Diverse oil and gas seeps in the southern Junggar Basin, NW China (piedmont Northern Tian Shan): Origins and links to tectono-sedimentary evolution

The southern Junggar Basin in NW China is a world-class location for the study of oil and gas seeps, with widely distributed, highly variable oil and gas seeps including gaseous mud volcanoes, liquid oil seeps, and solid bitumen deposits. However, the sources and origins of these seeps are poorly understood, as there are many potential source sequences of Permian to Paleogene age. The uncertainty surrounding the seeps means that it has not been possible to address their significance for oil and gas exploration in the area, and they are not understood in the context of the tectono-sedimentary evolution of the basin within the piedmont region of the northern Tian Shan Mountains (in the eastern part of the Central Asian Orogenic Belt, CAOB). In the present study, these issues were addressed using combined geological and geochemical methods. Results show that there are five main types of seeps in the region from west to east. Type A seeps originate from Jurassic source rocks and are best represented by the A'erqingou mud volcano. Type B seeps originate mainly from Paleogene source rocks but also contain a Jurassic-sourced component; they are best represented by the Dushanzi mud volcano. Type C seeps originate from a Cretaceous source and are best represented by the South Anjihai and Horgos oil sands. Type D seeps originate mainly from Jurassic source rocks but also contain a Triassic-sourced component; they are best represented by the Qigu oil seep. Type E seeps originate from Permian source rocks and are best represented by the Dalongkou bituminous deposit. The primary controls on seep distribution are both sedimentological (deposition of source rock) and tectonic (faulting). The position and burial history of sedimentary centres controlled the distribution and maturation of source rocks. Tectonic faults provided migration pathways and traps for the oil and gas. The results of this study will help to reduce uncertainty during exploration for oil and gas in the area. The findings also have general implications for petroleum systems and exploration in the piedmont areas of foreland basins worldwide.