Continental margins which flank western and eastern limits of India are broadly divided into West and East Coast. The width of the eastern Continental Shelf varies from 300 km near Bombay to approximately 80 kms near Porbandar, narrowing further towards south to 60 kms near Cochin. The Eastern Continental Shelf which extends from Cape Comorin to Calcutta, has the width varying from 2.5 km, about 150 kms south of i1adras to 2JO kms up to Ganges river delta, near Calcutta.
The continental western margin has been divided into six sedimentary basins. They are Klitchasin, Saurashtra basin, Bombav Offshore Basin, Konkan basin, Laccadive basin, and Kerala basin. Surface geological exposures along the western margins of India are known in Kutch and Saurashtra Peninsula and in a thin strip along the coast near Cochin. These basins being graben type, the stratigraphy of the basins extending into the shelf is inferred mostly from the seismic data.
The Eastern Continental margin of India has been also divided into six major sedimentary basins, which from South to North are Cauvery basin, Palar basin, Godavari-Krishna basin, Mahanadi basin, Benqa1 basin and Andaman-Nicobar basin. The surface geology of eastern continental margin is available near Cape Comorin, which is mostly Archaen rocks, with thick covering of Tertiary sediments, which are also found along the north western coast of Ceylon. Further north it consists of Cretaceous and Tertiary rocks overlain by Pleistocene and Recent sediments.
Reconnaissance and semi-detailed geophysical surveys have helped in delineation of big and small structures in western and Eastern margins of India. The first offshore exploratory work along the western continental margin. Was undertaken in the Bombay Offshore basin, which is a continuation of the Cambay basin on the land, where oil and gas fields were found earlier. Evaluation by drilling of important structures like Bombay High, Panna, Basseitn, Ratna, Heera, South Papti, etc. have resulted in the discovery of oil and gas fields. A full sedimentary section of more than 4 km. resting on Trappean/Pre-Cambrian basement has been met. Hydrocarbon bearing horizons range in are from middle Miocene-Eocene. Lot of facies variation has been noticed and which probably has been responsible for the hydro carbon accumulation/absence in a different structures in this basin. The other offshore basins, Kutch and Saurashtra are being evaluated as a result of the encouraging data obtained in the adjoining Bombay Offshore basin.
In the East Coast, big and small structures have also been delineated in the different sedimentary basins. Drilling carried out in the; Cauvery basin, Godavari, Bengal Offshore basins and Andaman and Nicobar islands has also indicated hydrocarbon shows both of smaller and bigger magnitude. These areas are now under active exploration for a thorough evaluation of their hydrocarbon potential in the prospective structures.