Quality of tight sandstone reservoirs in gravity-flow deposits of the deep-lacustrine Yanchang Formation (Ordos Basin, China) as controlled by diagenesis

The deep-lacustrine sandstones of the Triassic Yanchang Formation deposited by sediment gravity flows have an ultralow porosity and permeability. The sedimentary facies are complex and the diagenetic alterations and their controls on the reservoir quality are still poorly understood. They have been studied by thin-section analysis, scanning electron microscopy, back-scattered electron imagery, X-ray diffraction, cathodoluminescence microscopy, and analysis of fluid inclusions. Differences in the intensity of the diagenetic alterations lead to differences in reservoir quality. In particular, compaction reduced the primary porosity. Fine-grained turbidites lost most of their primary porosity. Sandstones with authigenic illite rims resisted compaction to some extent and preserved porosity, but pore-filling illite aggregates reduced the reservoir quality significantly; feldspar dissolution increased it slightly. Intergranular pores and feldspar dissolution pores result in the best reservoir quality. Four diagenetic evolution patterns resulted in good, moderate, or poor reservoir quality. These results help in evaluating reservoir quality and understanding the genesis of tight sandstones.