Enrichment factors and patterns of tight oil in the 8th oil group of the Yanchang Formation, Dingbian-Fuxian area, Ordos Basin

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.