Continental shale is abundant with unconventional oil and gas and has considerable prospect for exploitation. However, due to deposited environment differences, continental shale presents unique characteristics in terms of structure and mineralogy compared to marine shale. The mechanical property is elemental parameter for reservoir production, but relevant studies of continental shale are rarely reported. In this study, several groups of mesoscale (0.1–10 mm) uniaxial compression tests were conducted on continental shale with various bedding angles, sourced from the Yanchang Formation in the southern Ordos Basin, China. A new integrated real‐time loading‐observation‐acquisition system, specifically, a miniature tensile instrument‐light microscope (MTI‐LM) system, was utilized, which allowed us to visualize the failure process and simultaneously record the stress‐strain curves, providing snapshots for investigation of fracture initiation, propagation, and fracture surface features. The results demonstrate that as the bedding angle relative to the maximum compression direction increases, the uniaxial compressive strength (σ_c) and the percentage of bedding‐parallel fractures (F_p) fluctuate reversely, namely, higher σ_c corresponds to lower F_p. Additionally, a good consistency is revealed among the variations of σ_c, failure time and fracture surface roughness with bedding angles, suggesting stronger shale having rougher fracture surfaces and longer failure time at a certain loading rate. The meso‐mechanical anisotropy of shales is supposed to have an origin in heterogeneous component and microtexture, as indicated by different bridges connecting the opposite facies of fractures. The findings help reveal the mechanism underlying mechanical anisotropy and facilitate fracability evaluation of shallow shale gas reservoir.

Favorable prospects for the exploration of shale gas have been demonstrated in the Ordos Basin, China. Outcrop and core observations indicate that there are abundant laminas in the shale strata, which exert a great influence on hydro-fracture propagation, gas storage and fluid flow. In this study, the continental shale of the Chang 72 Member, collected from the south of Ordos Basin, was investigated to characterize the geometric heterogeneity. Laminas at multiple scales were observed and measured using conventional logging, borehole TV, core analysis, scanning electron microscopy, and the Particle and Crack Analysis System. These measurement tools correspond to the meter scale, decimeter scale, centimeter scale, millimeter scale and ten-micrometer scale, respectively, with measured thicknesses of 2.26 m, 2.09 dm, 1.70 cm, 1.48 mm and 11.70 μm, respectively. Fractal theory was used to analyze the power exponent distribution of the lamina thickness, with a resulting fractal dimension of 1.06. Finally, a geometric heterogeneity model was proposed for the Upper Triassic Yanchang Formation in the study area and verified by a modeled thickness of 26.30 m for the Chang 72 Member at the 10-m scale. The model facilitates cross-scale analysis and provides parameter guidance for heterogeneity characterization in the numerical simulation and model test of the shale gas reservoir.

Laminae are well developed in shale and generally influence fracture propagation during hydraulic fracturing. Hence, comprehensively understanding the tension and shear behaviors of shale laminae is crucial. There have been limited systematic studies thus far on the tensile and shear strength as well as fracture morphology of shale laminae. In this study, the Lower Silurian Longmaxi Shale (China) was investigated via Brazilian tensile and angle-varied plate shear tests. Five lamina types were tested, i.e., calcite (Cal), pyrite (Py), organic-enriched (Oc), the interface between Cal and Oc (Cal-Oc), and the interface between Py and Oc (Py-Oc) laminae. Results showed that the tensile strength was in the range 0.43–8.22 MPa, mainly in the order of Cal > Py > Cal-Oc > Py-Oc > Oc. The modes of fracture morphology were highly related to the occurrence, continuity, and mineralogy fillings of laminae. Shear strength parameters were within the range 22.50–29.64 MPa for cohesion and 37.29–43.60° for internal friction angle. Fracture surface roughness was strongly related to its cohesion. Calcite laminae considerably influenced the tensile fracturing of shale, suggesting that the geometry and properties of calcite lamina should receive more attention during the design of shale gas exploration.

The diagenesis of welded tuffs is a process in which volcanic debris undergoes degassing, compaction, and quenching, and vitreous rheologic, which indicates that the welding occurred in a high-temperature, high-pressure diagenetic environment and that different temperatures and pressures result in different degrees of welding in the welded tuffs, which can also result in differences in the mechanical properties of the rock. In this study, based on petrographic identification, mineral composition analysis, and pore structure characterization, uniaxial compression combined with linear accelerator CT and Brazilian splitting tests was carried out to investigate the influence of the welding degree on the strength and failure modes. The test results showed that although they had almost similar mineral composition and porosity, the uniaxial compression strength and tensile strength of the strongly welded tuffs were greater than that of the weakly welded tuffs. Their failure modes were also different. Fractures in the weakly welded tuffs developed gradually, while the strongly welded tuffs showed a higher brittleness with sudden failure. The results of this study shed light on the influence of the diagenetic environment on the mechanical properties of rock from a geological perspective and can provide a mechanical basis for rockfall risk evaluation in scenic areas of welded tuff.