Geochemical and Mineralogical Insights into Organic Matter Preservation in the Gondwana and Post-Gondwana Shale of the Lesser Himalayas, Nepal

The depositional environments, weathering and provenance, organic matter enrichment, and preservation in the Gondwana and post-Gondwana units of the Lesser Himalayas, Nepal, are studied through geochemical and mineralogical analyses using petrography, X-ray diffraction, XRF, and ICP-MS. Mineralogical findings indicate that shales comprise 55% to 72% clay, 25% to 55% quartz, and less than 10% carbonate minerals, with a significant presence of illite, suggesting a transition from fluvial to shallow marine environments during post-Gondwana deposition. The thin sections of the post-Gondwana sandstone reveal an increase in quartz, feldspar, and plagioclase content, with rounded to sub-angular quartz grains indicating moderate transportation before lithification, resulting from the Indo-Asian collision. Geochemical data, including major, trace, and rare earth elements (REE), along with bivariate discrimination diagrams, reveal distinct environmental changes; Gondwana sediments exhibit oxic, arid conditions with continental provenance, while post-Gondwana deposits indicate humid environments favorable for organic matter enrichment, primarily sourced from felsic-intermediate igneous rocks. The TOC is less than 1 wt.% in the Gondwana and is 0.75 to 2 wt.% in the post-Gondwana shale, indicating better organic matter preservation. The existing geological structural data and the research findings highlight the pivotal role of Himalayan tectonism in enhancing the thermal maturity and hydrocarbon generation potential of organic-rich post-Gondwana shales, attributed to their substantial organic matter content.