The origin of fluid in calcite veins and its implications for hydrocarbon accumulation in the Shunnan-Gucheng area of the Tarim Basin, China

The high-angle, structural genetic calcite veins of the Ordovician in the Shunnan-Gucheng area in the Tarim Basin provide a convenient study of fluid properties, hydrocarbon migration, and accumulation events in the fault zone of this area. Based on detailed core investigation, studies on fluid inclusions in calcite veins and comparison of geochemical characteristics of limestone matrix and calcite veins were carried out. Multiple types of hydrocarbons inclusion (e.g., dry asphalt inclusion,three-phase hydrocarbon inclusion, oil and water inclusion,gas-rich inclusion) were found in calcite veins. Furthermore, the coexisted aqueous inclusions with the gas–liquid phase had a high homogenization temperature (130–160 °C). The distribution characteristics of 18O-depleted (δ18O: −13.1‰ to −8.7‰), 87Sr-enriched (87Sr/86Sr: 0.708 879 to 0.710 432), relatively high total Rare Earth Elements (REEs), and positive Europium Anomalies indicated that the structural origin of calcite veins was mainly derived from the dissolution of surrounding rocks and diagenetic-hydrocarbon fluids. Participation signs of meteoric water were not proved. The rich hydrocarbon inclusions in the calcite veins indicated that the north-east strike-slip fault zone in the Shunnan-Gucheng area is an essential channel for hydrocarbon migration and accumulation.