Lietz, Juergen K.; German Geological Advisory Group with Petrobangla and Jahangir Kabir, Geologist Multiwell Drilling Project Petrobangla, Bangladesh

Within the last three years large efforts have been undertaken to explore the oil potential in Bangladesh. Prospecting for oil in an already well known gas province appease to be a challenge which is presently met by intensive seismic surveys, stratigraphical and geochemical investigations will exploration drilling.

Seismostratigraphic correlations nearly all-over Bangladesh appear to change some concepts held so far on the stratigraphic age of different sedimentary sequences. The results from seismic stratigraphy are partly in contradiction to the conventional stratigraphic framework will recent palynological results. Nevertheless they are presented in this paper as a palynological results. Nevertheless they are presented in this paper as a further approach to discuss still open questions on the hydrocarbon inventory of the Bengal Basin. The results of seismostratigraphy shed a different fight on different structural parts of Bangladesh regarding their oil potential.

Introduction

The discovery of a number of gasfields in Bangladesh mainly during the sixties proved the fact, that hydrocarbons have been generated in one of the biggest delta systems in the world. Nearly all the gasfields are confined to the eastern folded belt and represent anticlinal structural traps. Since these discoveries exploration geologists paid special attention to the question, whether chances to strike oil are realistic too.

In order to develop petroleum geological concepts the most basic requirement is the establishment of an overall stratigraphic framework. The set up of such a framework for the Bengal Basin has been always strongly hampered by the nearly nonpresence of any age diagnostic microfauna and by the problems in establishing any reliable lithostratigraphic framework within a deltaic system with high rates of subsidence and rapid facies changes (Brunnschweiler and Maroof Khan 1978, Brunnschweiler 1980, Maroof Khan 1980, Maroof Khan and Musharref Husain 1980).

More recently two independent approaches have been undertaken in order to establish a basinwide stratigraphic framework. One is based on palynological investigations of surface and Subsurface samples the other is the attempt of a basinwide seismostratigraphic correlation including the offshore area. Though conformity exists for both methods in certain areas contradiction is obvious for others. Reasons for these contradictions may he located within the limits of both methods and/or they may originate in the lack of a stratigraphic age scale which can be used as a calibration tool for both methods.

Further confirmations about the validity of one or the other method are necessary and as such the seismic method presented in this paper should be seen as a further approach for a better understanding of sill open questions.

SEISMIC STRATIGRAPHY

During the last four years intensive seimic surveys have been carried out in onshore Bangladesh with a total coverage of nearly 2000 km. These seismic surveys were mainly concentrated in the northwestern stable shelf/hinge-zone areas and in the noreheastern Surma Basin. Additional digital reprocessing of older six-fold analog data greatly improved the overall seismic data quality. Earlier during the fifties and early sixties onshore Bangladesh has been covered with a rather narrow grid of single-fold seismic surveys by STANVAC and PAKISTAN SHELL OIL. During the seventies the offshore areas were intersively suveyed by different international companies. In view of this data abundance the attempt was to set up a basinwide seismostratigraphic correlation scheme and to compare its results with the established stratigraphic framework.

METHODS

Seismic lines for the basinwide correlation were selected with the view to guarantee an utmost degree of continuous correlation together with the highest possible input of well information. Except for the lines shown in the attached correlation map, many others were used to narrow regional gaps or to improve poorer data quality. Poor analog data quality below 2.5 sec (two-way-time) caused, in some areas, the use of phantom horizons. Locally the same was necessary on digital sections but mainly where correlation problems were related to fault induced interferences.

All together seven reflection horizons were picked based on characteristic reflection patterns and geological input. They define seven major seismic sequences. Following only a brief description of these sequences is given.

Seismic sequences
Sequence G

Sequence G represents the oldest non-crystalline rocks in the stable shelf area. They comprise coal-bearing Gondwana sediments and Jurassic volcanic and volcanoclastics of the Rajmahal Group, This sequence has been drilled in the wells Jalangi (India), Singra and Kuchma.

Sequence F

The lower boundary of Sequence F is reflection horizon R7 which can be traced over the northwestern shelf area. It unconformably overlies sequence G and marks a widespread transgressive event. The upper limit of sequence F is reflection horizon R6 which is mapped as an erosional event. Sequence F has been drilled in the wells Jalangi, Singra, Kuchma and Bogra.

The age of Sequence F is assumed to range from ? Upper Cretaceous to Paleocene or Eocene.

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