Migration differentiation of hydrocarbon components in the 7th member of the Triassic Yanchang Formation,central Ordos Basin
The shale sequence of the 7th member of the Yanchang Formation(also referred to as the Chang 7 Member)in the Ordos Basin consists of mudstones,shales,and sandstones.To delve into the migration differentiation of shale oil components in different lithologies,we investigate the migration differentiation of typical hydrocarbon components in the central Ordos Basin.The findings show that the rocks exhibit different contents of the four group components:saturated hydrocarbons(SAT),aromatic hydrocarbons(ARO),polar nitrogen-,sulfur-,and oxygen-containing compounds(NSO),and asphaltenes(Asph).Specifically,the sandstones manifest high SAT content and low NSO+Asph content.Conversely,the shales and mudstones exhibit low SAT content and high NSO+Asph content.Regarding n-alkanes,the sandstones demonstrate high n C11-15 content and low n C23-39 content,while the shales and mudstones display low n C11-15 content and high n C23-39 content.Compared to the sandstones,the shales and mudstones show more significant variations in the Pr/Ph,Pr/n C17,and Ph/n C18 ratios.These rocks manifest low C27/C29 regular sterane ratios and low Ts/Tm and Ts/(Ts+Tm)ratios,while the sandstones show opposite trends.They demonstrate substantial variability in 22S/(22S+22R)ratios for C32 homohopane,in contrast to the relatively concentrated ratios for the sandstones.Besides,the sandstones are characterized by high gammacerane index values varying in a broad range,whereas the shales and mudstones exhibit the opposite trend.Notably,the shales and mudstones generally have 8β(H)-drimane/8β(H)-homodrimane ratios exceeding 1,which,however,are typically less than 1 for the sandstones.A comprehensive analysis indicates that the variations in the distributions of the various hydrocarbon group components within the source rocks can be attributed to both differential adsorption and migration differentiation.
migration differentiationhydrocarbon componentshaleChang 7 Member of Yanchang FormationTriassicOrdos Basin
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