Paleo-fluid characteristics and preservation of hydrocarbons in the Sangzhi-Shimen synclinorium and adjacent areas

The paleo-fluid characteristics and hydrocarbon preservation in the Sangzhi-Shimen synclinorium and adjacent areas are discussed using comparisons of C, O, and Sr isotopes between the calcite veins and surrounding rocks in the Sinian and Triassic formations and combined with the tests of fluid inclusions in calcite veins. The ⁸⁷Sr/⁸⁶Sr values of surrounding rocks in the Lower Paleozoic are higher than those of contemporary seawater and calcite veins, which suggests that the surrounding rocks had been reformed by fluid with high ⁸⁷Sr/⁸⁶Sr values before fracturing. The calcite veins close to thermal springs in the north of Sangzhi-Shimen synclinorium have high ⁸⁷Sr/⁸⁶Sr values, low δ¹³C and low δ¹⁸O values, which can be explained by the mixing of meteoric water during calcite vein deposition and suggests that reverse faults might have been open when they were formed. The salinity and homogenization temperature of fluid inclusions shows that the residual cap formations above the Upper Cambrian Loushanguan Formation are less than 3 km thick with poor sealing capacity. The high salinity of calcite inclusions in the Middle Cambrian Kongwangxi Formation showed that gypsum salt cap layers are distributed widely in the study area, and the gypsum salt layers buried over 2 km deep have excellent sealing capacity. The preservation conditions of the Tianerya, Xiangjiaya, Chefang anticline traps, and the Qiaotou, Zhongping buried structural traps are favorable. The dolomites in the Sinian Dengying Formation and Cambrian Qingxudong Formation are exploration targets except for those in the Tianerya anticline.