Characteristics of Ordovician paleokarst inclusions and their implications for paleoenvironmental and geological history in Halahatang area of northern Tarim Basin

The Halahatang area has become a key region for petroleum exploration in the Tarim Basin in recent years owing to its well-developed and extremely heterogeneous Ordovician paleokarst carbonatites. This study explores the geochemical characteristics of the paleokarstification products, various periods of paleokarstification, and paleoenvironmental conditions of the Ordovician carbonates in the Halahatang area. After careful analysis of the rock fabric, the stable carbon and oxygen isotopic composition of the calcites filling paleokarstic fissures were systematically examined, and experimental analysis on fluid inclusions was conducted. The delta O-18 values were generally negative, ranging from -4.14 to -15.71aEuro degrees with an average value of -9.95aEuro degrees. These values reflect a paleokarstification environment changing from shallow burial to deep burial, the rocks of which were later affected by corrosion. The delta C-13 (PDB) values range from 2.12 to 4.09aEuro degrees with an average value of 0.65aEuro degrees. This confirms the complexity of the paleokarstification and revealed four different paleoenvironmental conditions: a marine environment with syndiagenesis, weathered crust exposed to a freshwater karst environment, a shallow-burial karst environment, and a deep-burial high-temperature environment. The homogenization temperature of the inclusions indicates the existence of four paleokarstification and calcite infilling periods: buried karstification during the late Hercynian, the Indosinian-Yanshanian, and the Himalayan and karstification by meteoric water during the Caledonian. The results of this study provide new evidence for the prediction and exploration of karst reservoirs.