Distribution, Origin, and Impact on Diagenesis of Organic Acids in Representative Continental Shale Oil

This investigation focuses on the prevalent continental oil shale within the Triassic Chang 7, a member of the Yanchang Formation in the Ordos Basin and the Permian Lucaogou Formation in the Junggar Basin of western China, and delves into the impacts of hydrocarbon generation and the derived organic acids on the physical attributes of oil shale reservoirs. Water-soluble organic acids (WSOAs) were extracted via Soxhlet extraction and analyzed by a 940 ion chromatograph (Metrohm AG), supplemented with core observations, thin-section analyses, pyrolysis, and trace element assays, as well as the qualitative observation of pore structures via FIB-SEM scanning electron microscopy. The study discloses substantial disparities in the types and abundances of organic acids within the oil shale strata of the two regions, with mono-acids being conspicuously more prevalent than dicarboxylic acids. The spatial distribution of organic acids within the oil shale strata in the two regions is non-uniform, and their generation is inextricably correlated with the type of organic matter, thermal maturity, and depth at which they are buried. During diverse stages of diagenesis, the hydrocarbons and organic acids produced from the pyrolysis of organic matter not only exert an impact on the properties of pore fluids but also interact with diagenetic processes such as compaction, dissolution, and metasomatism to enhance the reservoir quality of oil shale. The synergy between chemical interactions and physical alterations collectively governs the migration and distribution patterns of organic acids as well as the characteristics of oil shale reservoirs. Furthermore, the sources of organic acids within the oil shale series in the two regions demonstrate pronounced dissimilarities, which are intimately associated with the peculiarities of their sedimentary milieu. The oil shale of the Yanchang Formation was formed in a warm and humid freshwater lacustrine basin environment, while the oil shale of the Lucaogou Formation was deposited in a brackish to saline lacustrine setting under an arid to semi-arid climatic regime. These variances not only illuminate the intricacy and multiplicity of the sedimentary attributes of oil shale but also accentuate the impact of the sedimentary environment on the genesis and distribution of organic acids, especially the transformation and optimization of reservoir dissolution by organic acids generated during hydrocarbon generation—a factor of paramount significance for the precise identification and effective development of the “sweet spot” area of shale oil. These areas, characterized by an abundance of organic matter, their maturity, and superior reservoir properties, are the foci of the efficient exploration and development of continental shale oil.