Geostatistic recognition of genetically distinct shale facies in upper Triassic chang 7 section, the Ordos Basin, North China

Abstract

Abstract The Ordos basin in North China may be one of the nonmarine basins with highest hydrocarbon generation per volume of sedimentary fill. It has great potential for unconventional oil and gas exploration, especially shale oil. When evaluating and predicting shale oil resources, the difference between different regions and different layers of shale is very obvious, so it cannot be calculated simply by average. A geostatistic decision tree was constructed based on a training set of 22 parameters (geochemistry, petrology, and sedimentology) for 110 shale samples from the basin, which can be used to implement genetic classification of subsequently collected shale samples. Three shale facies are identified by the decision tree. The properties of each facies are different, such as TOC, kerogen type and sulfur content. Genetic properties indicate formation conditions and provide base for the geostatistic assignments. The differential distribution of shale facies in the basin reflects their origins from different formation mechanism. Elements ratios, isotopes and biomarkers can be used to infer the sedimentary environment for each shale facies. Facies 1 mainly occurs in the upper Chang 7 section and shows evidence of quartz-rich source rock deposited under suboxic–anoxic condition. Facies 3 occurs at the lower Chang 7 section and shows evidence of clay-rich and pyrite-rich shale deposited under reducing dysoxic conditions with numerous higher plants input. Facies 1 and 2 constitutes the upper Chang 7 with interactive thick layers; Facies 2 and 3 constitute the lower Chang 7 with interactive thin layers. Petrology, geochemistry and sedimentology of the shale facies suggest: Facies 1 represents the suboxic–anoxic upper Chang 7 proximal quartz-rich shale, which has lower TOC and sulfur content than other facies. The organic lamina is straight but rare. Kerogen is type I and dominated by sapropel. The content of saturated hydrocarbon in chloroform asphalt “A” extract is the highest. Facies 2 has higher TOC value, sulfur content and soluble hydrocarbon content than facies 1. It was formed in unstable hydrodynamic environment under weak anoxic condition. It is characterized by abundant laminas with various deformation structures and more alginite and vitrinite in kerogen. Anoxic lower Chang 7 distal sulfur-rich shale source rock make up organic-rich facies 3 (∼7.0–17.0 wt % TOC). This facies received more higher-plant input than the other facies. Well preserved algae shells are common in numerous and flat laminas. The content of chloroform asphalt of facies 3 is higher than that of other facies, but among which the saturated hydrocarbon content is the lowest, the aromatic, non-hydrocarbon and asphalt are the highest, suggesting the fluidity of hydrocarbon is the worst. Compared to others, this facies was formed in the environment where temperature is relative lower, water is relative deeper, productivity and salinity are relative higher, and the reducibility is relative stronger. A deeper understanding of shale classification and distribution is important for the accurate resource evaluation and optimal exploration planning of shale oil.