A Comparison of Various Diagenetic Controls by Zeolites on Reservoir Quality: Permian-Triassic Conglomerates, Junggar Basin, Northwestern China

Zeolites are widely distributed in the Permian-Triassic conglomerates of the Junggar Basin and play an important role in reservoir quality. A better understanding of the matching relations between reservoir qualities and zeolitic diagenesis is of great importance for oil exploration in basins. In this study, Permian-Triassic conglomerates from southeast Junggar (SEJ), middle Junggar (MJ), and northwest Junggar (NWJ) were investigated, using casting thin section observations, SEM analysis, EPMA analysis, porosity, and permeability measurements. The Permian-Triassic conglomerates of all three areas have abundant volcanic lithics. The conglomerates from NWJ and MJ contain higher contents of sedimentary lithics and feldspars than those from SEJ. Zeolites of SEJ and NWJ originate from the alteration of volcanic lithics, occurring as pore filling in stage B of eodiagenesis. The zeolitic diagenetic process of SEJ is volcanic lithic-clinoptilolite-analcime-heulandite-laumontite, while that of NWJ is volcanic lithic-analcime. Zeolites of MJ originate from albitization of plagioclase, occurring as pore filling and replacement in stage A of mesodiagenesis. Its diagenetic process is a replacement of plagioclase by laumontite. Diverse zeolitic diagenesis played different roles in reservoir quality. Zeolite cementation and compaction both destroyed the reservoir qualities of SEJ and NWJ. In MJ, the reservoir quality was mainly destroyed by compaction, resulting in strong heterogeneity. In later diagenesis, the dissolution of zeolite cements improved reservoir qualities. SEJ is located farther from hydrocarbon generation sags than NWJ and MJ. As a result, zeolite dissolution pores are poorly developed in SEJ. Previous research has revealed that analcime dissolves more easily than laumontite. Zeolite dissolution pores developed better in NWJ than in MJ. In comparison of the conglomerates with zeolite dissolution pores in the Junggar Basin, NWJ conglomerates have the best reservoir quality, indicating that zeolitic diagenesis can provide implications for oil exploration in the Junggar Basin.