Controls of diagenetic alteration on the reservoir quality of tight sandstone reservoirs in the Triassic Yanchang formation of the Ordos Basin, China

Abstract

The Ordos Basin is rich in tight oil resources, and the Upper Triassic Yanchang Formation tight oil reservoir is one of the most important exploration targets. In the study area, the Chang 63 tight sandstone reservoir exhibits lacustrine deep-water lobe-shaped delta deposits with low porosity, low permeability and a complex diagenetic alteration history. The reservoir quality is extremely heterogeneous because of the heterogeneity of the depositional lithofacies and diagenetic alterations. This research integrates multiple techniques to investigate the depositional lithofacies, the characteristics and distributions of various diagenetic alterations, and their controls on reservoir quality. Four lithofacies are identified, and the various lithofacies have different diagenetic characteristics and diverse reservoir qualities. The Chang 63 tight sandstone reservoir has experienced significant diagenetic alteration. Mechanical compaction and authigenic illite are the most important causes of the decrease in reservoir quality. Carbonate cementation preferentially occurs in sandstones of lithofacies “i“ and “ii” near the mudstone-sandstone contact surfaces. Sandstones within 1 m of adjacent mudstone-sandstone contact surfaces are usually tightly cemented by carbonate cement. Illite can be divided into two types according to its morphology. Both types of illite lead to a decrease in reservoir quality. Ultimately, by combining the relationship between depositional facies and lithofacies with the distribution of different diagenetic processes, five evolutionary patterns are identified for reservoir quality in the lacustrine deep-water lobe-shaped delta deposits can be summarized by ratings of best, moderate, relatively poor, poor and worst.