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The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of M > 6 are highlighted since the 14th century until the beginning of the 20th.

Deformation in intraplate domains is usually considered as a consequence of tectonic events at plate boundaries. Nevertheless, the occurrence of intraplate earthquakes such as the recent Le Teil event in the south of France along the Cévennes Fault System (CFS), on 11 November 2019, Mw = 4.9, questions whether this far-field deformation only occurs during tectonic pulses at plate boundaries, or if it corresponds to low-intensity but regional continuous deformation through time. To address this question, we have coupled U–Pb geochronology of fault-related calcites with structural analysis along a major fault system (the CFS) in the South-East Basin, France. We evidence (1) an Albian activity of the CFS and (2) a continuous compressional activity of the CFS and satellite structures during the whole Eocene and probably during the Late Cretaceous – Palaeocene, including periods (e.g. Lutetian) usually considered as phases of tectonic quiescence. We thus demonstrate that the tectonic reactivation of this intraplate fault system is not restricted to periods of high rates of deformation at plate boundaries.

Following the Mw4.9 Le Teil surface rupture earthquake that occurred on the north-eastern Cévennes fault system (NCFS) in France, several investigations were carried out to understand the origin of the earthquake rupture. A few studies performed local modeling of the NCFS structures in three dimensions integrating the rheology of the sedimentary layers within the hypocenter zone. However, the geometry of the NCFS at the scale of the Southeast French Basin is poorly constrained and it remains difficult to locate its trace beneath the Quaternary sediments of the Rhône river valley. To address this issue, Électricité de France (EDF) carried out a deep reflection seismic survey along the NCFS. This new set of seismic profiles was interpreted using a geological data base including surface data, well data, and previous seismic data that were reprocessed. The resulting 3D structural model allows us to reconstruct a polyphase geological history during the past 320 Ma, which we divide into three major tectonic phases. We show that all structures in the basin in the study area were initiated as normal faults during the Lower Jurassic and the Lower Cretaceous. During the Upper Cretaceous, these structures were reactivated, acting as a major transfer fault zone during the Pyrenean shortening phase, then as normal faults during the Oligocene extension. The morphology and faults at the top-Carboniferous basin initiated during the Lower Jurassic strongly shaped the final structure of the NCFS during the subsequent tectonic phases. Our new results allow updating the historical geology of the Vivaro-Cévenol region and our knowledge about the structures that have affected the Southeast Basin since the Mesozoic. In the context of the Le Teil earthquake, our new structural model provides important constraints for continuing paleoseismological works that will better assess the seismic hazard in this region.

Multiple studies had been conducted to improve the geological understanding and enrichment patterns of the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin’s complex tectonic belt. However, the influence of different structural conditions on shale gas occurrence and shale reservoirs remained unclear. To address this issue, low-mature shale samples from the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin were selected for thermal simulation experiments, based on the actual burial evolution history of the region. Overmature shale samples with the maximum burial depth were then obtained. Next, based on the maximum burial depth sample, the thermal simulation conditions were adjusted to simulate the evolution of the shale reservoir under different structural preservation conditions. Finally, organic geochemistry, FE-SEM, nuclear magnetic resonance, low-temperature gas adsorption, high-pressure mercury injection, and other experiments were conducted to analyze the characteristics of the shale reservoir and the differences in shale gas occurrence under varying structural preservation conditions. As structural preservation conditions get worse, both the organic and inorganic pore sizes in the shale decrease, and organic pore morphology evolved from round-subround to narrow, elongated shapes. Meanwhile, organic and inorganic pore porosity, organic pore proportion, and fluid pressure within the pores decreased as structural preservation conditions get worse, which suggested that better structural preservation conditions were conducive to maintaining fluid pressure in shale pores, which in turn supported pore preservation. Fluid pressure changes had a greater impact on organic pores. FHH model calculations showed that shale pores formed under poorer structural preservation conditions exhibit more complex pore structures and greater heterogeneity. Finally, a pore structure development model was established based on the characteristics of shale pore structure under different preservation conditions.

Pyrite, as a characteristic mineral in organic-rich marine shale, is a significant index for the interpretation of paleoredox conditions. In this study, based on drilling cores and focused ion beam-scanning electron microscopy (FIB-SEM), the occurrence, diameter and particle size distribution of pyrites from 32 samples obtained from the Wufeng–Longmaxi Formation in the southeast Sichuan Basin were analyzed. The results show that pyrite displays various occurrences at the macro-scale and micro-scale. At the macro-scale (mm–cm), pyrite laminations, nodular pyrites and lenticular pyrites can be found from drilling cores. At the micro-scale (nm–µm), the common occurrences of pyrite are pyrite framboids, euhedral pyrites and infilled pyrite framboids. According to the formation mechanism of pyrites, pyrites can be divided into syngenetic pyrites and diagenetic pyrites. The infilled pyrite framboids are categorized as diagenetic pyrites. The mean pyrite framboid diameters (Mean, D) range from 2.94 µm to 5.33 µm (average of 4.26 µm), with most samples showing pyrite framboid diameters from 3.5 μm to 4.8 μm. Most of the diameters of the framboid microcrystals (Mean, d) are less than 0.4 µm. Therefore, according to the (Mean, D) and the (Mean, d), the pyrite framboids can be divided into three sizes: syngenetic framboids (SF, D < 4.8 µm, d ≤ 0.4 µm), early diagenetic framboids (EDF, D > 4.8 µm, d > 0.4 µm) and late diagenetic framboids (LDF, D < 4.8 µm, d > 0.4 µm). Additionally, box-and-whisker charts of the diameter, standard deviation/skewness value of the mean diameter of pyrite framboids (Mean, D) and the ratio of trace elements indicate that the sedimentary water body was a euxinic–dysoxic environment. Euxinic conditions dominated the Wufeng Formation to the lower part of the Long11-3 section, which is beneficial for the preservation of organic matter. However, the middle-upper part of the Long13-Long12 sub-member is a dysoxic sedimentary environment.

Pyrite is one of the important components of shale and plays a crucial role in shale gas enrichment. However, currently there are just a few studies on this subject matter. Therefore, the characteristics of pyrite in organic-rich shale section of the Longmaxi Formation and its impact on shale gas enrichment was studied in this paper by using outcrops, drilling cores, thin sections and test data. Result shows that pyrite occurred in different forms (macro-micro scale) in the Longmaxi Formation in the southeast Sichuan Basin. The formation and content of pyrite has a close relation with TOC content. Pyrite may catalyze the hydrocarbon generation of organic matter. Interparticle pores within the pyrite framboids and organic matter pores in the pyrite-organic matter complex are well-developed in the Longmaxi Shale, which serves as a major reservoir space for shale gas. Pyrite can promote shale gas enrichment by absorbing shale gas on its surface and preserving free gas in the interparticle pores and organic matter pores. In addition, as a kind of brittle mineral, pyrite can improve the brittleness of shale reservoir and increase the micro-nano pore system in shale reservoir, thereby improving the transmission performance of shale reservoir and boosting shale gas recovery.

Determination of gas adsorption capacity under geological conditions is essential in evaluating shale gas resource potential. A quantitative determination of gas adsorption capacity was proposed through 1) investigating controlling geological factors (including both internal ones and external ones) of gas adsorption capacity in organic-rich marine shale with geochemical analysis, XRD diffraction, field-emission scanning electron microscopy, and methane sorption isotherms; 2) defining the relationship between gas adsorption capacity and single controlling factor; 3) establishing a comprehensive determination model with the consideration of all these controlling factors. The primary controlling factors of the sorption capacity for the studied O3w-Lower S1l shale are TOC, illite and quartz, temperature, pressure, Ro, and moisture (water saturation). Specifically, TOC, thermal maturity, illite, and pressure are positively correlated with sorption capacity, whereas, quartz and temperature contribute negatively to the sorption capacity. We present the quantitative model along with application examples from the Wufeng-Lower Longmaxi Shale in the southeast Sichuan Basin, west China, to demonstrate the approach in shale gas evaluation. The result shows that the comprehensive determination model provides a good and unbiased estimate of gas adsorption capacities with a high correlation coefficient (0.96) and bell-shaped residues centered at zero.

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.

Fractures are important for shale-gas reservoirs with low matrix porosity because they increase the effective reservoir space and migration pathways for shale gas, thus favouring an increased volume of free gas and the adsorption of gases in shale reservoirs, and they increase the specific surface area of gas-bearing shales which improves the adsorption capacity. We discuss the characteristics and dominant factors of fracture development in a continental organic matter-rich shale reservoir bed in the Yanchang Formation based on observations and descriptions of fracture systems in outcrops, drilling cores, cast-thin sections and polished sections of black shale from the Upper Triassic Yanchang Formation in the SE Ordos Basin; detailed characteristics and parameters of fractures; analyses and tests of corresponding fracture segment samples; and the identification of fracture segments with normal logging. The results indicate that the mineral composition of the continental organic-matter-rich shale in the Yanchang Formation is clearly characterized by a low brittle mineral content and high clay mineral content relative to marine shale in the United States and China and Mesozoic continental shale in other basins. The total content of brittle minerals, such as quartz and feldspar, is c. 41%, with quartz and feldspar accounting for 22% and 19% respectively, and mainly occurring as plagioclase with small amounts of carbonate rocks. The total content of clay minerals is high at up to 52%, and mainly occurs as a mixed layer of illite-smectite (I/S) which accounts for more than 58% of the total clay mineral content. The Upper Triassic Yanchang Formation developed two groups of fracture (joint) systems: a NW–SE-trending system and near-E–W-trending system. Multiple types of fractures are observed, and they are mainly horizontal bedding seams and low-dip-angle structural fractures. Micro-fractures are primarily observed in or along organic matter bands. Shale fractures were mainly formed during Late Jurassic – late Early Cretaceous time under superimposed stress caused by regional WNW–ESE-trending horizontal compressive stress and deep burial effects. The extent of fracture development was mainly influenced by multiple factors (tectonic factors and non-tectonic factors) such as the lithology, rock mechanical properties, organic matter abundance and brittle mineral composition and content. Specifically, higher sand content has been observed to correspond to more rapid lithological changes and more extensive fracture development. In addition, higher organic matter content has been observed to correspond to greater fracture development, and higher quartz, feldspar and mixed-layer I/S contents have been observed to correspond to more extensive micro-fracture development. These results are consistent with the measured mechanical properties of the shale and silty shale, the observations of fractures in cores and thin-sections from more than 20 shale-gas drilling wells, and the registered anomalies from gas logging.

The Jiaoshiba shale gas field and Pengshui area are both located in the southeast Sichuan Basin with only 100 km apart. Although these two areas obtained shale gas flow from the Lower Silurian Longmaxi marine shale, there is a huge difference of production in these two areas where the Jiaoshiba shale gas field is more productive than the Pengshui area. In order to figure out the reason that caused this difference, this study analyzed the Longmaxi marine shale in these two areas by using drilling cores, and mineralogical and geochemical data. The results show that the Jiaoshiba shale gas field has a higher quality shale reservoir (higher content of quartz, higher porosity, higher permeability and higher TOC (Total Organic Carbon)) in the Lower Silurian Longmaxi Formation than that of the Pengshui area. The Lower Silurian Longmaxi marine shale in the Jiaoshiba shale gas field contains a higher total gas content (3.65 m3/ton in average) than that of the Pengshui area (1.19 m3/ton in average). Through structure analysis, this study found that the Jiaoshiba shale gas field and the Pengshui area located in the different tectonic units. The Jiaoshiba shale gas filed is located in the East Sichuan fold belt (wide spaced anticlines area) at the west side of the Qiyue mountain fault where less fractures are developed and the pressure coefficient is more than 1.5, resulting in much more free gas preserved in the shale reservoirs in the Lower Silurian Longmaxi Formation (the ratio of free gas content and absorbed gas content is 1.6), while the Pengshui area is located in the Hunan-Hubei-Guizhou thrust belt area at the east side of the Qiyue mountain fault where much more fractures are developed and the pressure coefficient is less than 1.0, leading to less free gas preserved in the Longmaxi marine shale (the ratio of free gas content and absorbed gas content is 0.6), that is why the shale reservoir in the Pengshui area has a lower total gas content than that in the Jiaoshiba shale gas field.