Organic geochemistry of the Vindhyan sediments: Implications for hydrocarbons

摘要

The organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance, source and thermal maturity, which are essential prerequisites for a hydrocarbon source rock. In the present study, evaluation of organic matter in the outcrop shale samples from the Semri and Kaimur Groups of Vindhyan basin was carried out using Rock Eval pyrolysis. Also, the adsorbed low molecular weight hydrocarbons, methane, ethane, propane and butane, were investigated in the near surface soils to infer the generation of hydrocarbons in the Vindhyan basin. The Total Organic Carbon (TOC) content in shales ranges between 0.04% and 1.43%. The S_1 (thermally liberated free hydrocarbons) values range between 0.01-0.09 mgHC/gRock (milligram hydrocarbon per gram of rock sample), whereas the S_2 (hydrocarbons from cracking of kerogen) show the values between 0.01 and 0.14 mgHC/gRock. Based on the T_(max) (temperature at highest yield of S_2) and the hydrogen index (HI) correlations, the organic matter is characterized by Type Ⅲ kerogen. The adsorbed soil gas, CH_4 (C_1), C_2H_6 (C_2), C_3H_8 (C_3) and nC_4H_(10), (nC4), concentrations measured in the soil samples from the eastern part of Vindhyan basin (Son Valley) vary from 0 to 186 ppb, 0 to 4 ppb, 0 to 5 ppb, and 0 to 1 ppb, respectively. The stable carbon isotope values for the desorbed methane (δ~(13)C_1) and ethane (δ~(13)C_2) range between-45.7‰ to-25.2‰ and-35.3‰ to-20.19‰ (VPDB), respectively suggesting a thermogenic source for these hydrocarbons. High concentrations of thermogenic hydrocarbons are characteristic of areas around Sagar, Narsinghpur, Katni and Satna in the Son Valley. The light hydrocarbon concentrations (C_1-C_4) in near surface soils of the western Vindhyan basin around Chambal Valley have been reported to vary between 1-2547 ppb, 1-558 ppb, 1-181 ppb, 1-37 ppb and 1-32 ppb, respectively with high concentrations around Baran-Jhalawar-Bhanpur-Garot regions (Kumar et al., 2006). The light gaseous hydrocarbon anomalies are coincident with the wrench faults (Kota-Dholpur, Ratlam-Shivpuri, Kannod-Damoh, Son Banspur-Rewa wrench) in the Vindhyan basin, which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils.