Characteristics and oil-bearing properties of deep-water gravity-flow deposits in the 7th-9th oil groups of the Yanchang Formation, Zhidan area, Ordos Basin

Based on the centimeter-scale characterization of cores from 15 cored wells, grain size analyses of 117 sampled wells, observations of thin sections from 11 wells, and production test data from 23 wells, we systematically investigate the characteristics of deep-water gravity-flow deposits in the 7th‒9th oil groups of the Triassic Yanchang Formation (also referred to as the Chang 7‒9 oil groups) in the Zhidan area, Ordos Basin, as well as their controlling effects on hydrocarbon distribution. The results indicate that the study area primarily exhibits three types of deep-water gravity-flow deposits: sandy debris-flow deposits, turbidity current deposits, and slide-slump deposits, with the first type predominating. The sandy debris-flow deposits contain thick-bedded massive sandstones, with rip-up clasts and mud-coated intraclasts observed. They are generally well sorted and rounded, and their grain-size cumulative probability curves show short-tailed two-segment or one-segment patterns with a coarse-skewed distribution, exhibiting slightly coarser grains than those observed in the turbiditic sandstones. The turbidity current deposits feature incomplete Bouma sequences, displaying flame structures, sole marks, and small single-layer sand thicknesses. Their grain-size cumulative probability curves primarily show a fine-skewed one-segment pattern. The slide-slump deposits are typically characterized by the presence of convolute beddings, deformation structures, and synsedimentary stepped faults. Analyses of reservoir properties reveal that the sandy debris-flow deposits generally possess better physical properties than the turbidity current deposits. Furthermore, the oil-bearing grades of the sandy debris-flow deposits are dominated by oil stains to oil immersion, resulting in relatively high oil saturation and high daily oil production during production tests. In contrast, the turbidity current deposits exhibit poorer oil-bearing properties, primarily limited to oil stains to oil traces. These findings suggest that the sandy debris-flow deposits are more favorable to hydrocarbon enrichment and production than the turbidite-flow counterparts. Overall, the comprehensive analysis indicates that within the Chang 7‒9 oil groups, Zhidan area, Ordos Basin, the widely distributed sandy debris-flow deposits represent the most favorable oil-bearing sand body type and should therefore be prioritized as significant targets for future hydrocarbon exploration.