A study on the hydrocarbon generation potential of the coal-bearing source rocks in the Pinghu Formation of Pingbei area, the Xihu depression

The source rocks of the Pinghu Formation in the Pingbei area of the Xihu depression consist of a set of coal-bearing deposits of marine-continental transitional facies. Organic matter abundance of the coal and dark mudstone were evaluated mainly based on potential hydrocarbon generation amount according to the evaluation criteria established. Results show that both the coal rocks and dark mudstones have high hydrocarbon generation potential and medium to high abundance of organic matter. The coal rocks are featured by S₁+S₂＞90 mg/g, HI＞100 mg/g,“A”＞1.0%, while the mudstones characterized by TOC＞1%, S₁+S₂＞2 mg/g, “A”＞0.05%, “HC”＞150×10⁻⁶. It is identified that Kerogen of the coal rocks belongs to typeⅡ₁-Ⅱ₂ according to the elemental data, and the content of hydrogen-rich macerals is very high. The total content of resinite, sporophyte and cuticle in exinite is up to 10%～20%, and the content of hydrogen-rich vitrinite is as high as 20%, indicating a high oil-generating potential. The content of hydrogen-rich macerals in mudstone is lower than that in coal rocks. The kerogen belongs to typeⅡ₂ with higher gas-generating potential. Threshold depth of oil generation and expulsion in the Pingbei area were determined by Ro values and (S₁+S₂)/TOC values, and they are 3 100 m and 3 500 m respectively. The results of thermal simulation show that the coal rocks are prone to generating both oil and gas, with higher proportion of oil. Oil generation rate can reach 130 mg/g TOC and Gas generation rate about 125 mg/g TOC. The mudstone is gas-prone for a long history of gas generation. Data suggests that the oil generation rate is 50 mg/g TOC only, while the gas generation rate can reach 185 mg/g TOC. Coal-derived oil is widely existed in the Xihu depression according to the heavier crude oil carbon isotope and correlation of biomarkers between oils and source rocks.