BIOMARKER SIGNATURES OF THE EARLY CAMBRIAN NIUTITANG FORMATION FROM YANGTZE REGION SOUTH CHINA: IMPLICATIONS FOR NATURE OF SOURCE ROCK

Early Cambrian rocks are extensively distributed throughout the South China block. Niutitang Formation (early Cambrian) is an essential hydrocarbon source rock in the south part of China. It is chiefly comprised of shales (silty shales, carbonaceous shales, calcareous shale, etc.). In this study, total organic carbon content (TOC), solvent extraction, and gas chromatography-mass spectrometry (GC-MS) of the early Cambrian sediments of the Niutitang Formation are discussed to elucidate the depositional settings, organic richness, thermal maturation level, source of organic matter, and the nature of hydrocarbons. These rocks contain good to excellent quantities of total organic carbon content. However, little extractable organic matter (EOM) has been noticed in the Niutitang Formation. The small EOM in these sediments are due to elevated thermal maturation of the organic matter. Various biomarker signatures such as C30-4-methylsterane, C27-C29 steranes, dinosteranes, monomethyl alkanes, and ß-carotane are clues for the eukaryotic and prokaryotic source of organic matter. Vitrinite reflectance using the methyl-phenanthrene index, triaromatic steroids, alkylated aromatic isomers and diverse hopanoid parameters indicates the OM in the Niutitang Formation is high-overmature nature. Likewise, these rocks comprise a larger quantity of polyaromatic fluoranthene (Fl), and pyrene (Py) designates the existence of high mature OM and are a clue for hydrothermal activities. The presence of C30 demethylsteranes, lower pristine/phytane, and higher gammacerane index suggest the prevailing conditions during deposition of these early Cambrian rocks were anoxic and saline. As these rocks are deposited in anoxic water settings and contain massive thickness, a higher level of organic matter and elevated thermal maturation level are proposed to be favourable for gaseous hydrocarbons for an unconventional reservoir in south China.