Clastic rock diagenesis and its influence on reservoirs: A case study of Zhenjing area in the Ordos Basin

The diagenetic types and diagenetic sequences of the Yanchang Formation in Zhenjing area of the Ordos Basin were studied based on core observation, casting thin section, scanning electron microscopy and catho-doluminescence. (1) Dissolution pores were not only from feldspar dissolution, but also the secondary dissolution of the feldspar alteration to form zeolite. Microfractures are siliceous microcracks caused by the strong compaction of quartz. The formation of intracrystalline pores and intergranular pores was closely related to the inhibition of compaction by dolomite formed early in diagenesis. (2) Due to the change of diagenetic environment, the early formed chlorite membranes released a large number of iron and magnesium ions, forming leaf-like chlorites during hydrolytic dissolution, which blocked pores. (3) The evolution characteristics of the diagenetic sequence in Zhenjing area reveal that the early carbonate cement has a high degree of self-formation. Chlorite membranes are the products of initial diagenesis, and leaf-like chlorites generated during the secondary diagenesis. Zeolites replaced plagioclases during the early phase, and then, the dissolution of zeolites provided better reservoir space. Feldspar dissolution yielded page-like kaolinites.