34 / 2023-12-25 13:06:53

Geological and Geochemical characterization of Gondwana and Post Gondwana shales: implication for unconventional gas within the Lesser Himalaya, Nepal

characterization; shale gas; Lesser Himalayas, Nepal; over-mature

Shale is an unconventional gas source as well as reservoir rock and studied significantly in many parts of the world with different perspectives. Few studies for unconventional shale gas characterization have been conducted in the Gondwana and Foreland basin of Himalaya orogen, and this is the first research attempt in the characterization of Gondwana and post-Gondwana shale within Lesser Himalayas, Nepal. In this study, we conducted geological fieldwork in the Surkhet and Tansen areas, followed by geochemical and petrographical laboratory analysis. The total organic carbon (TOC) content of 11 shale samples ranges between 0.5 to 1.5 wt.% indicating fair to good gas potentiality. The Pyrolysis temperature, Tmax is above 480 ⁰C, vitrinite reflectance, and other pyrolysis parameters suggest Type III-IV kerogen and over-mature to dry gas stage. Petrographic analysis signifies the maceral was primarily composed of the vitrinite group and XRD analysis confirms the major minerals are clay minerals (Illite, chlorite, and kaolinite), quartz, calcite, and plagioclase. SEM, MICP, and gas adsorption analysis reveal the shales contain micropores with limited meso to macropores. This study characterizes shales are overmature due to thick burial depth and the tectonic activity during the formation of the Himalayas, causing the dry gas stage. The Eocene shale of the studied area is similar to that of equivalent Subathu shale, India, in pyrolysis parameters, TOC content, and mineralogical composition. Sufficient thickness (>30m) with fair to good geochemical results of Eocene shale for unconventional gas than Gondwana shales in Nepal Himalaya, supports for future geological and geophysical exploration in Eocene Foreland shale strata.