Causes And Sources Of Key Tectonic Hydrocarbon In The Northern Margin

Fukang sag is located in eastern area of Junggar basin, with a huge amount of resource but low proved,so the northern part of the sag is one of hot spots for exploration. The oil and gas distribution of oilfields around the depression are complex, the types of reservoirs are various, the hydrocarbon sources remain controversy. Based on the comprehensive evaluation and analysis of source rocks of Fukang sag, a comprehensive analysis of hydrocarbon characteristics, formation and source of each geologic structure in northern margin was made, and the hydrocarbon formation was cleared, which establishes the foundation andindicates directions for the next step exploration.There are two sets of hydrocarbon source rock in Fukang sag, which is Pingdiquan group in Permian system and the Jurassic system. Because the source rock of Permian system has a deep burial and no drilling operation was carried out in the deep area of Fukang sag, the analysis and evaluation on the source rock of the slope area of Fukang sag was made. It is considered that the thickness of the source rock of Pingdiquan group of Permian system is large, while both the abundance of organic matter and the hydrocarbon generation potential are great. The types of organic matter aremainly Ⅰ、Ⅱ1, which belong to fine hydrocarbon source rock. Although the abundance of hydrocarbon source rock of Jurassic system is high, the type of organic matter is mainly III, which has a comparatively fair hydrocarbon generation potential. Because the buried depth of Fukang sag is large and the evolution degree of source rock is high, both the two sets of source rocks belong to mature stage and have strong hydrocarbonsupplying ability.Gas reservoirs mainly exist in the Baijiahai arch, which is mainly dry gas. The carbon isotope of methane is various, while the types of mature and high-overmature stage gas are both captured. The ethane carbon isotopes is strong, which belongs to partial humic type organic matter source product. Comprehensively considering the evolution history of source rock and geological tectonic feature, the high-overmature-stagenatural gas is mainly originate from the source rock of Carboniferous system in Dongdaohaizi sag.The crude oil of Baijiahai arch is the mature-stage product, while the pristane to phytane ratio and carbon isotope are between the sapropelic crude oil and sapropelic organic matter sourced oil, which has the mixing characteristics of the sourced oil of both Permian system Pingdiquan group and Jurassic system, which is also the mixing product of them. Combined with drilling and logging data of the transitional zone, the hydrocarbon show of which is bad, hence the possibility of the crude oil migration from Fukang sag to Baijiahai arch is excluded. Combined with geological tectonic feature of Baijiahai area and the evolution history of source rock of peripheral sag, it is estimated that the crude oil of Baijiahai area is sourced from mixture of the products of Permian system Dongdaohaizi sag and Jurassic system.There are significant differences in crude oil characteristics between different structure around the depression. The pristane to phytane ratio of the crude oil of Shabei oilfield is high, and the carbon isotope is strong, which belongs to the product of Jurassic system of Fukang sag. The pristane to phytane ratio of the shallow position of the slope areas is high and the carbon isotope is strong, which is also the product of Jurassic system of Fukang sag. Both the pristane to phytane ratio and the carbon isotope of the deep position are comparatively low, which is the product of Pingdiquan group of Permian system of Fukang sag.