Molecular geochemistry study of carbonaceous shale from Pranhita-Godavari valley, India: Implication for source rock properties, depositional environment and thermal maturity

Biomarkers are biological remnants of extinct species which are generally found in sediments. The preserved molecular structures provide crucial insights into the organic matter's origin and precursor materials and serve as valuable proxies for reconstructing the ancient sedimentary environment. These biomarker serve as "molecular fossils" that offer important clues about past environments and life. This paper presents a detailed biomarker study of the organic-rich shales from open cast coal mines of Pranhita-Godavari (P-G) valley in South Central India. The Gondwana Basins of India, including the PG, are the primary coal producing regions, contributing approximately 98-99% of the nation's total coal reserves. The Barakar and Lower Kamthi Formations of the PG basin host significant reserves of power-grade coal. The shales collected from three sub-basins, namely Godavari in northern, Kothagudem and Chintalpudi sub-basin in southern part of PG.  The presence of n-alkanes in the range of n-C₁₂ to n-C_35, along with acyclic isoprenoids i.e. pristane and phytane in majority of samples indicates that the organic matter was derived from terrestrial higher plant sources deposited in a reducing environment, except for a few. The Pr/n-C₁₇ versus Ph/n-C₁₈ ratios indicate that the P-G samples predominantly contain terrigenous type III kerogen with a minor contribution from mixed type II & III kerogen. Important polycyclic aromatic hydrocarbons (PAH) such as naphthalene, phenanthrene, and their alkylated derivatives, dibenzothiophene were also observed in the samples of   P-G Valley. The DBT/P ratio from several shales is high to very high (>2), indicating the marine contribution during the sediment deposition. The other important PAH identified are the pyrene, benzo[a] anthracene, fluoranthene, and benzo[a] pyrene are found in the aromatic fraction of PG shale. These aromatic biomarkers have the potential to provide better understand the origin, depositional environment, thermal maturity, and potential for hydrocarbon generation of coal deposits. The P-G Basin shalse, characterized by excellent organic richness and thermal maturity, exhibits a complex origin, reflecting contributions from both terrestrial and aquatic organic matter sources. Furthermore, the depositional environment in this basin witnessed a significant transition, shifting from marine/aquatic to terrestrial conditions during the Permian period.