Abstract

Detailed synthesis of results of exploration enables us to delineate the basin configuration, stratigraphy and tectonic framework of Bengal basin. The Basin has an area of 89000 square kilometers and a fill of 10-15 km thick sediments. A number of hydrocarbon indications have been registered but without any commercial discovery. On the basis of available seismic and well data the following five types of plays have been delineated, namely:

Pre-Trappean prospects Pre-Trappean prospects Palaeocene/Cretaceous pinchout prospects Palaeocene/Cretaceous pinchout prospects Reef prospects Oligocene pinchout prospects Miocene/Pliocene deltaic prospects

Subsidence history, geothermal and geochemical data indicate not only maturation of source-prone formations but also possible 'live-basin' conditions below a depth of 2900 m. Present paper gives a brief account of exploration efforts put in and possible source rock, maturation and entrapment possible source rock, maturation and entrapment conditions along with types of plays in Bengal basin.

Introduction

Bengal basin, having an area of 89000 square kilometers and sedimentary fill of 10-15 km, is the northernmost of the east coast basins of India (Figure 1). Indian Shield and Shillong massif form the western and northern limits of Bengal Basin. Eastwards the Basin extends into Bangladesh and is bounded by Arakan Yoma geanticlinal uplift. Southwards Basin plunges into Bay of Bengal beneath the continental shelf. Tectonically the basin can be divided into four structural elements i.e. basin margin fault zone, shelf, hinge zone/slope break and basin deep (Figure 2). This paper gives a brief account of exploration efforts put up, types of plays tested, generalised stratigraphy, source-rock conditions and possible entrapment and types of plays.

GENERALISED STRATIGRAPHY

Penetration of 25 wells, sampling Basin margin, shelf, Hinge and Basinal area and fairly large geological and geophysical surveys have unfolded the subsurface stratigraphy of Bengal Basin. A brief account of the same is given in Figure 3.

EXPLORATION HISTORY

Standard Vacuum Oil Company

Stanvac Oil company of USA initiated exploration of hydrocarbons in Bengal Basin by geological and photogeological surveys in 1949. This was photogeological surveys in 1949. This was immediately followed by aeromagnetic survey of shelf and geosynclinal trough of Bengal basin. This helped in preliminary demarcation of Basin configuration and gave an idea about sedimentary fill.

INDOSTANVAC PETROLEUM PROJECT (JOINT VENTURE)

Preliminary studies by Stanvac from 1949-53 led to signing a joint-venture project in December 1953. The Contract covered an area of 25900 square kilometers in NW part of Bengal Basin. Detailed gravity survey with 12000 stations was followed by 500 line kilometer of refraction survey. Based on 8000 line kms of conventional seismic survey no major anticlinal feature could be delineated but a number of smaller structural anomalies in the form of fault closure, basement high, nosing in hinge area, purely stratigraphic and structural- cum-stratigraphic prospects were mapped. Ten such prospects tested by ISPP drilled within concession prospects tested by ISPP drilled within concession area (Figure 5) are as follows:

Structural Prospects

In total, seven purely structural prospects in the form of fault closure, dome, anticlinal closure and nosing were drilled by ISPP (Figure 5). Burdwan-1 was drilled on the down-thrown side of fifty m.sec fault closure at Eocene level. Closure was also observed at Plio-Pleistocene and Rajmahal trap-top level, throw of the fault being 60 m.sec. The well, drilled to 2720 m depth ended in Rajmahal trap. Asphalt was recovered from 1960 m level (bottom of Sylhet Lst) and zone produced saline water on testing. The fault closures tested by the two other wells, namely, Jalangi-1 and Debagram-1 had throws of 180 and 90 m.sec. respectively. Jalangi was drilled on down-thrown side, whereas Debagram on up-thrown side of fault. Jalangi-1 had a small oil show in a diamond core taken in Middle Eocene, but the zone was found to be impermeable on testing. Two shows in side-wall cores below Sylhet limestone produced gas-cut water on testing. Debagram-1 had also oil and gas shows in Pliocene sediments in wire-line core. Bolpur-1 well was drilled on 50 m.sec. fault closure at Eocene level formed by basement high on the eastern side of the prominent regional NNE-SSW trending topographic scarp. The structure appears to be a drap over a basement high accompanied by thining of sediments at crest. Well drilled to 1524 m ended in Rajmahal trap. No important show except flourescence in Middle Eocene sediments was observed. Other two wells Ranaghat-1 and Port Canning-1 were drilled on doubtful closure and nosing at Plio/Miocene levels in Hinge and Basinal part respectively. Ranaghat-1 had a number of gas shows from Plio-Miocene section. Surface flow of gas during DST was also observed from one of the zones. Port-Canning ended in the Mio-Pliocene transition zone. Total of six gas shows were observed and four zones on testing were found to be impermeable. Testing of two zones had to be abandoned due to influx of salt water associated with high pressure.

Stratigraphic Prospect

ISPP had attempted two purely stratigraphic prospects namely W Ranaghat-1 and Memari-1 (Figure prospects namely W Ranaghat-1 and Memari-1 (Figure 5). Vest Ranaghat-1 - Located up-dip of Ranaghat-1 was drilled to test the possible stratigraphic entrapment of gas from a good show encountered in basal Pliocene section, Ranaghat-1 and also to test possible reservoir facies below Mio/Pliocene possible reservoir facies below Mio/Pliocene unconformity. Drilled to 2604 m well ended in U Miocene. One of the two zones tested proved impermeable and another produced gas cut water. Another feature Memari-1 was a time thinning of 20 m.sec. within Eocene and located directly over a broad and gentle nose at Eocene base (J IV B) level.

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