This study, for the first time, investigates the source, keroger pattern, evolution degree, sedimentary environment, and evaluates the hydrocarbon generating ability of marine-continental transitional facies shale from the Ebao area of the Qilian Basin in western China. The organic-rich shales of the marine-continental transitional facies were subjected to total organic carbon (TOC), rock pyrolysis, the biomarkers and kerogen microscopy analyses. The kerogen microscopy analyses indicated that the sample from ZK001 well and ZK2002 well had a mass of vitrinite and small number of exinite, so kerogen type was mainly humic. According to vitrinite reflectance, the organic matter was in the stage of maturity to high maturity. The organic analyses indicated that the shale samples possessed the features of the higher TOC and a mixed of terrestrial and marine sources and maturity to high maturity. These features were compatible with the characterized of kerogen microscopy characteristics. A lot of organic materials in the studied shales form ZK001 well and ZK2002 well could be because there were fine protection under salt lake facies and hypoxic marine environment. Therefore, the studied shales of marine-continental transitional facies shale had very good hydrocarbon generating ability as a result of the existence of a lot of high maturity organic materials of humic.
The hydrocarbon accumulation history of 2536.5 me2697.0 m of Well Sutan 1was studied, and the geological and geochemical data including the homogenization temperature of fluid inclusion and the geochemical characteristics of the crude oil were analyzed. The results showed that the crude oil had the properties of a high degree of evolution. The precursor type was mainly high terrestrial plants. Among them, crude oil from depths of 2620.67 m, 3640.57 m and 2697 m had been charged with crude oil with aquatic organism. The crude oil was deposited in weak oxidizing to oxidizing environment with fresh to slight saline water, which is better for preservation and transformation of organics. The characteristic of fluid inclusion with Well Sutan 1 showed that there were at least three occasions of filling of oil and one of filling of gas. Also, petroleum accumulation existed in the crude oil on two occasions. The reservoir forming time of the first charge with oil was about 142 Ma, and the second charge with oil was about 107 Ma, which is tectonic inversion of the Yanshanian Orogenic Movement in the Late-Early Cretaceous.
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