Oil Potential and Sweet‐Spot Evaluation of Saline Lacustrine Hybrid Shale: Lower Ganchaigou Formation, Yingxiongling Sag, Qaidam Basin

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ABSTRACT Shale oil sweet‐spot evaluation in saline lacustrine basins is challenged by complex mineral composition, hybrid sedimentation and strong reservoir heterogeneity. Here, the Lower Ganchaigou Formation in the northern Yingxiongling Sag was investigated using X‐ray diffraction (XRD), Rock–Eval pyrolysis, scanning electron microscopy (SEM) and porosity measurements to clarify the controls on oil content and mobility. The studied shales are mainly composed of quartz, dolomite, calcite and clay minerals, and can be divided into mixed shale (64.15%), calcareous shale (32.08%), felsic shale (2.64%) and clay‐rich shale (1.13%). Intercrystalline pores, dissolution pores and laminar fractures provide the dominant storage space, with effective porosity ranging from 1.2% to 7.4%. Although the shales have relatively low organic richness, with an average total organic carbon (TOC) content of 0.37%, Type II and Type III kerogen and immature to early mature thermal evolution favour a low‐TOC but relatively mobile shale oil system. Oil saturation and mobility vary substantially among lithologies, and calcareous shale is the most favourable interval for hydrocarbon enrichment because of its coupled hydrocarbon‐generating potential, carbonate‐rich mineral framework, and connected pore‐fracture network. Rather than directly applying OSI thresholds developed mainly for marine or organic‐rich shale systems, this study calibrates an OSI‐S 1 classification scheme for low‐to‐moderate TOC, high‐mobility saline lacustrine shale. The scheme delineates 28 sweet‐spot intervals, including ten Tier I targets, and provides a practical framework for selecting favourable shale oil intervals in the Lower Ganchaigou Formation.

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