Feldspar Extinction of Tight Gas Reservoir of the He8 Member in Central Part of Ordos Basin and Its Genesis, China

Analyses of casting slices, cathodoluminescence and SEM present that the He8 member tight gas reservoirs existed numerous feldspars before diagenesis in central part of Ordos Basin. However, today the reservoirs mainly consist... [ view full abstract ]

Analyses of casting slices, cathodoluminescence and SEM present that the He8 member tight gas reservoirs existed numerous feldspars before diagenesis in central part of Ordos Basin. However, today the reservoirs mainly consist of quartz sandstone and lithic quartz sandstone with low feldspar and high compositional maturity. The phenomenon is obviously different from the periphery reservoirs of basin with a relatively high content of feldspar. Based on reservoir characteristics, combined with acidolysis simulating experiment, burial history, thermal history and diagenesis characteristics, the reasons of feldspar extinction of He8 member sandstone reservoirs in central part of Ordos Basin were studied. The results show that feldspar extinction is closely related to the dissolution, and its corrosion occured under high temperature conditions. Of interest, abnormal thermal event has prompted the hydrocarbon generation of coal measures source rocks of the Upper Palaeozoic and released a lot of organic acid and CO2, which led to the dissolution of feldspar. The central part of Ordos Basin where tight gas reservoirs contain almost no feldspar content, the corresponding Ro value is relatively higher. The occurrence of feldspar mainly consists of three types, including nearly complete dissolution, carbonate filling and kaolinization. Feldspar extinct event plays a constructive role in the formation of secondary porosity, and it is of great significance for both He8 member and the underlying coal strata to predict favorable zone of tight sandstone gas reservoir in Ordos Basin.