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The enrichment of organic matter in high-quality marine shale is generally controlled by factors such as the redox conditions of sedimentary environments, productivity levels, terrigenous input, and ancient productivity. However, the controlling effect of the sedimentary environment on organic matter enrichment in intracratonic sag is still unclear. This study takes samples from the Qiongzhusi formation shale in southern Sichuan Basin as the research object, focusing on trace elements as well as rare earth elements in different stratigraphic intervals. The provenance of the Qiongzhusi formation shale is mainly terrigenous, with sediment sources mainly consisting of sedimentary rocks and granites. The primary sedimentary environment transitions from a continental margin setting, influenced by rift-related tectonic activity and sediment influx from adjacent landmasses, to an open oceanic environment characterized by mid-ocean ridge processes and oceanic plate subduction zones. During sedimentation, saline water was present, with predominant sedimentary environments ranging from shallow water to deep water continental shelves. The shale in the study area is characterized by a higher content of silicates and a lower content of carbonate minerals. Its siliceous sources are mainly influenced by biogenic and terrigenous debris, indicating higher ancient primary productivity and representing a favorable target for shale gas exploration.

The exploration and development potential of shale gas reservoirs in the Sichuan Basin is enormous; however, it also faces difficulties such as complex structures, strong heterogeneity, and unclear geophysical response characteristics. Fine prediction of geostress is an important part of shale gas exploration and development, which directly affects the implementation effect of reservoir evaluation, well trajectory design, and fracture reconstruction. The existing geostress prediction techniques lack high-precision seismic data constraints, making it difficult to accurately reflect the planar distribution characteristics of geostress in the block with rapid changes in complex tectonic zones. At the same time, the geophysical response characteristics of geostress in the Sichuan Basin are unknown, and the geostress seismic prediction technology lacks theoretical basis. This paper combines numerical simulation and physical experiments and defines the characteristics of the geophysical response of shale gas reservoirs in the Sichuan Basin changing with the stress field, and technical countermeasures for geostress seismic prediction have been established to provide technical means for accurate prediction of the geostress field in the shale gas block. Based on the geostress sensitive parameters obtained from prestack seismic inversion, the geostress field prediction of a shale gas work area in the Sichuan Basin is realized.

Horizontal wells are extensively utilized in the development of unconventional reservoirs. However, the logging responses and formation evaluation in horizontal wells can be impacted by factors like anisotropy and tool eccentricity. To investigate the influence of tool eccentricity on acoustic logging response, physical simulation experiments of array acoustic logging were conducted in a scaled borehole formation model under different tool eccentricity conditions. The experimental data were analyzed, and the findings revealed that when the receiver array is parallel to the borehole axis, the P-wave slowness and S-wave slowness remain unaffected by tool eccentricity. However, the amplitudes of the P-wave and S-wave decrease significantly with increasing tool eccentricity, following an approximate negative exponential pattern. Additionally, when the transmitter is centered and the receiver array intersects the borehole axis at an angle, the wave velocities increase significantly with tool eccentricity, with the P-wave velocity showing a faster increase. Conversely, when the transmitter is eccentric and the receiver array intersects the borehole axis at an angle, the wave velocity decreases notably with tool eccentricity, and the P-wave velocity decreases even faster. These findings contribute to a better understanding of the impact of tool eccentricity on array acoustic logging response in horizontal wells and offer guidance for developing correction schemes to address this effect.

Abstract Paper-based electronics have attracted much attention due to their softness, degradability, and low cost. However, paper-based sensors are difficult to apply to high-humidity environments or even underwater. Here, we report a fully paper-integrated piezoresistive sensing system that exhibits flexibility, waterproofing, air permeability, and biocompatibility. This system consists of hydrophobic paper as the substrate and encapsulation layer, conductive paper with a double ‘zig-zag’ and dotted surface structure as the sensing layer, and silver paste films as the interconnects. The structural design of the sensing layer helps to increase the contact area in adjacent layers under pressure and further improves the pressure sensitivity. The piezoresistive system can be worn on human skin in the ambient environment, wet environment, and water for real-time monitoring of physiological signals with air permeability and waterproofing due to its hydrophobic fiber structure. Such a device provides a reliable, economical, and eco-friendly solution to wearable technologies.

When the thermal maturity of the Longmaxi Formation in the southern Sichuan Basin is too high, the pore structure of shale becomes poor. Therefore, to investigate the effect of organic matter thermal maturity on shale pore structure, a study was conducted. Using the Longmaxi Formation shale in the southern Sichuan Basin as an example, the intrinsic relationship between shale porosity, pore structure parameters, organic matter laser Raman maturity, and organic matter graphitization degree was examined using X-ray photoelectron spectroscopy, particle helium porosity measurement, organic matter micro-laser Raman spectroscopy, and gas adsorption experiments. The results indicate that thermal maturity is the macroscopic manifestation of the graphitization degree of organic matter, and the correlation coefficient between the two is 0.85. A thermal maturity of 3.5% (with a corresponding organic matter graphitization degree of 17%) aligns with the highest values of shale porosity, pore volume, and pore-specific surface area across all pore size conditions. The evolution model of shale pore structure can be divided into two stages. The first stage is characterized by a thermal maturity between 2.0% and 3.5% (with a corresponding degree of graphitization of organic matter between 0% and 17%). During this stage, the number and connectivity of micro-macropores increase with increasing thermal maturity. The second stage is marked by a thermal maturity between 3.5% and 4.3% (with a corresponding degree of graphitization of organic matter between 17% and 47.32%). Basement faults are present, leading to abnormally high thermal maturity, poor preservation conditions, continuous generation of micropores, better connectivity, and a reduced number of pores. Medium macropores with good connectivity suffer from gas loss in the fracture network, leading to the collapse and disappearance of pores. The results mentioned in the statement have an important guiding role in the efficient exploration of shale gas in the Longmaxi Formation in the southern Sichuan Basin.

The oil content in shale is still a contentious issue due to the evaporative losses of free hydrocarbon. Considering the objective of the evaluation of oil content in shale and the demand of fast analysis at wellsite, a newly developed sealed rock thermal desorption method is introduced to quantify the content of free hydrocarbon in rock samples which combined the technique of sealed crushing at low temperature and improved the traditional rock pyrolysis (Rock-Eval). Comparison experiments have been conducted on Jurassic Da'anzhai shale from the Sichun Basin. The S0 value ranges from 0.001 to 0.046 mg/g, with S1 value from 0.165 to 4.648 mg/g by routine method of rock pyrolysis. The S0 value by the sealed thermal desorption method, which ranges from 0.026 to 0.984 mg/g, is about 1-2 order of magnitude higher than that of Rock-Eval. By improving the heating program, the sealed thermal desorption method can obtain the hydrocarbon content per unit mass of rock at temperatures of 5, 5-90 and 90-300℃, which not only obtains abundant oil-bearing data, but also shortens detection time. Combined with parameters such as mud gas measurement, shale geochemical parameters, and reservoir fluid properties, the "sweet spots" of shale oil in the Da'anzhai Member of the study well were evaluated, providing a new experimental method for evaluating the oil content of shale oil.

The complex fractures aggravate stress sensitivity and heterogeneity of the reservoir and seriously restrict effective development. Therefore, it is of great significance to study and quantitatively evaluate the stress sensitivity of the fractured reservoir. Taking the typical block of the Longmaxi shale reservoir in southern Sichuan as the engineering background, one uses the finite element method to develop a numerical model of a two-dimensional fracture closure variation subjected to the non-hydrostatic stress field. It explores the influence of different fracture occurrences and rock mechanical parameters on stress sensitivity. The theoretical model verifies the numerical simulation results to reveal the stress sensitivity mechanism of the fractured reservoir. The results show that the influence of the dip angle of fracture on the stress sensitivity depends on the anisotropy of applied in-situ stresses. The stronger stress sensitivity occurs in low-dip angles where the lateral pressure coefficient is less than 1. One defines the lateral pressure coefficient. On the contrary, the stronger stress sensitivity occurs in high-dip angles where the lateral pressure coefficient is more significant than 1. It is because the normal stress differences under different stress fields apply to the fracture. Under a given stress condition, the stress sensitivity of fracture negatively correlates with aspect ratio, elastic modulus, and Poisson’s ratio. Pressure maintenance may be more critical in a reservoir with a low aspect ratio and rich in soft minerals. The theoretical predicting model of fracture permeability under different conditions is established based on the linear elastic theory. The relative error between theoretically predicted results and numerical simulation ones is less than 10%, which verifies the accuracy of numerical simulation results. The fundamental reason for stress sensitivity in the fractured reservoir is the fracture geometry and mineral deformation change. The research results are of great significance for establishing the productivity equation considering the stress sensitivity, accurately evaluating the variation of reservoir seepage capacity, and formulating reasonable drainage and production system.

Deep shale gas (3500–4500 m) is the important replacement field of shale gas production growth in the future China. The research on key parameters of deep shale-gas reservoir is critical to determine its basic geological characteristics and establish a suitable development mode. In order to clarify the adsorption characteristics and controlling factors of deep shale gas in Longmaxi Formation, the main tests such as high-pressure methane adsorption, low-temperature nitrogen and carbon dioxide adsorption coupled with the adsorption fitting model and comparative analysis were conducted. The results show that the adsorption isotherms of deep shale gas also have a downward trend in spite of the higher pressure, and there is no obvious difference in adsorption characteristics, which is mainly due to the similar characteristics of microscopic pore-structure between deep shale and shallower shale. It is found that different adsorption models can well fit the experimental adsorption curve of deep shale gas, but the absolute adsorption capacity converted from excess adsorption capacity shows the same fitting result, i.e., DA-LF model > DR model > Langmuir model. Furthermore, DR model based on micropore filling theory is more suitable for characterizing the adsorption law of deep shale gas combined with the correlation analysis between pore structure and adsorbed-gas capacity. In addition, TOC is the key material factor controlling the adsorption capacity, and specific surface area of micropore is the key spatial factor. Compared to shallower shale, the deep shale shows higher siliceous content, lower calcite content, lower TOC content and lower adsorbed-gas content (the proportion of adsorbed-gas is about 30%).

AbstractThe optimal design of hydraulic fracturing parameters is the key to commercial exploitation of unconventional reservoirs. Hydraulic fracturing test is one of the main methods for optimizing fracturing parameters. It is known that scale effect exists between laboratory experiments and field treatments of hydraulic fracturing. However, studies on how to eliminate the scale effect are rarely reported. In this work, we conduct sensitivity analysis on rock mechanical parameters and fracturing parameters at different scales by using the dimensionless analysis method. The initiation and propagation process of field hydraulic fracturing is reproduced through laboratory tests, and fracturing parameters are analyzed by using numerical simulation. Our results show that the fracture propagation in the laboratory is inconsistent with that in the field fracturing. The fracture initiation and propagation in the field can be reproduced in experiments by using samples with high modulus and low toughness as well as high-viscosity fracturing fluid. Microcracks are created before the breakdown pressure is reached, and hydraulic fractures extend perpendicular to the direction of the minimum principal stress. The Carter’s leak-off coefficient has little effect on breakdown pressure and propagation pressure, but the injection rate and the horizontal principal stress have significant effects on breakdown pressure. This study provides a theoretical basis and guidance for the design of fracturing parameters both in the laboratory and in the field.

The characteristics of lithofacies in the shale reservoir of the Wufeng and Longmaxi formations located in the Luzhou gas field were studied using the three-end-member method (mineralogical components). In addition, the microscopic characteristics and pore genesis of different lithofacies were studied and compared using TOC, FESEM, and digital core images. The results of the present study showed the development of five lithofacies: 1) quartz-rich argillaceous shale; 2) quartz/clay-mixed shale; 3) calcareous/clay-mixed shale; 4) calcium-rich argillaceous shale; and 5) clay-rich siliceous shale. The degree of pore development between lithofacies was highly heterogeneous. In addition, data indicated that total shale porosity increased with the increase in TOC. Thus, the pores were mainly related to organic matter. The enrichment of framboidal pyrite in quartz-rich shale (S1, S-3) and mixed shale facies (M-2, M-3) is essential during the formation of high-quality reservoirs. On the other hand, the content of organic matter in clayey shale was low, which does not favor the development of high-quality reservoirs. The differences in lithofacies pore structures are controlled by the sedimentary environment. The strong retention and reduction environment are the most favorable features for the formation of organic-rich siliceous shale lithofacies, which promote the preservation of organic matter and the development of reservoirs.

Pyrite is a mineral that is commonly found in shale gas reservoirs. Its genetic mechanism and impact on pore and organic matter in shale gas reservoirs are critical for shale gas exploration. This study selects the Longmaxi shale (Lower Silurian) in the Changning area of the southern Sichuan Basin by comprehensively using a scanning electron microscope (SEM), X-ray diffraction (XRD), total organic carbon (TOC), and image processing technology. The type and characteristics of pyrite in shale reservoirs are studied, the sedimentary environment and genetic mechanism of pyrite are analyzed, and the influence of pyrite formation on organic matter enrichment and reservoir formation is evaluated. The results showed that pyrite in shale primarily forms framboidal pyrite, euhedral pyrite, and subhedral pyrite, with particle sizes ranging from 1 to 15 μm. The maximum framboid diameter (MFD) is less than 20 μm, with the average particle size distribution of 3–5 μm. These parameters indicate the vulcanization and blocking environment. The reducing environment promotes organic matter enrichment and preservation. Framboidal pyrite has two genetic sequences: rich organic matter and poor organic matter. The development of organic matter will limit the continuous radial growth of pyrite and is conducive to the protection of pores, and the formation of pyrite can reduce the activation energy of kerogen reaction and catalyze the hydrocarbon generation of organic matter, resulting in higher gas content. The framboidal pyrite content can be used to predict high-quality shale gas reservoirs.

Marine–continental transitional shale is one of the most promising targets for shale gas exploration in the Lower Yangtze region. To investigate the sedimentary environments and the regularity of the enrichment of the Longtan shale, multiple techniques including core and thin-section observations, geochemical and elemental analyses, X-ray diffraction, scanning electron microscopy (SEM), and low-pressure nitrogen adsorption (LPNA) were used to analyze the sedimentology, mineralogy, and pore structure of the Longtan shale. The core descriptions and thin-section observations showed that the Longtan shale was deposited in marine–delta transitional environments including delta-front, shore swamp, mixed tidal flat and shallow shelf environments. The Sr/Cu, V/Cr, CIA, EF (Mo), EF (U), and other major and trace element results indicated warm and moist climates and water-reducing conditions in the Longtan period. Both the climate and water conditions were favorable for organic matter production and preservation. The geochemical results showed that the Longtan shale was in the overmature stage (Ro values ranging from 2.4% to 3.57%) and that the average total organic carbon (TOC) content was 5.76%. The pore system of the Longtan shale consisted of inorganic pores with a small number of organic pores and microfractures. The porosity and specific surface area were mainly affected by the TOC and clay mineral contents. An effective combination of brittle mineral particles, organic matter, and clay minerals provided the necessary conditions for pore preservation. The organic pores, intergranular pores in clay minerals, and brittle mineral pores formed the main network system for the Longtan shale. In summary, the lithological combinations, organic geochemistry, and pore structure system were all affected by the sedimentary environments.

The Luzhou area in the southern Sichuan Basin has experienced multiple tectonic movements, forming a complex fault system; the activity has an important impact on the enrichment of shale gas in this area. In order to reveal the influence of the fracture system on the differential enrichment of shale gas, this paper takes the southern Sichuan Basin as the research object. The structural evolution process and fracture development characteristics of the different tectonic units in Luzhou area of southern Sichuan were characterized by conducting a seismic profile analysis, structural recovery using a back-stripping method, and core hand specimen description. We clarified the control effect of the structural deformation and fracture on the differential enrichment of shale gas, and we established a differential enrichment model of shale gas in the Luzhou area. The results show that: (1) The Luzhou area has undergone the transformation of a multi-stage tectonic movement. There are many sets of detachment structures in the longitudinal direction, and the plane structural form is a thin-skin fold-thrust belt composed of wide and narrow anticlines in the north–south direction. (2) The faults in the study area are affected by the Himalayan tectonic movement. The high-angle reverse faults are developed, and the number of large faults is small. The second and third faults are mainly developed. The second faults are only developed at the high position of the structure, which has a significantly destructive effect on shale gas reservoirs, while the third and fourth faults have no significant destructive effect on shale gas reservoirs. (3) In the study area, the types of cracks are categorized into transformational shear cracks, bed-parallel shear cracks, intraformational open cracks, lamellation cracks, shrinkage cracks, and abnormal high-pressure cracks. The thickness of the shale rock mechanical layer, brittle mineral content, and organic matter content jointly control the crack development degree in the shale of the Wufeng–Longmaxi Formation. (4) The uplift erosion, structural deformation, and fracture development caused by the structural evolution have affected the preservation of shale gas, resulting in the differential enrichment of shale gas reservoirs in the region. Based on the enrichment factors of shale gas, we established a differential enrichment model of shale gas in typical structural units and optimized the favorable enrichment areas, which are important contributions for guiding shale gas exploration and development in the Sichuan Basin.

In order to find out the enrichment mechanism and forming type of deep shale gas, taking the Longmaxi Formation shale in the Desheng–Yunjin Syncline area of Sichuan Basin as an example, we determined the mineralogy, organic geochemistry, physical property analysis, gas and water content, and the influence of three factors, namely sedimentation, structural conditions, and hydrogeological conditions, on the enrichment of shale gas. The results show that Longmaxi Formation shale in Desheng–Yunjin Syncline area is a good hydrocarbon source rock that is in the over-mature stage and has the characteristics of high porosity, low permeability, and high-water saturation. The contents of clay and quartz are high, and the brittleness index is quite different. According to the mineral composition, nine types of lithofacies can be found. The development characteristics of Longmaxi Formation shale and the sealing property of the roof have no obvious influence on the enrichment of shale gas, but the tectonic activities and hydrodynamic conditions have obvious influence on the enrichment of shale gas. The main control factors for shale gas enrichment in different regions are different. According to the main control factors, the gas accumulation in the study area can be divided into three types: fault-controlled gas, anticline-controlled gas, and hydrodynamic-controlled gas. The fault-controlled gas type is distributed in the north of the Desheng syncline and the north of the Yunjin syncline, the anticline-controlled gas type is distributed in the south of the Desheng syncline and the south of the Yunjin syncline, and the hydrodynamic-controlled gas type is distributed in the middle of the Baozang syncline. This result is of great significance for deep shale gas exploration.

Currently, Luzhou in the Sichuan Basin is a focal point for shale-gas exploration and development in China. However, a lack of detailed research on the mineral composition of the Wufeng Formation-Longmaxi Formation (WF-LF) shale is hindering the extraction of deep-buried shale gas in the Luzhou shale play. Herein, a field emission scanning electron microscope (FESEM) equipped with the Advanced Mineral Identification and Characterization System (AMICS) software was employed to analyze the mineral composition of the WF-LF shale from six wells in Luzhou. Quartz was the dominant mineral type, (16.9–87.21%, average 51.33%), followed by illite, calcite, dolomite, and pyrite. Our study revealed that (1) quartz content showed a moderate positive correlation with the total organic carbon (TOC) content, indicating that the quartz found in the shale is mostly of biological origin; and (2) the sum content of siliceous minerals and carbonaceous minerals was moderately positively correlated with the brittleness index (BRIT) in well SS1H2-7 and in the well group of RS8 and RS5, indicating that the siliceous minerals and carbonaceous minerals had an active effect on reservoir compressibility. Finally, according to the mineralogical features of each sublayer, we identified four types of reservoirs to determine their scope for exploration.