Geological and Geochemical Characterisation of Shale as an Unconventional Gas Source and Reservoir Within the Lesser Himalaya, Nepal

Abstract

The Gondwana and the Foreland shales in the Tansen and Surkhet areas within the Lesser Himalayas, Nepal, have been studied in detail, focusing on unconventional gas potentiality. The geochemical analysis indicates that the Eocene shales have a total organic carbon (TOC) content of up to 1.56%, and the Gondwana shales exhibit ≤ 0.75%. Pyrolysis results show Tmax values above 480°C, and the modified van Krevelen diagram and Hydrogen Index (HI) versus Tmax cross-plot confirm the presence of type III (gas-prone) kerogen. Vitrinite reflectance (Ro) values ranging from 1.06% to 2.96% indicate high thermal maturity. Mineralogical analysis reveals a distinct contrast between the shales, with the Gondwana shales showing higher silica content within a brittle-ductile zone and the Eocene Foreland shales are more richer in clay minerals (56%) and quartz (31%), with superior porosity (2.1% to 8.3%) and organopores, enhancing their reservoir potential. Scanning electron microscopy (SEM) and petrographic observations further highlight the dominance of vitrinite macerals and organic pores in the Eocene shales. The Eocene Foreland shales of the Bhainskati Formation have favourable carbon content and pyrolytic properties and present a promising target for unconventional gas exploration compared to the Gondwana Sisne Formation. The over-maturity of the shale resulted from the thrusting over the Lesser Himalaya in the early Miocene. The Main Boundary Thrust, activated from 11 Ma, is active and other tectonic activity impacts gas preservation. Regionally, the Bhainskati Formation is more thermally mature than the Patala Formation in Pakistan and shares similarities with the Subathu Formation in India. Despite having lower TOC content than globally established shales, the Eocene shales warrant further exploration for detailed exploration of their potential as unconventional gas reservoirs.