Offshore South East Asia Conference 9 - 12 February, Singapore.
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 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 clastics, unconformably overlain by Upper Cretaceos-Palaeocene carbonate-clastics (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 dispirism in deltaic sequences, by updip seals of porous clastics and carbonates, and by onlapping by major shale sequences porous clastics 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 Phocene; 25 wells have been drilled in Cauvery Basin, 7 offshore and the reminder 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. Palaeo current data indicates ESE or SE palaeo-slopes, and marine tongues of Upper Permian (in Paiar 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 Yomis - Andaman-Nicobar Islands - Indonesia suggests that the Bay of Bengal is a part of the Indian Plate rotated counter-clockwise and subducted under S.E. Asia.
Four sedimentary basins, viz., the Cauvery Basin, the Palar Basin, the Krishna-Godavari Basin and the Mahanadi Basin occur along the east coast of India from south to north. They all extend onto the continental shelf and excepting probably the Palar Basin, they originated as pull apart basins during the Gondwana breakup (India and Antartica) when the Bay of Bengal also came into being. The Cauvery, Krishna-Godavari and Mahanadi Basins have the horst and graben type basement configuration, aligned more or less in a NNE-SSW direction, which influenced the deposition of locally derived clastics in the depressions till the end of Mesozoic when they got filled up. During Tertiary, these basins had a gentle southeasterly tilt when mainly deltaic sediments thickening towards the sea (east) were deposited.
Among these four coastal basins, the Cauvery Basin provides relatively more data from various surveys and the 25 deep wells drilled - both onland and offshore. In the Krishna-Godavari Basin, our observations are based on a fair amount of seismic coverage and the sub-surface data obtained from the six deep wells. A limited amount of geophysical work has been carried out in the Mahanadi Basin and two deep wells have been drilled in the offshore by Oil India Limited, a sister Organisation of ONGC. In the Palar Basin, only surface geological mapping of the scantily exposed land pan has been carried out and no sub-surface information is available. Consequently, the Cauvery and Krishna-Godavari Basins are dealt with in detail in this paper while only general reference is made for the other two basins.
The Cauvery Basin extends over an area of 25,000 square kilometers onland and another 15,000 square kilometres in the adjoining continental shelf between 9 degrees N and 12 degrees N latitudes in the southernmost part of the east coast of India. It occupies the coastal plains of Tamil Nadu and Pondicherry States and extends into the waters of the Gulf of Mannar, Palk Strait and the Coromandal coast. The basin is nearly 400 kilometres long, in a NE - SW direction and is about 125 kilometres wide.
Geological investigations in the Cauvert Basin were commenced by the Oil and Natural Gas Commission in 1958 followed by onshore geophysical surveys in 1960. The 'Karaikal High' was the first structural feature delineated and the first exploratory well (Karaikal-1) was drilled in 1964. It indicated the presence of a small quantity of oil at the Eocene level which led to further exploratory drilling in the area, but without much success. Eighteen exploratory wells and 10 stratigraphic wells have so far been drilled onland but no commercial Find could be established.
Offshore geophysical surveys commenced in 1964 when a Soviet contract ship 'Akademic Archangelesky' covered 3880 line kilometres during 1964-66. Detailed offshore seismic surveys could be carried out only after 1975 when ONGC acquired its own survey ship 'Anweshak'. Seven wells have been drilled in the offshore till now. Hydrocarbon shows were obtained from the basement in two wells and the latest one in the Palk Bay tested oil production from the Upper Cretaceous.