Microbial dolostones of the Sinian Qigebulake Formation in the Kepin area, northwestern Tarim Basin, represent an important target for deep to ultra-deep hydrocarbon exploration. Based on integrated analyses of outcrop sections, drilling cores, thin sections, scanning electron microscopy (SEM), and petrophysical data, this study systematically investigates the lithofacies characteristics, reservoir space types, and controlling factors of microbial dolostone reservoirs. (1) Five major lithofacies types were identified, including stromatolitic dolostone, clotted dolostone, foamy laminated dolostone, granular dolostone, and crystalline dolostone. These lithofacies mainly developed in an inner-ramp depositional setting and vertically formed a shallowing-upward sedimentary succession from tidal flat to microbial mound and shoal facies. Reservoir spaces are dominated by secondary dissolution pores, including framework dissolution pores, intergranular and intragranular dissolution pores, vugs, fractures, and karst cavities. The reservoirs are characterized by medium porosity, low permeability, and strong heterogeneity. (2) Sedimentary facies, microbial dolomitization, and karstification jointly controlled the development of relatively favorable reservoir intervals. Early microbial-induced dolomitization enhanced the rigidity of microbial frameworks and facilitated the preservation of primary pores, whereas meteoric karstification associated with the terminal Sinian Keping Movement significantly improved reservoir quality through large-scale dissolution enlargement and fracture-cavity development. SEM observations reveal abundant microbial mineralization textures, including cauliflower-shaped, dumbbell-shaped, and spheroidal dolomite morphologies associated with EPS remnants, providing direct evidence for microbial mediation during dolomite precipitation. (3) Reservoir intervals with relatively favorable physical properties are mainly distributed in the middle-upper microbial mound intervals and upper karst-modified zones of the Qigebulake Formation, forming a favorable source–reservoir–seal assemblage with the overlying Yuertusi Formation black shales. This study provides new insights into the formation and preservation mechanisms of deep microbial dolostone reservoirs and offers important implications for ultra-deep hydrocarbon exploration in the Tarim Basin.
Liu et al. (Wed,) studied this question.