Diagenetic Controls on Tight Sandstone Reservoir Quality: A Case Study from the Huaqing Oilfield, Ordos Basin

Tight sandstone oil reservoirs in the Yanchang Formation of the Ordos Basin have great resource potential. Reservoir quality plays a crucial role in the aggregation and development of tight sandstone oil. The quality of tight sandstone reservoirs is indirectly controlled by diagenesis, which is directly manifested in the differences in pore structure characteristics. At present, there are many studies on the relationship between reservoir quality and diagenesis in the tight sandstone reservoirs of the Yanchang Formation, but there is still no unified method for classifying the diagenetic facies types and characterizing the full-scale pore structure. In this study, based on the large field of view cast thin section analysis and X-ray diffraction (XRD) technology, the clustering analysis method was utilized to classify the diagenetic facie types. Combining low-temperature nitrogen adsorption (LTNA), high-pressure mercury intrusion (HPMI), and constant-velocity mercury intrusion (CMI), a new method based on a weighted approach to characterize the full pore size of tight sandstone reservoirs was proposed. Four types of diagenetic facies are mainly developed in the study area: felsic weakly compacted diagenetic facies (A), dissolution diagenetic facies (B), carbonate cemented diagenetic facies (C), and tightly compacted diagenetic facies (D). The full pore size distribution curve of diagenetic facies B shows a multipeak distribution, in which the peak with the largest value is located from 0.4 to 0.8 μm. The full pore size distribution curves of diagenetic facies A, C, and D all show a single-peak pattern, with peak positions corresponding to decreasing pore diameters from 0.1 to 0.2 μm, 0.07 to 0.1 μm, and 0.007 to 0.13 μm, respectively. The presence of brittle minerals, such as quartz, and the occurrence of dissolution favor reservoir quality. Cementation destroys reservoir storage space, but this occurs when the cement content exceeds a certain threshold. Micropores and mesopores are more clearly related to diagenesis than nanopores. Diagenesis affects reservoir quality by controlling the development of micropores and mesopores. Finally, the correlation between diagenetic facies and the wireline logs was established, and it is clear that the development of diagenetic facies A and diagenetic facies B is associated with better reservoir quality. The study provides insights into how the diagenetic facies of tight sandstone reservoirs are correlated with reservoir quality and pore structures. It has reference significance for sweet spot evaluation in tight sandstone reservoirs.