Research on tight oil enrichment patterns under the condition of multi-source hydrocarbon supply is a key component of hydrocarbon enrichment theory. This study examines tight oil reservoirs in the 8th oil group of the Yanchang Formation (also referred to as the Chang 8 oil group) in the Dingbian-Fuxian area, Ordos Basin. By integrating data from drilling, well logging, core observations, thin-section observations, geochemical analysis, well tests, and production tests, we systematically evaluate the hydrocarbon generation potential of source rocks including the Zhangjiatan shales of the Chang 7 oil group, Lijiapan shales of the Chang 9 oil group, and the shales of the Chang 8 oil group. Accordingly, the general characteristics of tight oil reservoirs are clarified, the factors for tight oil enrichment in different oil plays are discussed, and thereby corresponding tight oil enrichment patterns are built. The results indicate that the Dingbian-Fuxian area contains three suites of good to excellent source rocks: the Chang 7, Chang 8, and Chang 9 oil groups. The widespread Zhangjiatan shales at the bottom of the Chang 7 oil group, among others, exhibit the greatest hydrocarbon generation potential, with an average total organic carbon (TOC) content of 6.1%. They are followed by the Lijiapan shales at the top of the Chang 9 oil group and shales in the Chang 8 oil group, which have average TOC contents of 5.3% and 3.9%, respectively and are distributed in the Ganquan-Fuxian-Zhidan area. Reservoirs in the Chang 8 oil group are dominated by fine-grained sandstones and siltstones, with measured peak porosity ranging from 4% to 12% (average: 7.9%) and measured permeability from 0.10 × 10^-3 μm² to 0.40 × 10^-3 μm² (median: 0.26 × 10^-3 μm²), establishing them as tight reservoirs. The tight-sandstone oil reservoirs in the area mostly show lenticular, zonal, and clustered distributions and are characterized by quasi-continuous accumulation. Shale tight oil enrichment in oil reservoirs in the 1st oil sub-group of the Chang 8 oil group (also referred to as the Chang 8¹ oil sub-group) is primarily controlled by the properties of the Chang 7³ oil sub-group, including the hydrocarbon supply capacity of source rocks, the thickness of the thin mudstones between source rocks and reservoirs, and the sealing performance of the strong floor. In the Chang 8¹ oil sub-group, the single-source direct oil play in the Chang 7³ oil sub-group, characterized by the configuration of upper source rocks and lower reservoirs, proves the most favorable, followed by the dual-source oil plays in the Chang 7³ and 8² oil sub-groups, where reservoirs are sandwiched between source rocks. In contrast, the dual-source oil plays in the Chang 7³ and 9¹ oil sub-groups, also characterized by reservoirs sandwiched between source rocks, represents the least favorable oil plays. Shale tight oil enrichment in the Chang 8² oil sub-group largely relies on the properties of the Chang 9¹ oil sub-group, including the hydrocarbon supply of source rocks, high-quality reservoir conditions, and the sealing performance of cap rocks. Within this oil sub-group, the single-source direct oil play in the Chang 9¹ oil sub-group, characterized by the configuration of upper reservoirs and lower source rocks, proves the most favorable. It is followed by the dual-source oil plays in the Chang 9¹ and 8² oil sub-groups, where reservoirs are again sandwiched between source rocks. Three distinct tight oil enrichment patterns are identified in the oil reservoirs of the Chang 8 oil group: (1) efficient enrichment pattern in the Chang 8¹ oil sub-group in the Dingbian-Wuqi area, driven by strong hydrocarbon supply from source rocks and high sealing performance; (2) complementary enrichment pattern in the Chang 8² oil sub-group in the Ganquan-Fuxian area, marked by the combination of excellent source rocks, high-quality reservoirs and cap rocks; and (3) synergistic enrichment pattern in the Chang 8 oil group in the Zhidan area, characterized by multiple hydrocarbon sources and high-quality reservoirs. This study aims to facilitate the efficient development and exploitation of tight oil in the Chang 8 oil group while advancing the tight oil enrichment theory.

The study intends to clarify the undefined dominant factors controlling the tight sandstone gas enrichment and challenges associated with play fairway mapping through the exploration of the 5^th member of the Permian Shiqianfeng Formation (hereafter referred to as the Qian 5 Member) in the Shenmu gas field, Ordos Basin. A combination of seismic, drilling, log, and laboratory data is applied to examine the characteristics of tight-gas reservoirs, analyze the impacts of various factors including faults, reservoirs, cap rocks, barriers, source rocks, and the elevation on tight gas enrichment, and map play fairways for tight gas exploration. The results indicate that the sand bodies of the Qian 5 Member originate from the distributary channels of the braided river delta plain subfacies and that inherited strike-slip faults play a role in improving the reservoir properties and transporting natural gas. The gas reservoir in the member exhibits a porosity mainly ranging from 6.0 % to 12.0 % (average: 9.1 %) and permeability from 0.20×10^-3 to 0.80×10^-3 μm² (median: 0.49×10.0^-3 μm²). The reservoir is of a lenticular tight gas reservoir, exhibiting a quasi-continuous distribution characterized by lateral contiguity and vertical superimposition. Primary factors controlling tight gas enrichment include faults and reservoir physical properties. In contrast, the thicknesses of cap rocks, barriers, and coal seams in source rocks produce relatively weak controlling effects on tight gas enrichment, and the elevation delivers no impact in this regard. In the case where the distance between a gas well and faults exceeds 9 km, the reservoir shale content is greater than 12 %, and the thickness of mudstone barriers exceeds 210 m, the degree of natural gas enrichment increases as the values of these parameters decrease. Conversely, under conditions where the porosity is below 9 %, the permeability falls below 0.5×10^-3 μm², the thickness of mudstone cap rocks is less than 120 m, and the coal seam thickness within source rocks is less than 15 m, the degree of natural gas enrichment improves with increasing values of these parameters. It is proposed to employ the gas enrichment indices to map the play fairways of tight-gas reservoirs. Using this method, a total of 14 Class Ⅰ and 22 Class Ⅱ play fairways primarily distributed in the eastern Shenmu gas field are identified.