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In shale plays, the in-situ effective stress is continuously being perturbed because of the production which impacts field operations. Therefore, mechanical properties of the formation and its response to stress field changes, with respect to mineral variations, is of great significance for the proper design of horizontal drilling and hydraulic fracturing. In this paper, clay-rich, calcareous and siliceous black shales are classified based on their XRD analysis, from the Wufeng and Longmaxi Formations in the Sichuan Basin of the Upper Yangtze Plate. Uniaxial quasi-static and cyclic loading tests, acoustic emissions, and CT scanning are combined to study mechanical behavior and fracturing mechanisms of the shale layer samples that own different mineral components. The results show that the strength and deformation response of the samples is a function of their mineral composition. The damage from cyclic loading is gradually accumulated in the samples which ultimately leads to fatigue and sample failure. The compressive strength of the samples under cyclic loading is lower than in the uniaxial quasi-static conditions. Furthermore, the fracturing of rock under uniaxial quasi-static loading mainly occurs after the peak strength is achieved while in cyclic loading, when a certain stress level is reached, fracturing begins, the damage accumulates up to the peak stress and the rock suddenly fails. Based on CT images from the samples after rupture, failure modes of the clay-rich, calcareous and siliceous black shales mainly constitute extensional and shear fractures, with some conjugate shearing. In comparison with uniaxial quasi-static loading, the fracture volume of the clay-rich, calcareous and siliceous black shales after cyclic loading increases by 12.2%, 25.9% and 32.9%, respectively. Overall, this study explains how cyclic loading that can be applied in the field via changing the injection pressure and other stimulation parameters, can effectively improve the complexity of the shale fracture network and lead to higher productivity of the formation. • Quasi static loading and cyclic loading are employed in the mechanical test. • The compressive strength of the cyclic loading is lower than that of uniaxial quasi-static loading. • The fracture volume of the cyclic loading is higher than that of uniaxial quasi-static loading. [ABSTRACT FROM AUTHOR]

Deep shale gas reservoirs, compared to shallow and intermediate shale gas reservoirs, usually exhibit lower fracture complexity, smaller stimulated volume, poorer conductivity, and faster production decline when the same injection scheme and construction parameters are used. As a result, specific injection scenarios for deep shale reservoirs must be investigated. In this paper, a series of laboratory fracturing tests were conducted to investigate the influence of injection scenarios on deep shale hydraulic fracture initiation and propagation. The black shale of the Lower Silurian Longmaxi formation in the Sichuan Basin was adopted, and four different injection schemes (constant fluid rate injection, cyclic injection, shut-in intermittent injection, and low-frequency pulse pressurization injection) were utilized in these experiments. The energy evolution during hydraulic fracturing with different injection schemes was analyzed by an array of acoustic emission (AE) sensors. Furthermore, the internal fracture geometry of the shale specimens after failure was determined using high-resolution CT scanning. Two parameters, fracture bulk density and fractal dimension were defined to quantitatively characterize the post-pressure fracture complexity. The results indicate that the rock breakdown pressure rises rapidly as the injection rate increases, for conventional monotonic rate injection. Although increasing the injection rate can serve the purpose of increasing the net pressure in the seam and enhancing the fracture complexity, it makes it easier to approach the equipment pressure limit. Unlike conventional hydraulic fracturing, which mainly induces tensile fractures near the wellbore, cyclic fracturing is more likely to cause fatigue damage to the rock, resulting in the formation of subcritical microcracks, which gradually grow into macroscale fractures. In addition, the cyclic injection method can reduce the rock breakdown pressure by approximately 24% compared with the conventional continuous injection method. The shut-in intermittent injection method can also reduce the breakdown pressure of shale by 22%, but this method is not conducive to enhancing the fracture complexity. Pulse fracturing creates the most complex fracture morphology among the four injection schemes tested. The rapid changes in wellbore pressure over a short time can easily result in the formation of multiple fractures around the borehole. Our experimental results and theoretical analysis can provide theoretical support for the optimization of injection schemes for deep shale hydraulic fracturing. • Laboratory tests were conducted to investigate the influence of injection scenarios on deep shale hydraulic fracturing. • All the experiments were monitored by an array of AE sensors. • The internal fractures geometry of the shale specimens after failure was determined by high-resolution CT scanning. • The cyclic fracturing can reduce the breakdown pressure by about 24% compared with the conventional continuous injection method. [ABSTRACT FROM AUTHOR]

In this paper, the original nature of organic matters is introduced for the first time to reevaluate the lower Permian (P 1) source rocks. The carbonate rocks in Maokou Formation (P 1 m) whose high organic carbon values were caused by external asphalt have poor hydrocarbon generation ability, but the argillaceous rocks in P 1 m have some, especially the recent discovered black shale in Gufeng deep-water facies in the north have good hydrocarbon generation ability. The black shale is 5–20 m thick and the measured average value of organic carbon can reach up to 7.24 wt%, which can greatly increase the hydrocarbon generation capacity in the north; The studies of natural gas sources of the lower Permian in eastern Sichuan basin show that the contribution of gas source is mainly Silurian. In the north, the lower Permian source rock is supplemented, while in the south, it's greatly affected by the coal-measure source rocks of Longtan Formation (P 2 l); Take the Wutan-1 well in Tanmuchang as an example to dissect the reservoir in syncline area. The Wutan-1 well has a tested daily production of more than 80 × 104 m3, and the paleotectonic evolution history shows that it was in the high position of the structure during the generation and expulsion peak of hydrocarbon. The fracture-vuggy lithologic reservoir of Wutan-1 well is favorable for hydrocarbon capture and later preservation. According to the above, the Tanmuchang syncline has a good spatio-temporal matching relationship, which is the key to hydrocarbon accumulation and exploration. This knowledge has expanded the exploration area of the Lower Permian Maokou Formation in eastern Sichuan, and more importantly, it brings much enlightenment to the exploration of the syncline areas in Sichuan basin and other basins at home and abroad. • The original nature of organic matters is introduced for the first time to reevaluate the lower Permian source rocks. • The black shale in Gufeng deep-water facies has hydrocarbon generation capacity in the north of the eastern Sichuan Basin. • The sources of natural gas in Maokou Formation are different between the north and the south in the eastern Sichuan Basin. • The syncline area of the Lower Permian Maokou Formation in eastern Sichuan has the potential for gas accumulation. [ABSTRACT FROM AUTHOR]