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.

The classification and evaluation of low-permeability tight reservoirs remain a pressing challenge in hydrocarbon exploration and exploitation. Traditional approaches are either tailored to conventional reservoirs or oversimplify the categorization of low-permeability tight reservoirs, limiting their practical applicability. Although some recently developed evaluation methods have shown promising results, their broad adoption is hindered by difficulties in parameter acquisition and high implementation costs. In this study, we examine the general reservoir characteristics in the study area using casting thin section observations, scanning electron microscopy (SEM), and high-pressure mercury injection (HPMI). Based on these results, combined with extensive data processing and multiple clustering algorithms, we propose a K-means clustering-based classification and evaluation method for low-permeability tight reservoirs. Furthermore, we mathematically define the classification boundaries using porosity-permeability cross plots. The proposed classification and evaluation system offers several advantages. By adopting an algorithm-driven approach, it overcomes the limitations of traditional experience-based criteria, thus providing more scientifically robust classification results. Although the proposed method integrates eight key parameters that capture reservoir physical properties and pore structure characteristics (i.e., porosity, permeability, sorting coefficient, median pressure, median pore radius, displacement pressure, maximum mercury saturation, and mercury withdrawal efficiency), the final classification and evaluation results rely solely on porosity and permeability, which are both the most indicative of reservoir quality and the most accessible. Therefore, this method addresses the limitations of traditional ones, including difficulty in acquiring evaluation parameters and challenges associated with widespread application. Employing mathematically defined classification boundaries, it avoids the oversimplified "one-size-fits-all" cut-offs inherent to traditional classifications. The method has been applied to the classification and play fairway prediction of low-permeability tight reservoirs in the Chang 7-9 oil groups in the lower Yanchang Formation, Dingbian-Fuxian area, Ordos Basin, providing a reliable basis for sweet spot evaluation in this area.

In the Fuxian area of the Ordos Basin, the 7th‒9th oil groups in the Triassic Yanchang Formation (also referred to as the Chang 7‒9 oil groups) hold great petroleum resource potential. Previous studies mostly hold that sandstones in these oil groups are dominated by deltaic deposits. Although gravity-flow deposits have been discovered in the Chang 7 oil group in recent years, some issues remain unclear, including their existence and distribution range across the Chang 7‒9 oil groups. Using core observations, grain size analysis, and thin section observations, we identify the primary types of gravity-flow deposits and their distributions, together with the comparison of reservoir characteristics and oil-bearing properties within. The results indicate that gravity-flow deposits are widely distributed across the Chang 7‒9 oil groups in the Fuxian area, including sandy debris-flow deposits, turbidite-flow deposits, and slide-slump deposits. The sandy debris-flow deposits, among others, feature great thicknesses and massive beddings, with rip-up clasts visible. Their grain-size cumulative probability curves exhibit three primary patterns: a coarse-skewed single-segment pattern, a short-tailed two-segment pattern, and a two-segment pattern with high suspended component content. In contrast, the turbidite-flow deposits are thin, with incomplete Bouma sequences observable in cores. Their grain-size cumulative probability curves primarily show a fine-skewed single-segment pattern. In contrast, the slide-slump deposits are characterized by wrinkling deformation, convolute beddings, and stepped faults. The comparison reveals that the sandy debris-flow deposits are generally superior to the turbidite-flow deposits, exhibiting high reservoir quality, favorable oil-bearing properties, and high daily oil production from well tests. Among the widespread gravity-flow deposits in the Chang 7‒9 oil groups in the Fuxian area, the sandy debris-flow deposits show the most extensive distribution, as well as the most favorable reservoir physical properties and oil-bearing properties. Therefore, they should be prioritized as significant targets for future exploration and development of tight oil in the Fuxian area.

Sandstones with high natural gamma-ray (GR) values (also referred to as high-gamma sandstones) occur in the 6th to 9th oil groups of the Yanchang Formation (collectively referred to as the Chang 6‒9 oil groups) in the Wuqi area, Ordos Basin. By conventional log interpretations, these sandstones are often mistakenly identified as siltstones or even mudstones, leading to an underestimation of the effective reservoir thickness. In this study, we conduct a fine-scale lithological division by combining core observations with grain size data. Accordingly, the logging responses, petrological characteristics, physical properties, and oil-bearing properties of high-gamma sandstones in the study area are summarized, followed by an analysis and exploration of their origin. The results indicate that the high-gamma sandstones exhibit significant conventional logging responses, including high GR values, elevated sonic interval transit time, and pronounced negative spontaneous potential (SP) anomalies. In the spectral GR logs, these sandstones show the characteristics of high uranium (U) and thorium (Th) concentrations, along with low potassium (K) concentration. Compared to common sandstones, the high-gamma sandstones generally contain higher contents of feldspar, mica, and clay minerals. Despite comparable porosity, the high-gamma sandstones show slightly lower permeability and oil saturation than common sandstones. Furthermore, their oil-bearing properties tend to decrease with an increase in the concentrations of radioactive elements such as U, Th, and K. The primary factors controlling the formation of the high-gamma sandstones include volcanic activity, sedimentary environment, and clay mineral type. The debris produced by volcanic eruptions supplies abundant radioactive materials for the study area. The northeastern and southwestern parts of the study area show differences in the migration, accumulation, and preservation conditions of radioactive elements, especially U and Th, which exhibit distinct degrees of enrichment under different sedimentary environments. Clay minerals exhibit varying adsorption capacities for radioactive elements. Specifically, illite and mixed-layered illite-montmorillonite demonstrate higher adsorption capacities, whereas kaolinite and chlorite exhibit limited adsorption capacities. Fine-scale investigations of the lithological and developmental characteristics of high-gamma sandstones in the Wuqi area hold great practical significance for increasing hydrocarbon reserves and enhancing productivity in the Ordos Basin, while also providing a key geological basis and theoretical guidance for the exploration and exploitation of similar reservoirs in other basins of China.

Shales are well-developed in the 8th to 9th oil groups of the Triassic Yanchang Formation (also referred to as the Chang 8‒9 oil groups) in the Ordos Basin. However, their mineral composition and pore and microfracture characteristics, as well as the impacts of these factors on shale oil enrichment and mobility, remain insufficiently understood. In this study, we comprehensively investigate the shale reservoir characteristics and their controlling effects on shale oil in the Chang 8‒9 oil groups using data from a series of analyses and tests, including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), CO₂ adsorption, low-temperature N₂ adsorption, high-pressure mercury injection (HPMI), and geochemical analyses. The results indicate that the shales in the two oil groups exhibit three lithofacies: felsic clayey shales, clayey felsic shales, and clayey-felsic mixed shales. The Chang 8 oil group is dominated by felsic clayey shales and clayey felsic shales, while all the three lithofacies types are relatively well-developed in the Chang 9 oil group. A comprehensive assessment of pore and microfracture characteristics, fracability, and shale oil mobility reveals that clayey felsic shales are the most favorable lithofacies, followed sequentially by clayey-felsic mixed shales and felsic clayey shales. The shale reservoirs in the two oil groups contain organic pores, intercrystalline pores, intergranular pores, dissolution pores, and microfractures. There are distinct positive correlations between total organic carbon (TOC) content and the volumes of micropores and mesopores. Compared to the Chang 7 oil group, organic pores are less developed in the Chang 8‒9 oil groups. Microfractures are well-developed in both oil groups, primarily including bedding-parallel fractures induced by abnormally high-pressure from hydrocarbon generation, followed by pressure dissolution-induced bedding-parallel fractures and shrinkage fractures in organic matter. Key factors contributing to the development of pores and microfractures in the study area include high TOC content, high organic matter maturity, and high brittle mineral content. Specifically, high TOC content and high organic matter maturity promote the development of organic pores and the formation of bedding-parallel fractures induced by abnormally high-pressure from hydrocarbon generation. In contrast, carbonate cementation inhibits pore development and fills fractures with cements. A high clay mineral content favors micropore development but adversely affects mesopore and macropore growth. Primary factors controlling movable oil enrichment include TOC content (< 4%), organic matter maturity (vitrinite reflectance (R_o) > 1.2%), brittle mineral content (> 60%), and fracture density (> 1.6 × 10⁴/m). As the assessment criteria for shale oil sweet spots in the study area shown, type Ⅰ sweet spots should have oil saturation index (OSI) values of greater than 100 mg/g, possess the highest shale oil mobility and are primarily distributed in the Fuxian, Ganquan, and eastern Zhidan areas. These areas represent the most favorable shale oil enrichment regions, holding promising exploration prospects. In contrast, type Ⅱ sweet spots are characterized by OSI values ranging from 70 mg/g to 100 mg/g, and exhibit a moderate mobility, while type Ⅲ sweet spots, with OSI values below 70 mg/g, show the lowest mobility. The type Ⅲ shale oil predominates across the study area.

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.

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.