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The east coast of India is considered to be a divergent margin formed during the fragmentation of Gondwanaland during the late Mesozoic. The four sedimentary basins located along this coast--Cauvery, Palar, Krishna-Godavari, and Mahanadi (from south to north)--have their seaward extensions into the Bay of Bengal where some of them have built a 5-6 km (16,000-20,000 ft) thick late Mesozoic to Holocene sedimentary section.
The formative tectonics of these basins are generally similar and date from Late Jurassic-Early Cretaceous interior fracturing of the continental crust with northeast-northwest-trending horst-and-graben faulting along the ancient Precambrian grain. In Palar, it appears to have been initiated in the Permian.
The basins have two or more cycles of deposition. During the first (rift) cycle in the Early Cretaceous (stage 1), nonmarine to paralic sandstones and shales were deposited in the interior grabens. With continued subsidence and marine encroachment during middle to Late Cretaceous (stage 2), the horst areas also became the repositories of sandstones, shales, and limestones, and finer clastics filled the graben areas. However, the basins filled gradually, followed by uplift and truncation. Fissure eruptions connected with the Deccan volcanic episode of Peninsular India cover parts of Krishna-Godavari onshore.
The second cycle (coastal margin), during the Tertiary, which is well developed in all basins except Palar, was superimposed unconformably on the horst-graben morphology of the Cretaceous basins. Paralic to shallow marine clastics and carbonates were deposited during the Paleocene-Eocene. Eocene carbonate banks of considerable areal extent appear to have supported limited biohermal activity in the three basins. In the Krishna-Godavari and, to some extent, the Mahanadi basins, deltas prograded to the outer edge of the continental shelf during the Neogene, with deep-sea equivalents along the paleoslopes.
Although no commercial discoveries have been identified to date, significant oil and/or gas shows have been encountered in some of the tests, with definite but lesser shows in others. The strata with important shows range from Cretaceous sandstones and fractured basalts to poorly consolidated Pliocene sands; others include weathered and fractured basement rocks and Eocene carbonates. Source facies have been identified in Upper Cretaceous, Paleocene, and Miocene shales. All important shows observed to date are located on horsts and other structural highs, but stratigraphic controls are very likely to be associated with sub-Upper Cretaceous unconformities, between Cretaceous and Paleocene and between Miocene and Pliocene. Oil and gas plays in each sequence appear to be limited by the fa lt block in which the particular sequence is most completely developed and each appears to contain indigenous source rock providing hydrocarbons to the reservoirs in the sequence. The major faults, with the possible exception of the major growth faults, appear to be sealing. The important plays are in the normal to transitional pressure regimes with a few gas plays in the overpressured sequences.
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