Organic Lithofacies Classification and Paleoenvironment of Lacustrine Shales in the Middle Jurassic Yangye Formation, Southwestern Tarim Depression

As a key preliminary strategic area for continental shale hydrocarbon exploration, the Southwestern Tarim Depression contains thick, high-TOC lacustrine shale on the Jurassic Yangye Formation, yet its strata still lack clear definition in precise lithofacies architecture and sedimentary evolution due to low exploration intensity. Based on core observation, thin section identification, XRD, XRF, TOC and elemental analyses, nine shale lithofacies are classified into four main types. Geochemical data reveal distinct depositional settings for each lithofacies. The laminated-bedded organic-rich argillaceous shale formed during the maximum lake transgression in a warm-humid freshwater deep lake with strong reduction. The bedded-massive organic-rich felsic shale developed in a relatively shallow freshwater lake affected by episodic terrigenous input. The bedded-massive medium-organic felsic shale occurred in a stable sub-humid freshwater semi-deep lake with obvious sediment dilution. The laminated-bedded organic-rich dolomitic-calcitic mixed shale mainly accumulated in an arid, evaporative closed lake with high salinity and strong water stratification. Vertically, the study area experienced three evolutionary stages: argillaceous shale, felsic shale and dolomitic-calcitic mixed shale. Lithofacies with TOC > 1.5% possess abundant organic matter and provide fundamental hydrocarbon-generating materials. This study provides references for reservoir evaluation and prospect selection in the study area.