In 2017, Well XWD1 drilled in the Wensu Salient, northwest Tarim Basin achieved the first hydrocarbon discovery in Proterozoic metamorphic rocks within the western basin of China. However, uncertainties regarding reservoir characteristics, hydrocarbon origins, and accumulation mechanisms have constrained further evaluation and exploration planning. Based on various hydrocarbon exploration data and experimental analyses from the region, this study conducts a comprehensive analysis of regional tectonic evolution, reservoir characterization, and hydrocarbon source correlation. Furthermore, it establishes a hydrocarbon accumulation model for metamorphic rock reservoirs and assesses their resource potential. It is found that: 1) the metamorphic strata have suffered extensive uplifts and denudations since the Paleozoic till the late Neogene, and extensively reformed by a northeast-trending boundary fault system, thus resulting fractured weathering crust reservoirs beneath the regional unconformity; 2) the metamorphic reservoirs exhibit ultra-low porosities typically around 4%, and low permeabilities ranging from 0.061 mD to 1.5 mD; 3) the discovered crude oil is characterized of typical terrestrial genesis associated with the northeastern Baicheng Sag, and was accumulated at approximately 17 Ma and 5 Ma; 4) the collected gas featured a mixed origin, which might derive from overmature marine source rocks in the Awati Sag, the marine oil cracking, and terrestrial source rocks in the northeastern Baicheng Sag, respectively; 5) the most promising metamorphic traps cover an area of 386.7 km², with a preliminary resource estimate of up to 57 million tons, implying significant exploration potential.

Low-temperature nitrogen adsorption can form a “hysteresis loop”, whose geometry and area can effectively reflect the pore structure of porous media and its retention effect on adsorbed gas. However, the role of the “hysteresis loop” in quantitative characterization of shale pore structure has often been ignored. The study aims to clarify whether shale can form “hysteresis loop” in low-temperature nitrogen adsorption-desorption experiment and the determinants on hysteresis loop’s area with experiments on the 7^th member of the Upper Triassic Yanchang Formation shale (Chang 7 shale) in the Ordos Basin. Various measures are applied in the study, including qualitative observation of pore structure under field-emission scanning electron microscopy (FE-SEM) and quantitative characterization of pore structure by low-temperature nitrogen adsorption test, hysteresis-loop quantitative analysis, total organic carbon (TOC) content analysis, pyrolysis and X-ray diffraction (XRD) experiments. The following results are obtained. First, whether shale can form a hysteresis loop in the low-temperature nitrogen adsorption-desorption experiment has an apparently positive correlation with the specific surface area, specific pore volume, clay mineral content, and pore structure fractal dimension, and an evidently negative correlation with the TOC content, while no apparent correlation with the average pore size, pore surface fractal dimension, highest pyrolysis peak temperature, and content of brittle minerals. Second, the hysteresis loop area depends on the development degree of the cylindrical pores with both ends open, ink-bottle pores, or parallel plate pores, the proportion of which to the total pores can be quantitatively evaluated by the hysteresis loop area. Third, the open-ended cylindrical pores, ink-bottle pores, or parallel plate pores in the samples from Chang 7 shale are mainly of the intergranular pores in clay minerals. Therefore, there is an apparently positive correlation between the hysteresis loop area and the content of clay minerals.