| The marine black shales from South China have the characteristics of long sedimentary age, high thermal maturity,and relatively low total organic carbon content. To recover initial gas-generation potential of these high and over mature shales, the commonly used evaluation parameters such as total organic carbon content, thermal maturity levels and types of kerogen are not suitable. In this article, the kinetics of hydrocarbon generation is applied to investigate the gas generation from kerogen samples and shale samples by using a half closed pyrolysis system and an anhydrous closed pyrolysis system. The quantitative evaluated pyrolysis products are described as C1, C2, C3, CO2, H2 gases, C6–C12 light hydrocarbons, and C10–C17 diamondoids. Based on the amounts and distribution characteristics of those pyrolysates at different maturity stages, the source and the evolution process of shale gas can be thoroughly investigated. It will provide experimental evidences for quantitative identification and evaluation of the gas generation potential of organic-rich shales with high maturity levels. The main conclusions are as follows:(1) The comparative study of shale and kerogen on the thermal evolution characteristics of hydrocarbon generation indicates that the result from the whole-rock pyrolysis can represent the actual thermal evolution characteristics of organic matters, while the result from the kerogen pyrolysis may be more helpful to understand the reaction mechanism in the evolution process.(2) According to the characteristics of hydrocarbon evolution, the generation process of methane in kerogen can be divided into four stages: the oil generation stage, the condensate generation stage, the wet gas generation stage and the dry gas stage. The secondary cracking of bitumens contributes 69.4% of the methane maximum yield in kerogen pyrolysis, including 15.7% from the bitumen(B1) formed in the early stage of oil window;31.9% from the bitumen(B2) produced in the peak stage of oil window;21.8% from the bitumen(B3) produced in the late stage of oil window. The contribution of primary cracking of kerogen accounts for 30.6% of methane generation in kerogen as well.(3) Diamondoids generated from kerogen within source rock experience a similar evolution process with that in oil during thermal maturation, including generation(0.8-2.1% Easy Ro for adamantanes and 1.0-2.5% Easy Ro for diamantanes), and destruction(2.1-3.0% Easy Ro for adamantanes and 2.5-3.5% Easy Ro for diamantanes) stages. Comparison of the diamondoid yields from different maturity kerogens shows that diamondoids in source rock are mainly generated from the secondary cracking of bitumens within the source rock at the late stage of thermal maturation(>1.3% Easy Ro), accounting for 75.6% of the maximum yield of adamantanes and 87.8% of the maximum yield of diamantanes, respectively. The deadline or upper limit of maturity for the generation of diamondoids from the primary cracking of kerogen is 1.3% Easy Ro. Some diamondoid isomerization ratios keep relatively constant in the formation stage of diamondoids, whereas a linear correlation with maturity occurs in their destruction stage, indicating that these diamondoid indices are a potential tool for the evaluation of thermal maturity of source rocks at high- and over- mature stages。(4) Shale can still maintain a certain capacity of gas generation after hydrocarbon expulsion effects at the early stage of maturation. The upper limits of methane generation in shale is 3.0% Easy Ro when the methane yield dropped at 1% of the primary methane maximum yield. Kerogen, expelled bitumen and residue bitumen contributes 22.7%, 57.6% and 19.6% of maximum yield of methane in shale, respectively. Based on the thermal maturation of shale, maximum yield of methane contributed by expelled bitumen can be divided into four parts. Quantitatively, among the 57.6%, 16.3% of the maximum yield of methane was contributed by the peak period of oil generation(i.e., 0.7-1.0% Easy Ro), 21.9% was contributed by the later period of the oil window(i.e., 1.0-1.3% Easy Ro), 12.0% was contributed by the expelled bitumen generated in wet gas stage(i.e., 1.3-2.0% Easy Ro), and 7.5% was from the expelled bitumen generated in the stage with Easy Ro at 2.0-2.5%.(5) The expulsion of hydrocarbons during the early stage of shale maturation(0.57-0.7% Easy Ro) is not significant, and can hardly influence the gas generation ability of shale in high-over maturation stage. Abundant soluble bitumen still exists in the shale matrix after the hydrocarbon expulsion. The soluble bitumen in the shale matrix, after interacting with kerogen and insoluble bitumen, becomes the major source of shale gas. The bitumen is a crucial precursor of C2-C5, and the yield of C2-C5 in the shale without bitumen would decrease faster than that in the shale with bitumen... |
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