Geochemical Evaluation of Part of the Cambay Basin, India

Abstract

In Broach-Jambusar and Ahmedabad-Mehsana blocks of Cambay basin, India, the hydrocarbon generated (HCG) and hydrocarbon expelled (HCE) per unit area of four Paleogene formations were computed at 38 locations to select the best targets and thus reduce exploration risk. Fractional generation curves, which show relation between vitrinite reflectance and fraction of original generative converted to hydrocarbons, were constructed for study areas and used to calculate HCG through remaining generation potential (S2 of Rock-Eval) and the thickness of the sedimentary section. HCE was estimated by subtracting volatile hydrocarbon content (S1 of Rock-Eval), representing the unexpelled in-situ-generated bitumen, from the computed value of HCG. HCG nd HCE, which combine source rock richness, thickness, and maturity, are useful for comparative evaluation of charging capacity of source rocks. Positive and negative HCEs characterize drainage and accumulation locales, respectively. In the study areas, the major generative depressions are at Sobhasan/Linch/Wadu and Ahmedabad in the Ahmedabad-Mehsana block and the Tankari and Broach depressions in the Broach-Jambusar block. In these areas, Paleogene source rocks have generated between 3 million and 12 million MT hydrocarbon/km2. The major known oil and gas accumulations, which are in middle to lower Eocene sandstones in vicinity of the generative depressions, overlie 2 million to 7 million MT hydrocarbon/km2 and HCG contours in both blocks and correlate well with negative HCE in the reservoir. Isopach maps of several major middle to lower Eocene reservoir sandstones in conjunction with HCG maps for Paleogene section help to delineate favorable exploration locales.