The forming mechanism and process of tight oil sand reservoirs: A case study of Chang 8 oil layers of the Upper Triassic Yanchang Formation in the western Jiyuan area of the Ordos Basin, China

Upper Triassic Yanchang Formation in the Ordos Basin, northwest China, comprise the typical tight oil reservoirs, which have porosity less than 10% and permeability less than 1 mD. The western Jiyuan area (WJA), at the western margin of th e Ordos Basin, has made significant progress in the tight oil exploration of the Yanchang Formation recently. Whereas the lack of recognition of tightening mechanism and process of reservoirs resulting in the misunderstanding of tight oil reservoirs prediction. This study focuses on the petrology, diagenetic processes, as well as forming mechanism and process of the Chang 8 (Ch-8) tight oil reservoirs located in the WJA. Four conclusions were reached. First, Ch-8 tight reservoirs, which are mainly composed of lithic arkose and feldspathic litharenite, experienced diagenesis, such as mechanical and chemical compaction, cementation, and dissolution. The reservoirs are at the late period of the mesogenetic A stage. Second, the tightness of the Ch-8 reservoirs is due to both sedimentary and diagenetic factors. The sedimentary environment where sand bodies are close to the provenance that has a rapidly increasing lacustrine level provides conditions for reservoir tightness. The initial porosity rapidly decreases due to compaction and cementation which produce porosity loss rates of 49.42% and 40.87%, respectively. By contrast, the effect of dissolution is limited, as it only increases porosity by 4.48%. Third, the tightening process of the Ch-8 reservoirs comprise four stages: compaction causes pores to reduce rapidly; followed by early cementation, which causes pores to reduce; dissolution, which causes pores to increase slightly; and late cementation, which causes tightness. Finally, it is likely that the Ch-8 reservoirs were already tight prior to hydrocarbon accumulation, thus demonstrating that they represent typical tight oil reservoirs.