Differential Diagenesis and Hydrocarbon Charge of the Tight-Sandstone Reservoir: A Case Study from Low-Permeable Sandstone Reservoirs in the Ninth Member of the Upper Triassic Yanchang Formation, Ordos Basin, China

Studies of hydrocarbon migration and enhanced oil recovery focus on the effects of reservoir heterogeneity on subsurface fluid flow and distribution. Differential diagenesis in clastic rock reservoirs is an important factor of internal-reservoir heterogeneity and its relationship to hydrocarbon charges is a key scientific issue for understanding hydrocarbon accumulation mechanisms in tight-sandstone reservoirs. This paper focuses on the ninth member of the Upper Triassic Yanchang Formation (Chang 9), located in the central and western Ordos Basin, China. The aims of the paper are to examine the differential diagenesis of sandstone reservoirs and to illustrate the process of organic/inorganic fluid–rock interaction using an integrated method of petrography, UV fluorescence spectra, fluid inclusion, and basin modeling analyses. The Chang 9 reservoir comprises four sandstone types: mechanically compacted sandstone, calcite-cemented sandstone, water-bearing sandstone, and oil-bearing sandstone. These four types of sandstone experience contrasting diagenetic evolutions. During early diagenesis, mechanically compacted sandstone and calcite-cemented sandstone undergo strong deformation and cementation, respectively. The water-bearing and oil-bearing sandstones experience similar diagenetic evolutions, but significantly different from those two tight sandstones in fluid activity and diagenesis magnitude. Three types of porous bitumen were identified in the oil-bearing sandstone, whereas no bitumen was identified in the water-bearing sandstone. According to the contact relationship between bitumen, cements, and dissolution pores, the related diagenesis sequence of the oil-bearing sandstones of Chang 9 was reconstructed. Three phases of fluid flow occurred in turn, with hydrocarbon charging in the process, but no hydrocarbon charging occurred in the water-bearing sandstones. The research findings, in terms of organic and/or inorganic fluid–rock interaction, can be used as a reference for the differential diagenesis and process of fluid–rock interaction in low-permeability sandstone reservoirs with a highly heterogeneous internal reservoir framework. Furthermore, this study could help in understanding the internal heterogeneity characteristics of a fluvial sandstone reservoir and its relationship with hydrocarbon charging.