Impact of Paleoclimatic Variability on Shale Oil Enrichment in Deep to Semi-Deep Lacustrine Facies: A Case Study of the Second Member of the Funing Formation, Qintong Depression, Subei Basin, Eastern China

Lacustrine depositional systems are more sensitive to climatic fluctuations than marine systems, producing significant stratigraphic and sedimentological heterogeneity that affects shale oil distribution. However, the mechanisms linking paleoclimatic variability to shale oil enrichment remain insufficiently constrained. This study investigates the coupling between hydrological variations, organic matter enrichment, and pore structure development under different climatic conditions in the Second Member of the Funing Formation from Well A1 in the Qintong Depression. Using a combination of total organic carbon (TOC) analysis, total sulfur (TS) analysis, X-ray diffraction (XRD), high-resolution geochemical profiling, field-emission scanning electron microscopy (FE-SEM), and helium porosimetry, we reveal how paleoclimate influences shale oil accumulation. The Second Member of the Funing Formation is subdivided into four stratigraphic units based on TOC variations, with units 1 and 3 corresponding to extremely arid conditions, unit 2 to arid conditions, and unit 4 to a humid climate. Both arid and humid conditions promoted elevated lacustrine primary productivity and moderate sedimentation rates, thereby favoring efficient organic matter accumulation. Increased water depth and optimal salinity also facilitated bottom-water anoxia, enhancing organic matter preservation. The study demonstrates that climatic variations in terrigenous clastic input and lake salinity affect the mineral composition of the lacustrine shale, influencing pore development. Under arid conditions, favorable mineralogical proportions enhances the formation of intercrystalline pores within the clay matrix, improving hydrocarbon storage potential. This research establishes that paleoclimate plays a critical role in shale oil enrichment by influencing both organic matter accumulation and pore structure. Among the analyzed units, unit 2, deposited under arid conditions, shows the most favorable source-reservoir characteristics and is identified as the most promising interval for future shale oil exploration.