| Source rock expulsion is a key factor to determine the formation of industrial oil and gas reservoirs. Recent studies and expulsions have shown great potentials of unconventional oil gas resources such as shale gas and shale oil, which indicates an important role of retained hydrocarbons in source rock. Therefore, evaluating hydrocarbon generation and expulsion are critical for understanding retained hydrocarbon in source rocks. To some extent, source rocks expulsion determines the reservoir formation of conventional(hydrocarbon expulsion) and unconventional(hydrocarbon retention). Pyrolysis such as Rockeval, Micro-Scaled Sealed vessel(MSSV), and gold-tube systems are widely used pyrolysis methods for retrieving kinetics of hydrocarbon generations. Previous studies are tending to use kerogen or powder for pyrolysis. However, these results are from kerogen which can only reflect thermal degradation behavior of organic matters and hydrocarbon process of kerogen with less information regarding of hydrocarbon expulsion. Powder due to inorganic framework is damaged, which represent the mixture of organic and mineral uniformly. Therefore, both of them can not reflect the process of expulsion accurately in actual geological conditions. Till to now, knowledge of retained hydrocarbon and its evolution in source rocks are also studied. Traditional method is evaluating the hydrocarbon generation and expulsion separately. Aiming for better understanding the whole process of hydrocarbon generation, retention, and expulsion. Four subjects including marine shale, lacustrine shale, coal, and coaly shale were chosen for study. Three pyrolysis methods including grain-based microscale sealed vessel(MSSV) pyrolysis, open-system Rockeval pyrolysis, and gold-tube pyrolysis were used to investigate the evolutions of hydrocarbon generation, retention, and expulsion, as well as their variations at different temperatures and maturities. These methods will provide an important experimental theory for effectively evaluating the unconventional potential of source rock. We have achieved the following conclusions:(1) Grain-based MSSV Pyrolysis in closed system: Quantities of extracted oil from residuals and yields of C1 to C5 gases were used to define oil, wet gas, and dry gas windows. Oil windows are defined as 0.6- 1.3 %Ro for shale and 0.5- 1.2 %Ro for coal, wet gas window are 0.9- 3.0%Ro for shale and 0.8- 2.7 %Ro for coal, and dry gas window are 1.3- 4.0 %Ro for shale and 1.2- 4.0 %Ro for coal, respectively. Coal shows relatively wider oil window than shale but similar gas window to shale. The maximum residual oil can reach 133.44 mg/g TOC, 69.84 mg/g TOC for marine and lacustrine shale, 10.03 mg/g TOC for coal, and 83.79 mg/g TOC for coaly shale, respectively. The results show that in oil window, marine and lacustrine shale residual oil show mainly saturates, aromatics, and resins but less asphaltenes while coal residual oil are mainly asphaltenes, aromatics, and resins but less saturates. In wet and dry gas window, marine and lacustrine shale residual oil is mainly made up of saturates, aromatics, and resins while coal residual oil is mainly made up of asphaltenes and resins. These results suggest that residual oil contents of marine shale, lacustrine shale, and coaly shale are higher with high proportions of saturated and aromatic hydrocarbons in low maturities which show high shale oil prospective than coal, while in high maturities the residual oil contents decrease quickly but still have higher potential for cracking gases which might become the source of shale gas. The residual oil in coal is low mainly in forms of aromatics, resins, and asphaltenes, which can only be the source of coal-bed methane as maturity increases.(2) Grain-based Rockeval pyrolysis in open-system: In this study, the natural structure with expulsion factor is represented by grain, while isolated kerogen indicates hydrocarbon generation of organic matters.The expulsion rates, expulsion efficiency, and the amounts of retention of different source rocks dramatically vary with each other at the same natural heating rate. The results suggest that the retention contents of Nenjiang formation is the highest among the four samples; that of the Maoming oil shale, Yanchang formation, and Shahejie formation become progressively lower both in the peak of hydrocarbon generation and after the second cracking. The results indicate that the retention ability of Nenjiang formation from Songliao basin is the strongest but the expulsion ability is the weakest among the four samples; by contrast, the expulsion ability of Shahejie formation from Dongying depression is the strongest but weakest in retention among the four samples.(3) A gold-tube pyrolysis method based on grain in closed system: With universal frequency factor(A=1*1013s-1), it is clear that the distribution of activation energies of expulsed hydrocarbons are narrow than that of retained hydrocarbons, which suggests that when generated hydrocarbon reaches a threshold value, the expulsion of hydrocarbon will occur and finish quickly. Both lacustrine shale from Yanchang formation and marine shale from Pingliang formation show higher energy for expulsed C6-13 showing its higher expulsion threshod than C14+ at the same maturation level. In comparison, lacustrine shale from Yanchang formation show higher activation energy for expulsed C6-13, C14+ hydrocarbons showing its higher expulsion threshold at the same maturation level than marine shale from Pingliang formation. This is coincided with the observation in both filed and laboratory.(4) The kinetic parameters of grain and kerogen of Yanchang formation obtained from Rockeval pyrolysis were used to simulate the hydrocarbon generation, retention, and expulsion in geological condition by combinning with Ordos basin burial history of well Xuntan1 located in Weibei uplift. The results show that both hydrocarbon generation rates and yields from kerogen are much higher than grain. To mid-Cretaceous, due to the uplift in the basin, the hydrocarbon expulsion is stopped and the corresponding expulsion efficiency is 22%. |
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