Geochemical Characteristics and Paleoenvironmental Significance of the Ordovician Paleokarst Reservoir in the Maigaiti Slope of Tarim Basin

The Ordovician carbonatites of the Maigaiti slope have formed the conditions for the development of large oil and gas fields with karst reservoirs. This study systematically analyzed the isotopic characteristics of carbon, oxygen, and strontium and enrichment trace elements regularity to examine the geochemical features of the paleo-karstification products, various periods of paleo-karstification, and paleoenvironmental conditions of the Ordovician. Affected by terrigenous clasts, the Lianglitage Formation is composed of limestone with a relatively high 87Sr/86Sr value and a frequently fluctuating sea level, which exhibits an overall increasing-to-decreasing trend variation. Of all samples affected by the atmospheric freshwater leaching effect, delta 18O values were negative, and delta 13C values varied greatly toward both ends. The fissure/cave infills were rich in Fe and Mn but poor in Sr, and the Sr/Ba value was considerably less than 1, which confirms the existence of ancient weathered crust and the development of an atmospheric freshwater karst environment. Four different paleo-karstification periods were identified according to the carbon and oxygen isotopic characteristics of calcite. Combined with the trace element characteristics of the infills, the paleo-karstification in the Yingshan Formation was subdivided based on three hydrological environment conditions. Erosion modification of buried acid compaction-released water from late-period corrosion pores, caves, and fissures formed by syndiagenetic paleo-karstification and weathered crust bare paleo-karstification due to atmospheric freshwater leaching can significantly improve the reserving and permeability characteristics of the karst reservoir to develop a large paleokarst reservoir.