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Abstract Physico-mechanical properties of shale are important parameters in evaluating the stability of potential wellbore and the design of hydraulic fracturing, which are primarily affected by their non-uniform spatial distribution of the microscopic physical–mechanical properties at particle scale. A series of constant strain rate experiments and stress-cycling experiments on shale specimens with different bedding dip angles were conducted to have a comprehensive understanding of the effect of the non-uniform distribution of microscopic failure stress on macroscopic physico-mechanical properties. According to the experimental results and Weibull distribution, we find that bedding dip angle and the dynamic load applying type affect the spatial distributions of microscopic failure stress. The values of crack damage stress (σ cd), σ cd/σ ucs (peak stress), ε cd (strain at crack damage stress), Poissons' ratio (ν), elastic strain energy (U e) and dissipated energy (U irr) of the specimens with more uniform distribution of microscopic failure stress are overall higher, while ε ucs (peak strain)/ε cd and elastic modulus (E) are lower. The dynamic load enables the spatial distributions of microscopic failure stress trend to be more homogeneous prior to the final failure with the increment of σ cd/σ ucs, ν, U e and U irr and the decrement of E.

Natural fractures play an important role for the exploration and development of low-permeability reservoirs in the Triassic Yanchang Formation of the Ordos Basin. Abundant hydrocarbon resources have been found in the Yanchang Formation of the Gufengzhuang area, showing good exploration prospects. However, only limited research results on fractures have been obtained, and the distribution law and active period are both unclear. Based on the analyses of core, imaging logging, carbon-oxygen isotope, homogenization temperature of fluid inclusion, acoustic emission, tectonic stress and fracture activity, the characteristics, formation stages and causes of natural fractures in the Chang 7 to Chang 9 members of the Yanchang Formation in the Gufengzhuang area were studied, and corresponding indications for petroleum accumulation were discussed. Natural fractures are well-developed in the Chang 7 to Chang 9 members of the Yanchang Formation, especially in the Chang 7 member. High-angle and vertical fractures are ubiquitously developed, mainly of NEE orientation, followed by NE, and finally NW and NWW. The fractures are generally unfilled or semi-filled by calcites, providing effective pathways for hydrocarbon migration. The fracture surface is flat, and obvious traces of oil migration such as oil invasion, oil stain and oil spot can be observed. The average values of oxygen isotopes of the Chang 7 to Chang 9 members are -15.8‰, -17.5‰ and -18.7‰, indicating that the factures were formed at depth of 1 675, 2 338 and 2 785 m. According to the homogenization temperature peaks of fluid inclusions of 90-100 ℃ and 120-130 ℃, the formation depth of fractures is 1 750-2 000 m and 2 500-2 750 m. The fractures in the Chang 7 to Chang 9 members were active during multiple stages, including the Indosinian, Yanshanian and Himalayan stages, with the Yanshanian and Himalayan as the main active stages. Effective fractures developed in multiple stages have improved the physical properties of low-permeability reservoirs in the study area. They provided important migration channels and storage spaces for the formation of multi-layer and deep-layer reservoirs such as Chang 7, Chang 8 and Chang 9, and affected the productivity of oil wells.

Based on geochemical analysis results of core samples from the Triassic Chang 7 Member of Well Feng 75 drilled in the northwest margin of Ordos Basin, combined with geological characteristics of this region, the formation environment of the black shale and its control on shale oil enrichment are comprehensively studied. From the Chang 73 to Chang 71 Sub-members, the black shale have organic carbon contents decreasing from 5.70% to 3.55%, hydrogen indexes decreasing from 345 mg/g to 269 mg/g, while the oxygen indexes increasing gradually from 6 mg/g to 29 mg/g, indicating that the sedimentary environment during the depositional period of Chang 72 and Chang 73 Sub-members was anoxic. Biomarkers in the black shale change regularly, and have an obvious “inflection point” at the depth of 2753–2777 m in the Chang 73 Sub-member, indicating that the input of terrigenous organic matter increased. However, there is a negative drift about 2% of organic carbon isotopic composition near the “inflection point”, which is in conflict with the results of biomarker compounds. This is because the extreme thermal and anoxic events caused by continental volcanic activity in the ancient Qinling region caused negative drift of carbon isotopic composition of the black shale in the Ordos Basin. The volcanic activity caused rise of temperature, polluted air, extreme weathers, more floods and thus more input of terrigenous organic matter, and gave rise to extremely anoxic environment conducive to the preservation of organic matter. Terrigenous organic matter is more conducive to the formation of light oil than aquatic organic matter, so these sections in Yanchang Formation are major “sweet spots” for shale oil enrichment.

In the Ordos Basin, the Chang 8 oil reservoir of the Triassic Yanchang Formation is the main target of oil exploration and development and there are many factors affecting the hydrocarbon accumulation. Well G20 is an important exploration well in the Yanchi area in the northwestern part of the basin, and the core of the Chang 8 reservoir was sampled and tested to determine the source rocks, trace element composition, mineral composition, reservoir physical properties, and oil-bearing properties. The results show that the rock retrieved exhibits delta plain subfacies; the range of the Chang 8 water body was large, with a gentle slope, and the climate was relatively dry. These findings suggest that this reservoir was deposited in an environment in which sedimentary sand body could easily form. The Chang 8 sandstone reservoir pores are dominated by intergranular pores and feldspar intragranular dissolution pores, indicating that the Chang 8 reservoir is a low-porosity and low-permeability reservoir. Chang 7 source rocks from this area have a type I-II1 hydrocarbon-generating potential, with an average total organic carbon (TOC) content of 5.99% and vitrinite reflectance (Ro) value of 0.48%. Combined with the regional sedimentary evolution, tectonic movement, and reservoir distribution, it is considered that due to the lack of lithologic traps or low-amplitude structural traps, G20 produced water in well testing. The Chang 8 oil reservoir in the Yanchi area can be divided into structural reservoirs and structural-lithologic reservoirs. The conventional oil and gas reservoir exploration ideas of “thick sand belt” and “reservoir sweet spot” are not applicable here. The lithology traps or low-amplitude structural traps and areas with good preservation conditions are the main directions for the next phase of exploration in the northwestern part of the Ordos Basin.