The influence of diagenetic heterogeneity on tight-reservoir properties in the Upper Triassic Yanchang Formation, Southeastern Ordos Basin, China

The Chang 7–8 sections of Yanchang Formation in the Ordos Basin are dominated by lacustrine tight sandstones but characterized by high hydrocarbon production. This setting provided an opportunity to study reservoir characteristics and diagenetic processes of tight sandstones, to facilitate future oil exploration. In this study, it was found that reservoir qualities were mainly controlled by the diagenetic heterogeneity at different parts of sandstones, based on a comprehensive analysis of thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), pressure-controlled mercury injection, X-ray computed tomography (XCT), and well log data. Carbonate cements tend to occur at the edges of sandstone bodies. Sandstone layers with a distance less than 0.5 m from the sandstone–mudstone interface, or with a thickness of less than 1.5 m, were rich in carbonate cements (carbonate content was more than 15%). Lower porosity (less than 4%) and permeability (less than 0.01 mD) trend to distribute in sandstones that have higher content of carbonate cements. The source of carbonate cements was most likely from adjacent mudstones or from the dissolution of unstable grains. Samples collected from the center of sandstone layers have higher porosity, pore connectivity, and permeability but with low carbonate cement content. Hydrocarbon accumulation was facilitated through vertical fractures between the layers. The dissolution of feldspar and lithic fragments by organic acid associated with oil charging improved reservoir properties in the center of sandstone layers, and generated material for carbonate cement precipitation at the edge of sandstone layers. The results of this study demonstrated that the dominance of different diagenetic processes led to a reduction of reservoir quality at the edge of sandstone layers but an increase in the center of sandstone layers. This research offered a new perspective on the study of reservoir properties and the diagenesis mechanism of tight sandstones, which could be conducive to tight oil exploration and development.