17α(H)-diahopane-based identification of tight oil charging models in the Yanchang Formation, Ordos Basin

The origin of tight oil in the 8th oil group of the Triassic Yanchang Formation (also referred to as the Chang 8 oil group) in the Ordos Basin remains controversial in previous studies. By integrating a range of experimental and analytical methods, including the total organic carbon (TOC) content measurement, Rock-Eval pyrolysis, biomarker identification, confocal laser scanning microscopy (CLSM), and nuclear magnetic resonance (NMR), we thoroughly analyze the distribution characteristics of 17α(H)-diahopanes in 33 source rock samples and 12 crude oil samples from the eastern Ordos Basin. Accordingly, the tight oil charging models in the Chang 8 oil group are explored. The results indicate that the source rock and crude oil samples all contained 17α(H)-diahopanes, which are highly indicative of oil-source correlation. The distribution characteristics of 17α(H)-diahopanes reveal that in well Y426, crude oil in the 1st sub-group of the Chang 8 oil group (the Chang 8¹ oil sub-group) might originate primarily from the black shales of the 3rd sub-group of the 7th oil group of the Yanchang Formation (the Chang 7³ oil sub-group), while that in the 2nd sub-group of the Chang 8 oil group (the Chang 8² oil sub-group) was likely derived from the black shales of the 1st sub-group of the 9th oil group of the Yanchang Formation (the Chang 9¹ oil sub-group). In contrast, in well Y1011, crude oil in the Chang 8¹ oil sub-group might be sourced primarily from the dark mudstones of the same oil sub-group, while that in the Chang 8² oil sub-group likely originated from the dark mudstones within itself. In well Y426, the Chang 8¹ and Chang 8² oil sub-groups consist primarily of interbedded thickly layered sandstones and thinly layered mudstones, featuring favorable physical properties. In this well, crude oil from the black shales of the Chang 7³ and Chang 9¹ oil sub-groups can migrate into the delta-front sand bodies of the Chang 8¹ and Chang 8² oil sub-groups, respectively, through high-permeability sand bodies. In well Y1011, the Chang 8¹ and Chang 8² oil sub-groups are dominated by interbeds comprising thickly layered mudstones and thinly layered sandstones, suggesting poor reservoir physical properties. In this case, it is difficult for crude oil generated by the black shales of the Chang 7³ and Chang 9¹ oil sub-groups to migrate into reservoirs of the Chang 8¹ and Chang 8² oil sub-groups. Additionally, stratigraphic heterogeneity exerts a significant impact on oil-source correlation results, underscoring the need to comprehensively identify the oil sources of the Chang 8 oil group by combining geological conditions.