Contrasting diagenetic evolution and hydrocarbon charge of tight gas sandstones in the lower Permian Shanxi Formation, southeastern Ordos Basin, China

Abstract

Reservoir quality of, and gas production rates from, tight sandstones in the Lower Permian Shanxi Formation vary greatly across the Yan’an Gas Field, southeastern Ordos Basin, China. Effective reservoirs are significant for the economic success of tight gas exploration and production. This study examined the relationships between diagenetic evolution, porosity reduction and hydrocarbon charge to reveal the formation and distribution of effective reservoirs in this field. The sandstones are very fine- to coarse-grained, quartz arenites, sub-litharenites and litharenites. The significant variations in the texture and mineralogical composition of the sandstones exert major controls on diagenetic heterogeneity and pore distribution. Five petrofacies (petrofacies-A to -E) are classified. The various petrofacies experienced contrasting diagenetic evolution and petroleum charge histories. The medium- to coarse-grained, quartz arenites (petrofacies-A) and sub-litharenites (petrofacies-C) experienced less ductile compaction and cementation and extensive grain dissolution, and remained relatively porous before early hydrocarbon emplacement. When peak gas generation and migration occurred, petrofacies-A and -C still had moderate porosity and were able to be charged. On the contrary, the fine-grained, tuffaceous quartz arenites (petrofacies-B), ductile grain-rich sandstones (petrofacies-D) and tightly carbonate-cemented sandstones (petrofacies-E) experienced limited diagenetic alterations with ductile compaction or carbonate cementation predominating and leading to extensive loss of porosity during eodiagenesis. Petrofacies-B, -D and -E had low porosity before early petroleum charge. Petrofacies-A and -C therefore constitute the only effective reservoir rocks of tight reservoirs of the Shanxi Formation. This study indicates that petrofacies is useful for reservoir characterization and modeling in the Yan’an Gas Field and other tight gas fields worldwide.