposed rock formations from older to younger is Upper Bhuban, Boka Bil, Tipam and Dupi Tila. Mostly argillaceous sediments with little coverage of arenaceous sediments characterise the structure. The axis of the structure runs along NNW-SSE direction. Some portion of western flank and younger formation of southern pitch has been eroded away due to bay of bengal.

This structure falls in the tropical climate zone and receives heavy shower during monsoon. The area is covered by dense mixed forest with gama, Chittagong teak and bamboo, shrubs etc are common. Among animals monkeys and elephants are abundant. Deer, pig etc is also common. The accessibility of the area is moderately fair. It can be approached from the sea beach, which is jeepable during low tide, and from the Teknaf-Cox's Bazar highway. There are many streams that criss-cross the structure.

Feni structure a subsurface anticlinal structure. It lies under Feni police station of feni district. It is situated at about 8 km south of Feni town. It is bounded by longitudes 91°20'E to 91°29'E and latitudes 20°50'N to 23°00'N. The structure is delineated by a seismic survey. It is an egg shaped symmetrical anticline without any fault. Amplitude of the structure is about 200m. The northern pitch is gentler than southern one. It is about 12 km in length and 5 km in width.

Tectonically, the structure is located in the folded flank of the Bengal Foredeep between Hatiya Trough and Tripura uplift. Both east and west of the structure is separated by synclines from sitakunda and bgumganj anticlines respectively. Structurally, it is about 150m higher than Begumganj anticline. The axis runs NNW-SSE direction. The anticline runs almost parallel to Begumganj structure.

Two wells have been drilled in the structure. Petrobangla drilled the first well in 1981 to a depth of 3,200m. The drilling was vertical up to Bhuban Formation through Dupi Tila, Tipam and Boka Bil formations. Production in this small field was suspended in 1998 after about five years of operation. [MA Baqui]

Hatiya structure lies in the southeastern part of Bangladesh and occupied a part of greater noakhali and vast area of Bay of Bengal. Tectonically, it is situated in the western platform flank of Bengal foredeep.

Based on the result of magnetic and gravimetric survey the southern part of Bengal Foredeep is divided into two sub basin, the Faridpur Trough and the Hatiya Trough, separated from each other by the Barisal-Chandpur High. Hatiya Trough is the most subsided and deepest part of Bengal Foredeep and it attained a maximum sedimentary thickness of more than 20 km. The structure is asymmetric in nature with gentle western flank. The eastern flank is steeper and probably faulted.

Result of seismic survey provides evidence of existence of some closed structural uplift within Hatiya Trough. Some commercial gas has been found at Shahabajpur and Begumganj above the high-pressure zone. In Shahabajpur the Upper Gas sand encountered at Boka Bil Formation and Lower Gas sand to Upper Bhuban Formation.

Hazipur structure located at Madhupur in the greater mymensingh district and lies on the so-called Hinge zone. Hazipur area was first brought under single fold seismic survey in 1959-60 by Stanvac oil company. Later on a 12 fold seismic survey was carried out to southwest between Ganges-Brahmaputra by Prakla Seismos. A zone of truncation of the upper Oligocene beds associated with the Mid-Tertiary unconformity was identified from seismic result. The Barial Group is missing from the shelf and is represented only near the hinge zone line and basin ward. Based on Stanvac's Single Fold Seismic Survey Hazipur Well-1 was drilled to a depth of 3816m in 1960 and terminated as dry hole.

The Barail Group (3,130m to 3,816m) at Hazipur is consisting of grey to dark grey, splintery shale and sandstones. Carbonaceous material is found scattered at the top of formation. Barial Group is an excellent reservoir rock but at Hazipur well no significant oil show were noticed except traces of oil show in between 3,126m to 3141m.

The Bhuban Formation (2,247m to 3,130m) is unconformably overlies the Barail Group with a wide spread of unconformity and the base of overlying beds usually conglomerate. The Boka Bbil Formation (1,397m to 2,247m) is unconformably overlies the Bhuban Formation, deposited in deltaic environment. Dupi Tila Formation (245m to 1,393m) is unconformably overlies the Boka Bil Formation. [SK Aminul Islam]

Inani structure lies in Cox's Bazar Sadar upazila of Cox's Bazar district. This structure is about 5 km south of Cox's Bazar town and bounded by 21°5'N to 21°25'N latitude and 92°0'E to 92°10'E longitude. It is represented by NNW-SSE trending low hillocks attaining maximum elevation 54.86m. Tectonically, Inani anticline is situated in the Chittagong Folded Belt of Bengal Foredeep. The structure runs along the coastline of the Bay of Bengal. The anticline is dissected into two parts by reju khal across the northern pitch. The southern pitch is represented by very low relief due to saddle separating Inani structure from Dakhin Nhila structure.

Inani is a narrow and elongated structure in which Tipam sandstone is characterised by steep zone in both the flanks. Maximum 70° dip was recorded in the steep zone. In the crestal part the dip varies from 3° to 12°. Tipam Sandstone meets Boka Bil Formation in a gradual low dip. The oldest exposed rock is Boka Bil Formaton. [Sifatul Quader Chowdhury]

Maheshkhali structure situated in Cox's Bazar district between latitude 21°28'N and 21°45'N and longitude 91°05'E and 92°01'E. The hill ranges forms a parallelogram along the eastern side of the island. The trend of the ranges is NNW-SSE. The highest elevation is 86.26m above mean sea level almost at the centre of the range.

Tectonically, the structure belongs to the western zone of the folded flank of the Bengal Foredeep. It is a simple asymmetrical anticline. The eastern flank dip towards northeast at an amount of 7° to 10°; the western flank towards almost due west at an angle of 30° to 40° indicating asymmetry. [Md Khurshid Alam]

Sitakunda structure one of the westernmost structures of chittagong and chittagong hill tracts. It is situated at the northwestern part of Chittagong district, between 22°34'N and 22°43'N latitude and 91°38'E and 91°41'E longitude. It is delimited in the north by the feni river, in the south by the karnafuli, in the east by the halda river and in the west by the sandwip channel. In a broader sense the Sitakunda Hill Range acts as a water divider between the Halda valley on the east and the Sandwip channel on the west. The structure is about 70 km long and 10-km wide and is situated at about 270 km southeast of dhaka.

The Sitakunda fold is an elongated, asymmetrical, box-type doubly plunging anticline. The axis of the fold is running in NNW-SSE direction, parallel to the general trend of regional strike. Both the flanks of the anticline merge into the alluvial plain of the Feni River in the north and the Karnafuli in the south due to its doubly plunging nature.

The structure has a gently dipping eastern flank and steeper western flank, which is suddenly truncated by the alluvial plain. This truncation is a result of a major faulting that runs parallel to the general strike of the anticline.

The exposed sedimentary rock sequences of the structure, except limestone, provide no difference in overall lithology from that of other structures of Chittagong and Chittagong Hill Tracts. The Sitakunda structure contains a thick sedimentary sequence of sandstone, shale and siltstone. Total thickness of the exposed sediments is about 6,500m.

Sitapahar structure It lies east of the Patiya and the west of the Gilasari and the Belasari structures. On the north of the Sitapahar anticline, Changotaung anticline is situated while bandarban anticline is located on the south. The Sitapahar anticline is a N20°W-S20°E trending structure plunging 6° in S18°E. The structure is about 40 km in length. Its steeper western flank is overturned in some places.

The Karnafuli river is the main channel of the drainage system in the area. The courses of most of the important tributaries are parallel to the main range trending NNW-SSE and appear to be controlled by structural trend of the area. One of the most significant geomorphological features of the area is the course of silchhari stream along the anticlinal axis. The stream is following the anticlinal valley and this type of valley is developed if comparatively weaker rocks are exposed along the axis. [Sifatul Quader Chowdhury]

Kamta structure A buried anticline located in gazipur district, which is about 20 km north-northeast of Dhaka city. Its size is not as big as Kailashtila. It is about 45 km west of Bakhrabad anticline. Tectonically, it lies in the southern part of Madhupur Tract. Average attitude of the area is 5-10m above mean sea level. The area is covered by Madhupur clay of Pleistocene age. There is no evidence of surface folding. The result of seismic survey confirmed the presence of a structure.

The shape of the structure is like footprints. It is a gentle brachi anticline with very gentle dips on the flanks. The length and width of the structure is about 13 km and 4 km respectively. The amplitude is about 28m. In the deeper horizon there are two culminations that are separated by a saddle. The axis runs almost NE-SE direction.

Kamta well was discovered by Petrobangla in 1981. Production in this small field was suspended after about six years of operation in 1991 because of excessive water flow. [MA Baqui]