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

摘要

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 fades 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.