Local high-salinity source rock and origin of crude oil in the xianshuiquan structure in the northwestern Qaidam Basin, China

摘要

Despite its importance as an oil-bearing structure in the northwest Qaidam Basin, the development characteristics of source rocks and the origin of crude oil in the Xianshuiquan structure have not been clarified. Based on total organic carbon, extracted chloroform bitumen "A", kerogen maceral, vitrinite reflectance, light hydrocarbon, biomarker compositions and seismic profiles, comprehensive analyses are performed on the origin and accumulation of crude oil in the Xianshuiquan structure. The results show that: 1) unlike the conclusions of existing research in the northwest Qaidam Basin, the Upper Youshashan (N-2(1)) member of the Xianshuiquan structure developed a set of high-salinity source rocks with a hydrocarbon generation capacity; 2) the crude oil in the shallow members (above the Lower Youshashan (N-1) formation) of the Xianshuiquan structure originated from the N-2(1) high-salinity source rock that developed locally in the Xianshuiquan structure, rather than from the generally believed N-1 source rock; 3) two sets of fault systems were developed in the Xianshuiquan structure and formed two types of petroleum system, deep and shallow, and the crude oil generated from the Upper Ganchaigou (E-3(2)) and N-1 source rocks did not migrate into the shallow members for accumulation. 4) The light hydrocarbons of crude oil are lost seriously and the preservation conditions of crude oil in the Xianshuiquan structure are poor. According to the above results, the hydrocarbon accumulation model of the Xianshuiquan structure is established, and it is proposed that the deep members of the Xianshuiquan structure have considerable exploration and development prospects. These results are of great significance to the exploration and development of the northwestern Qaidam Basin.