Geological conditions for tight oil enrichment and its exploration potential of gravity-flow deposits from the 7th to 9th oil groups of the Triassic Yanchang Formation, Ordos Basin: Discussion on the petroleum exploration paradigms of downwarped lacustrine basins

The lower oil play, namely the 7th to 10th oil groups, of the Yanchang Formation in the Dingbian-Wuqi-Zhidan-Ganquan-Fuxian area of northern Shaanxi within the Ordos Basin, has emerged as a key target for the exploration of the Yanchang oilfield in recent years. In this study, we investigate this oil play by integrating observations of approximately 2115 meters of cores from 125 wells, grain-size data from more than 400 wells, and a systematic analysis of logging facies. The results indicate that gravity-flow deposits are widely distributed across the 7th to 9th oil groups of the Yanchang Formation (also referred to as the Chang 7‒9 oil groups) in the study area. These deposits are dominated by sandy debris-flow deposits, followed by turbidity-current deposits. The gravity-flow deposits exhibit excellent source rock-reservoir-cap rock conditions and hold great potential for tight oil exploration, with possible petroleum initially-in-place (PIIP) estimated at about 2.5 × 10⁹ t. The type of source rock-reservoir-seal assemblages is identified as a major factor controlling the accumulation and enrichment of tight oil in the Chang 7‒9 oil groups in the study area, and theses assemblages therein can be classified into four categories with a total of 11 specific types. The configurations featuring reservoir interbedded with source rocks and vertically-stacked source and reservoir, among others, represent two most favorable assemblage categories for tight oil accumulation. Moreover, new evaluation criteria for source rocks of the Yanchang Formation in the Ordos Basin are established. These criteria highlight the predominance of excellent source rocks across the Chang 7‒9 oil groups in the study area. Within this interval, the Chang 7 oil group is recognized as containing the most favorable source rocks, followed by the Chang 9 and Chang 8 oil groups. Furthermore, a new sweet spot assessment method is proposed based on the grading of well production, and the analysis of enrichment factor (EF) and associated geological controlling factors. China's hydrocarbon exploration paradigms are undergoing significant shifts, with a shift from shallow-water deposits to deep-water gravity-flow deposits representing a major trend in the hydrocarbon exploration of downwarped lacustrine basins. Specifically, gravity-flow deposits exhibit favorable hydrocarbon accumulation conditions in downwarped lacustrine basins. Particularly, sandy debris-flow deposits exhibit a larger scale, favorable physical properties, and better oil-bearing properties compared to the turbidity-current deposits, establishing them as a major target for petroleum exploration and exploitation of deep-water gravity-flow deposits.