DISTRIBUTION OF OVERPRESSURE IN THE LOWER MIOCENE BHUBAN FORMATION IN THE BENGAL BASIN

The majority of the wells drilled so far in eastern Bengal Basin encountered overpressured zones at depths ranging from less than 1 km (Patharia–5) to 4.5 km (Muladi–1). Proximity to the Indian craton, the Himalayas, and the Indo–Burman ranges and rapid orogenic sedimentation have resulted in the generation of overpressure in the Bengal basin. In eastern Bengal basin, overpressure zones frequently have been reported from the Miocene sequences in exploratory wells. Incomplete dewatering of fine–grained sediments, clay diagenesis, and tectonic compression due to the convergence of Indian plate and Burmese plate are considered as possible causes for overpressure development in the Bengal basin. 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 Lower Miocene Bhuban Formation. In addition, clay dehydration also may have 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 study the distribution of 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.