Overpressure in the Eastern Bengal Basin, and Its Relation to Compressional Tectonics

Many sedimentary basins throughout the world exhibit some degree of non–hydrostatic fluid pressure, particularly overpressure. Proximity to the Indian craton, Himalayas, and Indo–Burman ranges and rapid orogenic sedimentation has resulted in the common occurrence of overpressure zones in the Bengal basin. In eastern Bangladesh, overpressure zones frequently have been reported from the Miocene sequences in exploratory wells. Incomplete dewatering of fine–grained sediments, clay diagenesis, and tectonic compression associated with the westerly movement of the Indo–Burman ranges are possible causes for overpressure development in the Bengal basin.

The majority of the wells drilled so far in eastern fold belts and foreland basin areas of the Bengal basin encountered overpressured zones in Lower Miocene Bhuban Formation (Surma Group) at depths ranging from less than 1 km (Patharia-5) to 4.5 km (Muladi-1). Depth to the top of overpressure zones increases toward the west. Based on similar studies on Tertiary deltaic sequences elsewhere, it seems that the overpressure zones were caused by compactional disequilibria of thick shale sequences in the Bhuban Formation. In addition, clay dehydration may have also contributed in increasing overpressure. Illitization and clay dehydration in shale sequences of the Bhuban Formation have been reported in a number of wells.

An integrated approach has been adopted to model the overpressured zones in eastern Bengal basin and their relation to compressional tectonics by analyzing geophysical and lithologic logs, and subsurface sediment core samples from various exploratory wells.