Diagenetic controls on the reservoir quality of tight reservoirs in digitate shallow-water lacustrine delta deposits: An example from the Triassic Yanchang Formation, southwestern Ordos Basin, China

Diagenesis is a very important factor to determine the quality of clastic reservoirs, especially in tight sandstone reservoirs. The tight reservoir of Triassic Yanchang Formation in southwestern Ordos Basin is the foremost hydrocarbon exploration target. The shallow-water lacustrine delta developed in Chang 8 reservoir of the study area is distributed in digitate shape, which are mainly composed of distributary channel, continuous mouth bar and together with natural levee above. The typical tight reservoir of Chang 8 member experienced a complex diagenetic evolution process, but the relationship among diagenetic alteration, deposits and lithofacies, as well as their control and influence on reservoir quality is still unclear. In this study, casting thin section, scanning electron microscope (SEM) and backscatter scanning electron microscope (BSE) combined with energy spectrum analysis, together with stable isotope analysis, X-ray diffraction analysis, and homogenization temperature test of fluid inclusions are used to analyze the reservoir characteristics including petrology, lithofacies, history and intensity of diagenesis. The control and influence of these characteristics on ultimate reservoir quality of digitate shallow-water lacustrine deltas are further evaluated. The results show that four lithofacies can be identified (namely, "i", "ii", "iii", "iv") with the decrease of grain size and increase of ductile components in the tight sandstones of digitate shallow-water lacustrine delta, respectively corresponding to preferential development positions in the deposits. There are similarities and differences on the diagenetic processes among various lithofacies. The strong mechanical compaction after burial primarily decrease the original pore space in all lithofacies, and the largest loss of pore space occurs in lithofacies iv sandstones (siltstone with wavy bedding). The chlorite is mainly in the morphology of pore-lining, but coating and rosette-like chlorite are rare. The ability to resist compaction in sandstones of lithofacies i and lithofacies ii is enhanced due to the moderate development of pore-lining chlorite, thus retaining partial pore space. Carbonate cements are abundant in relatively coarser-grained sandstones close to the sandstone-mudstone interface, and the occurrence of these cements is closely related to the adjacent mudstone, indicating that Fe and Mg ions necessary for carbonate cementation are most likely from adjacent mudstone where ions are released by mechanical compaction and subsequent hydrocarbon generation pressurization during the burial process. Weak carbonate cementation occasionally occurs at the architectural bounding surface far away from mudstone, which probably related to the transformation of clay minerals. Five typical diagenetic evolution patterns in digitate shallow-water lacustrine deltaic reservoir are summarized by considering the diagenetic processes with deposits and lithofacies. From pattern I to pattern V, the corresponding reservoir quality gradually deteriorates. The results in this study are of great help to deepen the understanding of diagenetic alteration process and reservoir heterogeneity of tight sandstones, and provide theoretical basis for exploration and sustainable development of similar lacustrine shallow-water lacustrine delta reservoirs, especially for bar fingers in shallow-water deltas.