Fluid Types and Geologic Models for Karst Reservoir Development Within the Penglaiba–Lower Yingshan Formations, Ordovician, Northern Tarim Basin

Abstract

The Ordovician strata in Northern Tarim host extensively developed carbonate karst reservoirs that contain abundant hydrocarbon resources. However, owing to extreme burial depths, pronounced heterogeneity, and limited seismic resolution within the Tarim Basin, the diagenetic fluid types and their specific influences on reservoir development remain poorly understood. Consequently, this study integrates core observation, thin-section identification (TSI), cathodoluminescence (CL), scanning electron microscopy (SEM), X-ray diffraction (XRD), stable isotopes (C, O, Sr), trace and rare earth elements (REE), fluid inclusion analysis (FIA), and in situ laser U-Pb dating (U-Pb). This multifaceted petrographic and geochemical approach characterizes the macro- and microscopic geological features of these karst reservoirs. By elucidating the types, timing, and phases of diagenetic fluids, this research evaluates fluid-driven impacts on reservoir quality and establishes a comprehensive genetic model for reservoir evolution. Results identify five distinct tectonic fracturing phases. Phases 1, 2, and 4 involved calcite infilling precipitated from seawater and meteoric freshwater, with fluid inclusion homogenization temperatures of 62–87 °C, 57–91 °C, and 94–126 °C, formed during the Caledonian–Hercynian, Early Hercynian, and Indosinian–Yanshanian periods, respectively. Phase 3 featured hydrothermal dolomite infilling during the Hercynian, with fluid inclusion homogenization temperatures ranging from 128 to 163 °C, whereas Phase 5 remained unfilled during the Himalayan. Constrained by the U–Pb age interval of 445.2–436.5 Ma acquired from vug-filling calcite together with cross-cutting petrographic relationships, multi-stage meteoric freshwater dissolution mainly occurred from Middle Caledonian Episode III (447–443.7 Ma) to the Early Hercynian (460–359 Ma). Reservoirs within the Penglaiba–Lower Yingshan Formations underwent a complex evolution comprising syngenetic-to-early diagenetic pore development, Middle Caledonian–Early Hercynian weathering crust karstification and dedolomitization, and Late Hercynian hydrothermal dissolution-infilling, ultimately resulting in the formation of tectonic-karst composite reservoirs.