Near surface hydrocarbon prospecting in Mesozoic Kutch sedimentary basin, Gujarat, Western India—A reconnaissance study using geochemical and isotopic approach

The Mesozoic Kutch sedimentary basin in the western continental margin of India has known accumulations of oil and gas. However, the multilayered thick Deccan Traps of Late Cretaceous overlie the Mesozoic targets, thereby imposing inherent problems in imaging sedimentary layers below the traps for exploratory studies. One of the unconventional techniques, the near surface geochemical prospecting study, has been carried out in the onland and offshore Kutch to understand the microseepage and sources of the light hydrocarbon gases (methane through butane) observed in the soils and sediments from the basin. The geochemical and isotopic compositions of the soil samples from the Kutch onland vary in the range of C1 (methane)=3–582 ppb, C2 (ethane)=0–39 ppb, C3 (propane)=0–15 ppb and the δ13C1 (methane)=−24.9‰ to −35.7‰. The offshore sediments showed the presence of butane along with the lower hydrocarbons and the concentration ranges as C1=36–475 ppb, C2=5–27 ppb, C3=5–27 ppb, iC4=2–6 ppb, nC4=0–4 ppb, and their δ13C values are characterized as δ13C1=−37.7‰ to −44‰, δ13C2=−24.6‰ to −30.5‰ and δ13C3=−26.1‰ to −34‰. The compositional ratios and carbon isotopic signatures of light gaseous hydrocarbons suggest their thermogenic origin. The δ13C1–δ13C2 signatures of samples from offshore Kutch indicate the gases to be non-associated, originating from sapropelic liptinitic organic matter. The concentration distribution pattern of light hydrocarbons indicates the areas Bhuj, Anjar, Mandavi and Mundra to have higher values for these gases. The anomalies are also coincident with the active faults, the Bhuj Fault (BF), the Katrol Hill Fault (KHF) and the Sandsra Dungar Fault (SDF) passing through the area, which may serve as a preferential migratory pathway for the light hydrocarbon gases. The results of this study support the generation, preservation, migration and the near surface manifestation of thermogenic hydrocarbons in Kutch basin, particularly in the northern and southern regions of the study area.