Macro-seepage based potential new hydrocarbon prospects in Assam-Arakan Basin, India

Active macro seepages of methane that occur in between the north bank of the Brahmaputra river and Himalayan foothill region of Assam Arakan Basin, India, indicate the presence of hydrocarbon accumulation in the subsurface, but the hydrocarbon prospects in this region are not well studied. We carried out an extensive field sampling, which included a total of 58 sediment core collections from an active gas seepage location and nearby areas at a depth of 2-2.5 m. Our sample locations are placed at 1 km intervals laterally. We performed laboratory investigations and mapped near-surface chemical alterations associated with active macro seepages and microseepages. The analysis of geochemical composition of hydrocarbon gases in the sediment indicates both the biogenic and thermogenic origins of seeped hydrocarbons. The stable isotope analysis of methane suggests the presence of thermogenic as well as mixed biogenic-thermogenic gases. The presence of such mixing of gases is caused by the secondary alteration processes during their migration through potential faults and fractures. The trace elements of the sediments show anomalous concentrations at different parts of the study area, with a wide range of concentrations for Ba (54 to 492 ppm), Cu (1-25 ppm), Cr (61-329 ppm), Ni (1-42 ppm), Pb (2-48 ppm), Th (2-32 ppm), U (4-39 ppm), V (19-133 ppm) and U (0.87-6.5 ppm). There are higher concentrations of adsorbed gases, trace elements, and microbes along the identified lineaments. Such higher concentration can be triggered by high hydrocarbon-oxidizing bacteria count, which is greater than 10⁴ cfu/gm of soil of bacterial growth around the lineaments. We identified potential hydrocarbon prospects based on the macro and micro seepage analysis using integrated geological, geochemical and microbial techniques in the study area.