Geology and Hydrocarbon Prospects of East Coast Basins of India and their Relationship to Evolution of the Bay of Bengal

Abstract

Abstract Four sedimentary basins, extending onto the Continental Shelf, occur along the east coast of India — the Mahanadi, Godavari-Krishna, Palar and Cauvery basins. All except Palar Basin have two structural stages; 1) pull-apart basins developed during the Gondwana episode and 2) filled up by Lower Cretaceous time with locally derived elastics, unconformably overlain by Upper Cretaceous-Palaeocene carbonate-elastics (in Cauvery Basin) succeeded by Eocene to Recent, mainly deltaic, sediments thickening seawards. Sedimentary sequences, up to 7 km in thickness, were deposited in the east coast basins and contain source as well as reservoir rocks, adequately matured at depths in excess of 1-5 kms. Traps are formed by basement drape, tilting and faulting, by growth-faults and diapirism in deltaic sequences, by updip seals of porous elastics and carbonates, and by onlapping by major shale sequences over the sloping basement. Two wells have been drilled in the Mahanadi basin with some hydrocarbon shows in the Eocene; four wells have been drilled in Godavari-Krishna basin, three offshore and one onshore, and flows of oil and gas have been obtained from them in the Lower Cretaceous and lower Pliocene; 25 wells have beeri drilled in Cauvery Basin, 7 offshore and the remainder onshore, and shows of oil or gas have been obtained from the Eocene in three of them onshore, and from the basement in two offshore wells. No commercial discoveries have yet been made, but the prospects of finding them are considered good. Palaeocurrent data indicates ESE or SE palaeo-slopes, and marine tongues of Upper Permian (in Palar Basin) and Jurassic and Lower Cretaceous ages suggest that Bay of Bengal developed by rifting, perhaps earlier in the south, extending north and north-eastward during the Mesozoic. Parallelism between the eastern edge of Indian shield along the east coast and the island-arc system of Arakan Yomas - Andaman-Nicobar Islands — Indonesia suggests that the Bay of Bengal is a part of the Indian Plate rotated counterclockwise and subducted under S.E. Asia.