Advances in shale oil exploration in the Triassic Yanchang Formation, Ordos Basin and their implications

Significant progress has been achieved in the shale oil exploration of the 7th oil group of the Yanchang Formation (also referred to as the Chang 7 oil group) in the Ordos Basin. Specifically, the 1st and 2nd oil sub-groups of the Chang 7 oil group (also referred to as the Chang 7¹ and Chang 7² oil sub-groups, respectively) are primarily characterized by interbedded-type shale oil, which has achieved commercial exploitation. Furthermore, breakthroughs in risk exploration have been made for shale oil in the 3rd oil sub-group of the Chang 7 oil group (also referred to as the Chang 7³ oil sub-group), with probable petroleum initially-in-place of 272 × 10⁶ t having been reported. In this study, we systematically review the recent advances in geological insights into and new technical breakthroughs in shale oil exploration in the Chang 7³ oil sub-group. Based on a summary of exploration implications, we propose future development directions of shale oil exploration in the Chang 7 oil group. The lithofacies, depositional origins, reservoir properties, oil-bearing properties, and hydrocarbon accumulation mechanisms of the Chang 7³ oil sub-group are determined in this study. Specifically, this oil sub-group primarily contains deep-lacustrine basin deposits, which are dominated by shales. These deposits mostly occur as thinly interbedded and lamellar structures vertically and exhibit thick, extensively distributed mudstones, shales, and silty mudstones laterally. Flume experiments and three-dimensional (3D) focused ion beam-scanning electron microscopy (FIB-SEM) reconstructions demonstrate that hyperpycnal flows act as the dominant mechanism governing the formation of sandy fine-grained sediments in the Chang 7³ oil sub-group. The development of these hyperpycnal flows was jointly controlled by frequent flood events and the steep-slope terrain in the southwest. In contrast, shales were formed primarily due to suspension settling and intense hydrodynamic reworking. Organic-inorganic diagenetic coupling identified in the shale sequences of the Chang 7³ oil sub-group, promoted intergranular and dissolution pores in sandstones and siliceous laminae, interlayer fractures, and intercrystalline pores in shales as reservoir spaces. The hydrocarbon accumulation in the Chang 7³ oil sub-group is predominantly governed by retention and micro-migration. All lithologies in this oil sub-group generally bear oil, with average oil saturation exceeding 55% and free hydrocarbons accounting for up to 80% of the total. Accordingly, we propose a hydrocarbon accumulation pattern of the oil-bearing compartment type, characterized by pervasive distribution of multi-type fine-grained sediments, co-existing micro/nano pore-throat systems as reservoir spaces, and favorable oil-bearing properties across the entire shale sequences. Therefore, it is recommended to consider the organic-rich shale-siltstone-tuff assemblages in the Chang 7³ oil sub-group as a whole exploration target. This will help advance systematic, integrated shale oil exploration and exploitation in the study area.