Favorable Exploration Lithofacies and Their Formation Mechanisms in Lacustrine Shales Deposited under Different Salinity Conditions: Insights into Organic Matter Accumulation and Pore Systems

Efficient identification of favorable exploration lithofacieswithcommercial value is of utmost importance in lacustrine shale petroleumexploration. Shales deposited under different salinity conditionsexhibit considerably different organic matter (OM) accumulation andmineral compositions. However, the systematic summarization of favorableexploration lithofacies and their formation mechanisms in lacustrineshale systems formed under different salinity conditions is stilllacking. Herein, lacustrine shale of the Lower Jurassic Qianfoya Formation(J(1)q) from the Langzhong Area of the Sichuan Basin is studiedto systematically reveal its OM accumulation mechanisms and the factorscontrolling its physical properties. By combining our data with thatpublished in the literature, the favorable exploration lithofaciesand their formation mechanisms in lacustrine shales deposited undervarious different salinities (i.e., freshwater, saline, and hypersaline)are discussed. Our findings indicate that the J(1)q shalecomprises four lithofacies, namely, organic-lean mixed shale (LM),organic-lean argillaceous shale (LCM), organic-rich mixed shale (RM),and organic-rich argillaceous shale (RCM). The pore systems of RCMand RM are dominated by OM-related pores, whereas those of LCM andLM are dominated by clay mineral-related pores. The J(1)qshale was deposited in an oxic environment with a semiarid to semihumidclimate, moderate weathering, and freshwater to brackish conditions.The OM content is primarily influenced by the relatively reductiveenvironment. The physical properties of RCM and RM are primarily determinedby the total organic carbon (TOC) content, whereas those of LCM andLM are determined by the clay mineral content. Additionally, a novelmodel for describing the change in the TOC content, pore system characteristics,and fracability of shale with an increase in salinity is proposed.The model shows that with an increase in salinity, the TOC contentfirst decreases, then increases, and finally decreases. Simultaneously,there is an improvement in the proportion of framework-grain relatedpores within the pore system. RCM, RM, and tight sandstone and limestoneinterlayers within freshwater lacustrine shale can be regarded asprimary sweet spots. In the saline and hypersaline lacustrine shalesystems, the main exploration targets are RM and organic-rich calcareousshale. These observations provide theoretical guidance for locatingsweet spots in lacustrine shale systems deposited under differentsalinity conditions.