Characterization of gravity-flow deposits in the Chang 7–9 oil groups in the Yanchang Formation, Ganquan County, Ordos Basin

This study aims to address the challenges associated with the identification of the sedimentary facies of gravity-flow deposits in the 7th to 9th oil groups of the Yanchang Formation (also referred to as the Chang 7–9 oil groups) in Ganquan County, Ordos Basin. The sedimentary environments and developmental patterns of gravity-flow sand bodies in these oil groups are analyzed to provide a theoretical basis for petroleum exploration in deep-water deposits. Specifically, the type, spatial distribution, and differences in oil-bearing properties of gravity-flow deposits in the study area are determined through centimeter-scale observations of cores from 12 cored wells, grain size data analyses of 97 samples from 91 wells, and microscopic examination of 93 thin sections from 16 wells, as well as single-well log facies analyses, well-correlation section analyses, and the analyses of reservoir physical properties and oil-bearing properties. The results indicate that deep-water gravity-flow deposits are widely distributed across the Chang 7–9 oil groups in Ganquan County. These deposits can be categorized into slide, slump, sandy-debris-flow, and turbidity-current types, among which the latter two are the most prevalent. Sandy-debris-flow deposits dominate in the Chang 7 and 9 oil groups, while both sandy-debris-flow and turbidity-current deposits occur in high proportions in the Chang 8 oil group. Both sandy-debris-flow and turbidity-current deposits exhibit two types of grain-size cumulative probability curves: one-segment curves and short-tailed two-segment curves. Nevertheless, the sandy-debris-flow deposits are slightly coarser, with the coarse ends (i.e., portions corresponding to coarse grain sizes) of grain-size curves typically beginning below 2.5 φ and a mean grain size of 3.10 φ. In contrast, the turbidity-current deposits are relatively finer, with the coarse ends of grain-size curves mostly starting above 2.5 φ and a mean grain size of 3.84 φ. Compared to the turbidity-current deposits, the sandy-debris-flow deposits exhibit more favorable reservoir physical properties and oil-bearing properties. Therefore, sandy-debris-flow deposits should be prioritized as the primary target for tight oil exploration in the lower hydrocarbon play of the Yanchang Formation in the study area, followed by turbidity-current deposits.