GENERATION AND DISTRIBUTION OF OVERPRESSURE IN MIOCENE STRATA IN THE EASTERN BENGAL BASIN

Overpressure zones have been frequently reported from Miocene Bhuban sequences of the Bengal basin, located close to the eastern Himalayas and Indo-Burman ranges. The majority of the wells drilled so far in the eastern Bengal basin encountered overpressure zones at depths ranging from less than 1 km to about 4.5 km.

The average geothermal gradient in the eastern Bengal basin area ranges from 15.8 to 30° C/km. In the western platform area, the average geothermal gradients are relatively high. The depth–pressure profile indicates that formation pressure gradient increases from 0.12 atm/m in the west and north to 0.18 atm/m in the east and southeast. The depth to the top of the overpressure zone is shallower where the top of the Miocene Bhuban Formation is also shallow.

The Bhuban sediments exhibit loss/gradual decrease of the illite/smectite mixed layer clay with increasing burial depth. Compaction–induced hydrofracturing and clay injection have been identified in several wells below the overpressure zone. Secondary porosity development is also a common occurrence in the Bengal basin as evidenced from feldspar dissolution.

⁴⁰Ar/³⁹Ar analysis of Miocene detrital muscovite grains provides cooling ages between ca 15 Ma and 554 Ma. The average cooling age and depth of the samples reveal that the estimated rate of deposition for the Miocene sequence was high.

Incomplete dewatering of fine–grained sediments, smectite dehydration, increase in pressure gradients toward the east, secondary porosity development, and high rates of sedimentation contributed toward the generation of overpressure in the eastern Bengal basin.