Reservoir characteristics and main controlling factors of karst shale oil in the Lower Ganchaigou Formation,Yingxi area,Qaidam Basin

Abstract

The Paleogene Lower Ganchaigou Formation (E₃²) in the Yingxi area, contains karst-type shale oil reservoirs characterized by strong heterogeneity and unclear formation mechanisms. Using an integrated approach of core observation, thin-section petrography, XRD, image logging, paleogeomorphic reconstruction, and fluid inclusion microthermometry, we systematically investigated reservoir characteristics and karst development. The reservoirs consist of saline lacustrine mixed sedimentary rocks with 40-50% carbonate minerals and interbedded sulfates and halides, providing the material basis for multistage karstification. Three karst mechanisms were identified: meteoric dissolution during shallow burial, organic acid dissolution during hydrocarbon generation, and thermochemical sulfate reduction (TSR) during deep burial. Notably, TSR-related dissolution is a key constructive process for developing high-quality secondary porosity, challenging previous interpretations emphasizing structural fractures. Reservoir is jointly controlled by paleoclimate, paleogeomorphology, and deep burial environment. We established a genetic model of ‘saline lacustrine sedimentation-penecontemporaneous dolomitization-multistage karst reworking’, providing insights for predicting deep unconventional reservoirs.