Although, most of the gas producing horizons in the Bengal basin are located in the Bhuban formation but, in spite of penetrating the entire Bhuban formation with repetition, the Sitakund well has failed to achieve any commercial gas discovery. In addition, Sitakund structure delineated with surface geologic mapping, prior to drilling, was strongly thought to be an ideal structural trap characterised by proper fold culmination, fold closure, and seal. The drilling expense of tens of crores of taka was also probably thought to be a well-merited investment. The puzzling outcome has been a major ingredient for further study and research. A geological review has been made to decipher the most likely reason for such failure.

The eastern folded belt and Sitakund structure started forming due to oblique plate convergence, trench migration, thrust jump, and progressive deformation resulting in westward propagation of the deformation front. Sedimentation progressed dominantly within Arc-Trench-Ocean bathymetry tectonic setting. Both shelf and slope sediments are the characteristics. The characteristic phenomenon of Sitakund structure that has been revealed from geophysical analysis is the two decollement zones (a planner zone having major detachment interfaces).

Intra-fold decollement at 5-6 km depth is characterised by dominantly shelfal mud and bounded by both east dipping and west dipping zone of thrust slices. While the brittle-plastic transition at 10-11 km depth is characterised by dominantly pelagic mud. The middle part of this structure is characterised by a transverse tongue-shaped tectonic depression of the brittle-plastic transition zone and was a major route for sedimentation. The movement along the intra-fold decollement has played an important role in attaining the geometric configuration of the structural traps.

The tension and intense shearing in the middle part of the structure where Sitakund-5 well was drilled has been revealed from land sat, converging geophysical anomaly pattern toward south, the occurrence of kink-band, knee fold, and the behaviour of rock materials around buckling layers underlain by decollement zone. This has further led experts to re-assess the Sitakund structure as a rotational double vergence fold.

The rotational nature has made the southern half of Sitakund structure as a very complex one. The southern half being intensely deformed and bounded by two major east dipping regional thrusts and some minor thrust slices, the structure of the southern half has been severely affected by tightening of fold elements, near vertical bedding attitude, intense shearing of the middle part due to the transformation of east dipping thrust in the southern half to west dipping thrust in the northern half, and deterioration of favorable trapping condition.

So, Sitakund well-5 location was wrongly selected although the drilling was performed on to the crestal region of the fold. Possibly, it was thought to be a simple, single vergence fold structure.

The curiosity of this research has further revealed that the structural configuration and the trapping conditions are not same throughout the Sitakund structure. The overlying seal rock, known as Boka Bil formation, does exist in the northern segment of the structure. The potential segments within the structure that suggest the possibility of the discovery of gas on commercial scale also do occur, similar to the occurrence of gas shows at Barabkund and Labanakhya chara, and oil seep at Khaiyachara area.

The segment in the northern half of the structure bounded by both east dipping and west dipping thrust slices, fold culmination, fold closure and with proper cap rock condition, are the favourable structural traps.

The oil show, oil and gas seepages in Sitakund structure strongly suggests the possibility for a commercial gas discovery, if not oil. The northern sector has been evaluated as a much better prospect though the subsurface by seismic imaging has not been done. However, seismic image acquisition in recent time but much after the drilling Sitakund-5 well has partially covered the southern domain of Sitakund structure. It is learnt that the area of seismic coverage has shown totally blank seismic image especially in the central part of the structure. If it is true, that further supports the high order intensity and complexity of deformation in the middle and southern segment of the structure.

The northern sector strongly merits imaging by conducting seismic survey. However, in order to avoid risky expenditure for seismic imaging, gravity survey could be conducted prior to seismic survey. This would reduce the area of seismic coverage through identifying lead and prospect. Structural configuration and trapping condition strongly suggest that the northern sector should encounter gas within a depth of 4000 metres.

We must bear in mind that even a gas reservoir with only zero point one TCF recoverable reserves would have present market value worth of more than one thousand five hundred crore taka. Should we not take a risk of spending about hundred crore taka for successfully achieving the entire activities of gravity survey, seismic survey, drilling, and appraisal to resolve this puzzle?

Dr. Aftab Alam Khan is a Professor in the Department of Geology, University of Dhaka.