INFLUENCE OF OVERPRESSURE ON FORMATION VELOCITY OF NEOGENE STRATA FROM EASTERN BENGAL BASIN, BANGLADESH

The Bengal basin, located at eastern India, received clastic wedges from the Himalayas in the north and the Indo-Burman Ranges to the east. Neogene strata of the basin have been drilled extensively in search of petroleum. Interpretation of sonic log data of anticlinal structures from eastern Bengal basin reveals significant variations of acoustic velocity of subsurface strata. Occurrence of overpressure has been reported in several exploratory wells in the Bengal Basin, Bangladesh.

Formation velocities vary as much as 6% to 48% in Miocene to Pliocene stratigraphic units. Average velocities of Mio-Pliocene Tipam, and Miocene Boka Bil and Bhuban Formations are respectively 2677 m/sec, 3410 m/sec and 3997 m/sec at the eastern Titas structure; 2942 m/sec, 3404 m/sec and 3958 m/sec at southern Bakhrabad; and 2577m/sec, 3812m/sec and 3919m/sec at the northeastern Rashidpur structure showing a gradual increase in velocity with depth. In contrast, the Sitakund anticline in SE Bangladesh reveals a decrease in velocity with depth from 2900 to 4000 meters. The top of over pressured zone in the Sitakund structure occurs at a depth of 1100 m whereas in other structures this depth is located around 3000 meters deep. Velocity fluctuations are influenced by the presence of gas-bearing horizons, with velocities of gas-producing strata 3 to 7% lower than those of corresponding strata at equivalent depth.

Decrease of velocity in the Sitakund structure is probably due to the presence of over pressured and under-compacted mudrocks of the Bhuban Formation. The Sitakund structure experienced greater uplift than the other three structures studied, and the depth to the top of the Bhuban Formation is therefore occurs at a shallower depth. Consequently the over pressured zone is also shallower at Sitakund than at the other three wells. Tectonic compression, associated with the Indo-Burman orogenic belts from the east, likely contributed the most toward formation of overpressure in Sitakund anticline.