The effective source rocks in the Cambay basin, India

Abstract

We evaluated the source potential of three Paleogene formations, identified the effective source rocks, and determined their maturation histories to derive exploration leads and inputs for petroleum system study. Kinetics-derived hydrocarbon expelled (HCE) values are 0-0.4, 4-16, and 16-32 t hydrocarbons (HC)/m2 for the Kalol (middle Eocene), Cambay Shale (middle Eocene-upper Paleocene), and Olpad (lower Paleocene) formations, respectively. Values are high in the south for the Kalol and Cambay Shale formations and in the north for the Olpad. Source rock (SR) data reveal six laterally extensive potential sources (PS) with kerogen type II/III in the middle Eocene-lower Paleocene. The PSs are SR4 at the base of the Kalol Formation; SR5 and SR6 at the top and middle, respectively, of the Cambay Shale Formation; SR7 and SR8/9 at the top of the Olpad Formation; and SR11/12 at the base of the Olpad Formation. In the north, SR11/12, SR8/9, and SR7 attained peak oil generation (vitrinite reflectance [Ro] ~0.75%) at 45, 5, and 1 Ma, respectively. The SR6 and SR5 attained generation threshold (0.5% Ro) recently. In the south, SR11/12, SR8/9, SR7, and SR6 attained 0.75% Ro at 50, 8, 7, and 3 Ma, respectively; SR5 and SR4 attained 0.5% Ro at 7 and 6 Ma, respectively. Chromatographic and isotope data of 108 oils and 43 PS samples show that SR7 and SR8/9 initially charged the Paleocene-middle Eocene reservoirs across the basin. Subsequently, SR6 and SR5 in the central Cambay and SR6, SR5, and SR4 in the south charged the reservoirs in their respective drainage areas. Conventional and basin-centered accumulations in the strata between SR7 and SR8/9 that contain free hydrocarbons under pressure are likely. Hydrocarbons generated in SR11/12, possibly lost in axial areas, may exist at margins.