Source Areas and Migration Trends of Oil and Gas in Bombay Offshore Basin, India

Abstract

The Bombay Offshore basin, located on the western Indian Shelf, is the southern extension of the onshore Cambay basin and contains oil and gas-bearing structures. Oil and gas have been generated mainly from the thick shale sequences deposited in local depressions within the basin. The Paleogene and Miocene prodelta muds deposited in one such major low, the Dahanu depression, appear to have been the main source beds for these hydrocarbons; the Paleogene and lower Miocene shale sequence in the eastern part of the Bombay-Ratnagiri Shelf, may have been a subsidiary source. The western clastic basin fringes the Bombay-Ratnagiri Shelf and may have provided some small oil and gas accumulations along the paleoshelf edge from local source areas. We postulate that the three main migration trends from the Dahanu depression are, (1) southwest toward the Bombay High and the H1-5 structure, (2) southward toward the Bassein-B-38 trend, and (3) northward toward South Tapti and North Tapti. Oil in the Anklesvar field of the onshore Cambay basin and in the Bombay High, Bassein, and B-38 structures contains a high wax content, suggesting that the prodelta muds of the Cambay Gulf Shelf and the Dahanu depression and its southern extension on the eastern part of the Bombay-Ratnagiri Shelf, may have provided for these genetically related oils a common system of source beds rich in terrestrial organic matter. Paleostructural analysis indicates that the early NNW-SSE-trending anticlinal structures, faulted down to the coast on their eastern flanks, adjoin the shale areas and were the main locales of oil and gas accumulation.