A geochemical evaluation method of fault sealing evolution and its controlling effect on hydrocarbon accumulation: A case study of the Hongche fault zone in the northwest margin of the Junggar Basin

Evaluation on fault sealing evolution is of guiding significance to hydrocarbon accumulation research, but there are few researches on the organic combination of fault sealing evolution and dynamic hydrocarbon accumulation. Taking the Hongche fault zone in the northwest margin of the Junggar Basin as an example, this paper establishes a geochemical evaluation method of fault sealing evolution by means of the geochemical test and analysis technologies such as biomarker of source rocks, characteristics of hydrocarbon generation thermal simulation products, chromatography and mass spectroscopy of crude oil, carbon isotopes, and light hydrocarbons. In addition, the controlling effect of the differential evolution of fault sealing in different geological periods on hydrocarbon migration and accumulation is revealed. The following research results are obtained. First, the lateral sealing of the fault in the Hongche fault zone controls the difference of crude oil properties between the upper and the lower block of the fault. Second, in the Late Triassic to the Early Jurassic, the fault didn't provide lateral sealing to oil and gas, so mature crude oil migrated from the source rock of Lower Permian Fengcheng Formation to the Carboniferous in the upper block, where it was biodegraded to high-density heavy oil. In the Late Jurassic to the Early Cretaceous, the fault sealed oil laterally instead of gas, so the highly mature oil of Fengcheng Formation and the mature oil of Middle Permian Lower Wuerhe Formation accumulated in the lower block. Third, the main reason why the associated gas in the reservoir of Hongche fault zone is generally dry gas is that the sealing of faults to different fluids is different in different geological periods. Fourth, the evolution of fault sealing can be divided into four stages: opening, weak sealing, medium sealing and strong sealing, which respectively control the four accumulation processes of oil and gas loss, oil reservoir biodegradation at the upper block, oil reservoir accumulation and natural gas loss at the lower block, and dry gas capture in the late stage. In the first three stages, mature and highly mature natural gas is lost, and in the late strong sealing stage, dry gas is accumulated in a large scale, so as to form a complex oil and gas distribution pattern with heavy oil, medium oil, light oil and dry gas coexisting. In conclusion, the geochemical evaluation method for the dynamic evolution history of fault sealing is of important guiding significance to the exploration and development of multi-source and multi-stage oil and gas in complex areas. © 2023 Natural Gas Industry Journal Agency. All rights reserved.