Types and controlling factors of Ordovician paleokarst carbonate reservoirs in the southeastern Ordos Basin, China

Abstract: With continuous improvements in oil and gas exploration technology and the increasing demand for oil and gas resources, the focus of oil and gas exploration has shifted from deep to ultradeep reservoirs. This study targeted the carbonate karst reservoirs of the Ordovician Ma5 Member in the southeastern Ordos Basin. Based on the analysis of cores, thin sections, cathodoluminescence, scanning electron microscopy (SEM), carbon and oxygen isotopes and trace elements, we discuss the development characteristics, types and influencing factors of paleokarst reservoirs. Petrology and geochemistry show three kinds of karstification in the study area: quasi-contemporaneous karst, epigenetic karst and deep burial karst. The quasi-contemporaneous and epigenetic paleokarsts were mainly controlled by freshwater, where the soluble components of the strata were preferentially dissolved. The deep burial karstification was dominated by the reaction product H2S from thermochemical sulfate reduction (TSR) between sulfate minerals and organic matter, during which hydrothermal fluid provided the required activation energy for the reaction. The observations of cores and thin sections show that the paleokarst reservoirs are composed of different kinds of dissolution pores and fractures and cemented by multistage calcite and siliceous cements. Influenced by freshwater leaching and hydrothermal fluid, the three paleokarst processes led to negative anomalies in δ18O and δ13C, while the low δCe, low δEu and rare earth element (REE) partition modes confirm that the intensity of freshwater leaching was stronger than that of hydrothermal fluid. The controlling factors of the paleokarst reservoirs included acidic fluids, paleogeomorphology and paleostructure in each diagenetic stage. According to the structural background and core characteristics, we establish a development model and spatial evolution model of the paleokarst reservoirs in the southeastern Ordos Basin.