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Shale belongs to low-porosity and low-permeability reservoir, so it is difficult for fluid to enter pores through displacement. However, in the process of shale hydraulic fracturing, most of the fracturing fluid does not flowback after entering strata. It indicates that reaction may take place when fluid enters into strata, resulting in low flowback rate of fracturing fluid. Imbibition experiment and physicochemical properties test show that there is imbibition effect in shale. There are two stages: one is the initial rapid water absorption, while the other is later osmotic water absorption. The first stage is affected by the seam network system, while the other stage is mainly affected by the content of clay minerals. As clay minerals swell by absorbing water, the original fracture network system gradually expands, and porosity grows, permeability improves and recovery rate increases. The physicochemical properties test shows that (1) imbibition is influenced by both capillary force and mineral components. The higher the clay content is, especially the content of chlorite and illite/smectite (I/S) , the stronger the imbibition capacity is. (2) Shale with strong imbibition capacity also has strong seam-making capacity, which not only provides convenience for hydraulic fracturing but is more conducive to exploitation. [ABSTRACT FROM AUTHOR]

The in-situ stress state of the eastern margin of the Sichuan Basin, where there is a complicated fault zone, is poorly understood. This study offers vital insights into the in-situ stress state based on information from 17 wellbores in the Wufeng-Longmaxi Shale near the eastern boundary fault (i.e., the Qiyueshan Fault) of the Sichuan Basin, China. Drilling-induced tensile fractures and borehole breakouts revealed a decrease in the angle between the S Hmax (maximum horizontal stress) orientation and fault strike when the wellbores are close to the damage zone of the Qiyueshan Fault. This stress rotation may be caused by the lateral contrast in the Poisson's ratio between the country rock and damage zone of the Qiyueshan Fault. A high angle between the S Hmax orientation and fault strike also appeared in the individual fault blocks of the Qiyueshan Fault as a result of local stress concentration. The higher magnitudes of horizontal stresses indicate stronger lateral compression in the Qiyueshan Fault Zone. Owing to the lateral compression of a "sandwich" consisting of a high-E (Young's modulus) roof and low-E shale intervals, natural fractures mainly appeared in the low-E shale intervals when the wellbore was close to the fault damage zone. The low fracture intensity in the low-E shale intervals of the wellbores located at the boundaries of the stress concentration blocks may be associated with an extensional stepover process. • Spatial variations in stress orientation and magnitude around the fault zone. • Lateral mechanical contrast decreases the angle between S Hmax and fault strike. • Stress concentration complicates S Hmax orientations in certain fault blocks. • Vertical mechanical contrast affects the natural fracture intensity of shale. [ABSTRACT FROM AUTHOR]

Landslide susceptibility mapping (LSM) is a useful tool to estimate the probability of landslide occurrence, providing a scientific basis for natural hazards prevention, land use planning, and economic development in landslide-prone areas. To date, a large number of machine learning methods have been applied to LSM, and recently the advanced convolutional neural network (CNN) has been gradually adopted to enhance the prediction accuracy of LSM. The objective of this study is to introduce a CNN-based model in LSM and systematically compare its overall performance with the conventional machine learning models of random forest, logistic regression, and support vector machine. Herein, we selected Zhangzha Town in Sichuan Province, China, and Lantau Island in Hong Kong, China, as the study areas. Each landslide inventory and corresponding predisposing factors were stacked to form spatial datasets for LSM. The receiver operating characteristic analysis, area under the curve (AUC), and several statistical metrics, such as accuracy, root mean square error, Kappa coefficient, sensitivity, and specificity, were used to evaluate the performance of the models. Finally, the trained models were calculated, and the landslide susceptibility zones were mapped. Results suggest that both CNN and conventional machine learning-based models have a satisfactory performance. The CNN-based model exhibits an excellent prediction capability and achieves the highest performance but also significantly reduces the salt-of-pepper effect, which indicates its great potential for application to LSM. [ABSTRACT FROM AUTHOR]

Geochemical homogeneity in shale is often assumed when tracing subsurface fluids and characterizing sedimentary basins. This study presents measurements of the bulk gas composition, stable isotopes, and noble gas volume fraction and isotopes for shale gas samples collected from gas wells in the Wufeng-Longmaxi Shale, the southern Sichuan Basin, China. The dryness [C 1 /(C 2 + C 3)] ranging from 166.3 to 251.2, combined with δ13C 1 and δDC 1 that vary from −28.8 to −27.3‰ and − 153 to −145‰, respectively, point to a late mature thermogenic origin of hydrocarbon gas. 3He/4He ratios of gas samples are around 0.01 times the air value suggesting dominantly crust-derived He. 21Ne/22Ne and 40Ar/36Ar ratios of many gas samples are higher than the corresponding air values indicating the mixing of crustal and atmospheric noble gases. Multiple dichotomous patterns are observed in noble gas signatures of forelimb and backlimb samples, and depression and crest samples. 20Ne/22Ne ratios of some crest samples are higher than that of depression samples in the backlimb, pointing to the presence of diffusion-driven fractionation that is likely caused by the long-distance migration from depression to crest. Elemental ratios of air-derived noble gas isotopes – 22Ne/36Ar, 84Kr/36Ar, and 132Xe/36Ar are compared to the recharge water values, suggesting the interactions of oil, gas, and water phases in the shale over geologic time. Forelimb samples generally display older ages than backlimb samples, indicating a larger flux of external radiogenic 4He due to the higher density of deep faults in the forelimb area caused by the basement-involved deformation. The basement-involved deformation also causes pore collapse especially in the forelimb leading to a lower porosity that results in a more pristine noble gas signature in the forelimb due to the reduced impact of younger recharge water. • Fractionated noble gas ratios show the presence of oil in the Wufeng-Longmaxi Shale. • Regional tectonic deformations in the Sichuan Basin modulate oil-gas-water interaction in the shale. • Long-distance diffusion-driven hydrocarbon migration occurred in the Wufeng-Long Shale. [ABSTRACT FROM AUTHOR]

To evaluate the risk of induced seismicity due to hydraulic fracturing, a detailed stress analysis in a seismically active region, i.e., the Changning shale-gas field in the southern Sichuan Basin, was conducted utilizing sonic wave velocity logs and high-resolution electrical resistivity image logs of shale gas wells. Wells with relatively higher magnitudes of horizontal stresses are spatially located in the vicinity of thrust faults, in agreement with the stress concentration at the upper tip of the basement-involved thrust fault. The prevailing orientation of maximum horizontal principal stress (S Hmax) is N110°–120°E on the southwestern Changning anticline, which aligns with the motion of the Sichuan Basin, as driven by the continuous eastward extrusion of the Tibet plateau, which initiated at approximately 50 Ma. However, the S Hmax orientation is N30°–70°E along the northern Changning anticline, implying an inheritance of the N–S geodynamic compression during the Jurassic to Late Cretaceous. Drilling-induced tensile fractures primarily appear in compliant layers that are rich in clay or organic matter in wells located adjacent to thrust faults, suggesting that stiff layers can efficiently inhibit the hydrofracture growth height owing to more intense tectonic stress near thrust faults. A natural fault with a strike of approximately 30° to the S Hmax orientation can be more easily activated by extreme pumping pressure in wells that are weakly affected by the stress concentration along the thrust fault. Nevertheless, a cyclic injection of fracturing fluid with a gradually increasing target pressure may effectively reduce the magnitude of induced seismicity. • Detailed stress analysis of Changning shale-gas field using geophysical data. • Wells with higher horizontal stresses spatially located near thrust faults. • Drilling-induced tensile fractures appear in layers rich in clay or organic matter. • Stiff layers inhibit hydrofracture growth due to more intense tectonic stress. • Cyclic fluid injection with increasing target pressure may reduce seismicity. [ABSTRACT FROM AUTHOR]

Abstract: Three faults were involved in the rupture process of the May 12, 2008 Mw7.9 Wenchuan earthquake, representing a rarely documented case of multiple faults rupturing simultaneously. Distinct from the other two major NE-trending surface ruptures, the ~7km long Xiaoyudong surface rupture is oriented NW340° and shows thrust- and left‐slip faulting. In order to understand what role Xiaoyudong rupture played in the tectonic evolution of Longmenshan thrust system, we conducted a detailed investigation including geomorphic feature observation, coseismic displacement measurement, paleo-rupture event trenching, and fault kinematic estimation of the Xiaoyudong area. Our results not only document an in situ paleoearthquake occurring at 3.2–2.3ka BP, but also present the Xiaoyudong rupturing event as an oblique thrust with minor components of left lateral slip. We also found vertical and sinistral slip rates estimated as 1.1–1.5mm/yr and 0.5–0.7mm/yr, respectively. Finally, we suggest that the Xiaoyudong rupture is not a passive tear fault but an active participator of slip partitioning on multiple faults within the Longmenshan thrust system, and moreover, it gave strong response to both the 2008 and the penultimate earthquake while it kept silence with the antepenultimate event. [Copyright &y& Elsevier]

Tectonism significantly affects the pore characteristics of organic-rich shales. Here, two sets of shales with similar mineral compositions and total organic carbon (TOC) content were used to evaluate the response of pore structure to tectonic deformation through gas adsorption, FE-SEM, and Nano-CT. The stable region shales have superior porosity, pore volume, pore diameter, surface porosity of organic matter (OM), and pore connectivity. However, the deformed region shales show obviously low storage space, pore size, and connectivity. The OM pores of the stable region shales were well preserved due to the support of residual overpressure in the closed shale system. The larger pore size and better pore connectivity enable gas molecule to be much easier to diffusion and flow, thus leading to high yield after hydraulic fracturing. Conversely, the OM pores of the deformed region shales are commonly compressed and even closed, because of the absence of overpressure-associated supporting mechanism in a relatively opened shale system. Shale gas is difficult to transport in this kind of shale reservoir with poor pore structure parameters, resulting in limited yield. Furthermore, our work also provides 3D pore deformation information of shale reservoirs under complex tectonic episodes. • Shale pores were quantitatively evaluated in 2D and 3D under different tectonic preservation condition. • Tectonic deformation influences the shale pore structure and connectivity. • Great preservation condition is beneficial to gas accumulation and engineering renovation. [ABSTRACT FROM AUTHOR]