Effect of Salinity on Source Rock Formation and Its Control on the Oil Content in Shales in the Hetaoyuan Formation from the Biyang Depression, Nanxiang Basin, Central China

A total of 75 shale samples from the Biyang Depression were analyzed with Rock-Eval, total organic carbon (TOC), organic maceral, and gas chromatography–mass spectrometry (GC–MS) techniques to reveal the effect of salinity on source rock formation and its control on the oil content. On the basis of the diversities in salinity and redox conditions reflected by the gammacerane/αβ C30 hopane (G/H) ratio, the pristine/phytane (Pr/Ph) ratio, and the extended tricyclic terpane ratio [ETR = (C28 + C29) tricyclic terpane/Ts)], three types (T1, T2, and T3) of shales were identified, which deposited in the brackish condition (G/H ratio < 0.3), the semi-saline condition (G/H ratio = 0.3–0.6), and the saline condition (G/H ratio > 0.6), respectively. The comparisons among T1, T2, and T3 shales revealed that the salinity had a significant effect on the paleoproductivity of the lacustrine system. Most algae thrive in semi-saline conditions (T2) but are restrained in saline conditions (T3), so that moderate salinity conditions (G/H ratio = 0.3–0.6) are most conducive to the accumulation of algae organic matter (AOM). Although they have similar thermal maturity (0.6–1.1% vitrinite reflectance) and kerogen type (type II), the T1, T2, and T3 shales were developed in different reducing environments indicated by the Pr/Ph ratio and contained variable abundance of organic matter, resulting in remarkable differences in the preservation condition of AOM and the shale oil content within the T1, T2, and T3 shales. The classification and evaluation criteria of the shale oil resource were performed according to three categories based on the oil content (S1) and TOC values. It was revealed that the T1, T2, and T3 source rocks were mainly distributed among these areas of the potential, enriched, and ineffective shale oil resources, respectively, suggesting that shales developed in semi-saline environments deserve to be the most desirable target of shale oil exploration and development at present. Thus, this work may help related industry practitioners acquire valuable information from lacustrine shale systems.