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The fluid–solid coupling effect is more pronounced in the process of deep coal seam development compared to shallow coalbed methane, exerting a greater influence on production, and cannot be disregarded. Throughout the extraction process, the interaction between effective stress and gas desorption triggers deformation within the coal seam, leading to dynamic changes in both porosity and permeability. This paper has developed a fully coupled gas flow and deformation model that contains the coal matrix and discrete fractures to describe the dynamic gas seepage behavior and deformation of deep coal seams within a coupled wellbore–hydraulic fractures–matrix system. The model's validity is corroborated through the examination of fracture aperture, employing the finite element numerical simulation capabilities of COMSOL Multiphysics. Subsequent to the model's validation, an in-depth investigation into the permeability and production variations under diverse parametric conditions is conducted. This analysis also encompasses the assessment of hydraulic fracture geometry's impact. The simulation outcomes reveal that the permeability alterations during coal seam development are subject to the counteracting influences of gas desorption and effective stress. Moreover, it is observed that an increase in the Langmuir volume strain constant and initial porosity correlates with enhanced production, whereas a diminution in the hydraulic fracture compression coefficient leads to increased cumulative production. Notably, the optimal production is attained when hydraulic fractures are oriented vertically yet asymmetrically relative to the horizontal well. [ABSTRACT FROM AUTHOR]

Abstract: Two sections of marine carbonate strata, Podu and Lengshuixi, from the Yangtze platform, South China, are investigated in terms of major geological events and carbon-strontium isotope responses. The results [...]

Based on field investigation and geochemical analysis, it is believed that the dolomites of the Longwangmiao Formation in the Tuhechang and Nanshanping sections, southeast Sichuan Basin, differ from other dolomites in the area, being mainly of reflux seepage origin. Tuhechang dolomite is of burial origin, and the dolomite in Nanshan-ping is of mixed-water origin. The Tuhechang dolomites occur in the lower part of the Longwangmiao Formation, and did not have the geological conditions suitable for forming reflux seepage dolomites. The temperature of inclusions of the Tuhechang dolomites shows a formation temperature higher than 105 °C. Its Mn element and Fe element content are relatively high (average 792.5x10-6 and 3 428.8x10-6 respectively), with good correlation. The average δ18O content is - 8.18‰, which is significantly lower than for dolomites in other sections of the study area. These all reflect a burial environment in the Tuhechang section. The main part of the Nanshanping section lies in a slope facies zone, and did not have the geological conditions for the formation of reflux seepage dolomite. Nanshanping dolomite contains an obviously low value for Na element, and δ18O is also obviously negative, both indicating that it was formed in a low-salinity fluid environment at a relatively low temperature. In addition, Nanshanping dolomite shows obvious U element enrichment, indicating that its formation was related to atmospheric fresh water, and it is therefore a mixed-water dolomite. These two dolomite types have relatively good physical properties, which is highly significant for reservoir development in the southeastern Sichuan Basin. [ABSTRACT FROM AUTHOR]

The in situ zircon U‐Pb‐Lu‐Hf isotope records from end‐Permian volcanic interlayers in southwest China, integrated with previous studies, restructure the evolutionary history of the Yangtze Craton from Precambrian to Late Paleozoic. This includes early continental crust formation before ∼3.0 Ga and massive juvenile crustal growth at 2.6–2.4 Ga; large‐scale crustal reworking at ∼2.1–1.7 Ga; Neoproterozoic crust addition at ∼1.1 to 0.7 Ga; collision and subduction along the craton margin between ∼700–541 Ma; Early Ordovician to Late Silurian magmatism; and large tectono‐thermal events in the Middle Carboniferous to end‐Permian. Some zircons with TMD2 ages from 4.40 to 4.01 Ga and lower initial 176Hf/177Hf values of 0.280592 to 0.280726 may imply the existence of Hadean crust relics beneath the Yangtze Craton and their provenances could be associated with Hadean crustal remelting. This study further clarifies that the Precambrian‐age zircons between the end‐Permian volcanic interlayers, the complexes in the western margin of the Yangtze Craton, and the sedimentary Kangdian Basin, may share an affinity based on similar U‐Pb age spectra and Hf isotope features. It also shows that the Neoproterozoic tectono‐thermal event may be associated with large‐scale tectono‐rifting activity, which is different from the Grenville‐age continental collision between Yangtze and Cathaysia blocks in South China. The above findings support the inference of a widespread Archean basement extending to the western Yangtze Craton and a provenance in the Kangdian Basin that is derived from the weathering and erosion of Paleoproterozoic continental crust. [ABSTRACT FROM AUTHOR]

The pore-throat size determines the oil and gas occurrence and storage properties of sandstones and is a vital parameter to evaluate reservoir quality. Casting thin sections, field emission scanning electron microscopy, high-pressure mercury injection and rate-controlled mercury injection are used to qualitatively and quantitatively investigate the pore-throat structure characteristics of tight sandstone reservoirs of Xiaoheba Formation in the southeastern Sichuan Basin. The results show that the pore types include intergranular pores, intragranular dissolved pores, matrix pores, intercrystalline pores in clay minerals, and microfractures, and the pore-throat sizes range from the nanoscale to the microscale. The high-pressure mercury injection testing indicates that the pore-throat radius is in range of 0.004–11.017 µm, and the pore-throats with a radius >1 µm account for less than 15%. The rate-controlled mercury injection technique reveals that the tight sandstones with different physical properties have a similar pore size distribution (80–220 µm), but the throat radius and pore throat radius ratio distribution curves exhibit remarkable differences separately. The combination of the high-pressure mercury injection and rate-controlled mercury injection testing used in this work effectively reveals the total pore-throat size distribution in the Xiaoheba sandstones (0.004–260 µm). Moreover, the radius of the pore and the throat is respectively in range of 50–260 µm and 0.004–50 µm. The permeability of the tight sandstones is mostly affected by the small fraction (<40%) of relatively wide pore-throats. For the tight sandstones with good permeability (>0.1 mD), the larger micropores and mesopores exert a great influence on the permeability. In contrast, the permeability is mainly influenced by the larger nanopores. Furthermore, the proportion of narrow pore-throats in tight sandstones increases with reducing permeability. Although the large number of narrow pore-throat (<100 nm) makes a certain contribution to both reservoir porosity and permeability, they have contribution to the former is far more than to the latter. [ABSTRACT FROM AUTHOR]

A mature understanding of the sandstone gas reservoir in the Xiaoheba Formation in the southeastern Sichuan Basin remains lacking. To assess the reservoir characteristics and the origin of the high-quality reservoir in the Xiaoheba Formation, this paper uses systematic field investigations, physical property analysis, thin section identification, scanning electron microscopy and electron microprobe methods. The results indicate that the Xiaoheba sandstone is an ultra-tight and ultra-low permeability reservoir, with an average porosity of 2.97% and an average permeability of 0.56×10−3 μm2. This promising reservoir is mainly distributed in the Lengshuixi and Shuangliuba regions and the latter has a relatively high-quality reservoir with an average porosity of 5.28% and average permeability of 0.53×10−3 μm2. The reservoir space comprises secondary intergranular dissolved pores, moldic pores and fractures. Microfacies, feldspar dissolution and fracture connectivity control the quality of this reservoir. The relatively weak compaction and cementation in the interbedded delta front distal bar and interdistributary bay microfacies indirectly protected the primary intergranular pores and enhanced late-stage dissolution. Late-stage potassium feldspar dissolution was controlled by the early-stage organic acid dissolution intensity and the distance from the hydrocarbon generation center. Early-stage fractures acted as pathways for organic acid migration and were therefore important factors in the formation of the reservoir. Based on these observations, the area to the west of the Shuangliuba and Lengshuixi regions has potential for gas exploration. [ABSTRACT FROM AUTHOR]

The remains of trilobites and other species were unexpectedly discovered in a volcanic ash layer beneath the Permian–Triassic Boundary (PTB). Based on a biostratigraphic investigation of the Zhongliangshan section in Chongqing, South China, the quantity of the species gradually decreased with subsequent volcanism. This finding provides an opportunity to further understand the disappearance of trilobites and the evolution of the mass extinction event. The temporal coincidence between the volcanic eruption event and the loss of trilobites and other species supports the idea of a cause-and-effect relationship. The species remains in the ash bed appeared before the disappearance ofClarkinayiniand the climax of the negative carbon isotope excursion, which implies that the onset of the mass extinction occurred at the end-Permian. The explosive volcanic events caused massive releases of CO2, toxic gases and volcanic ash and resulted in loss of habitat for certain species in the Tethys domain. This phenomenon may have led to the abrupt death of trilobites and the catastrophic collapse of biodiversity. [ABSTRACT FROM AUTHOR]

Well-documented outcrops around the Emeishan Large Igneous Province (ELIP) in South China, eastern Tethys, encompassing the end-Guadalupian mass extinction have been investigated. Correlatable sections recording the event exhibit very similar lithological characters, positive-then-negative C isotope excursions and massive biotic demise. Detailed analyses of the fossil record and carbon isotopic variations were carried out on the Guadalupian–Wuchiapingian Boundary sections over the inner, middle, outer zones of the ELIP and its margin. Due to a pronounced decrease in marine habitat area and the environmental and ecological change over this part of the Tethys, the biota crisis records show the loss of numerous tropical invertebrate taxa, and exhibit fewer genera and smaller testing sizes and low productivity. The biota crisis was a sustainable and gradual reduction in diversity over the Capitanian. The associated carbon isotopic data reveal unusually high δ13C(carb) values before the late Capitanian, representing higher primary productivity (or buried rate) and more13C-enriched CO2released by hydrothermal carbonate breakdown from the upper crust into the sediments at that time. Subsequently, an accelerated negative excursion across the boundary and the gradual excursion with low carbon isotope amplitude favours an increased influx of light12C sourced by the volcanism around the eastern Tethys. The very similar time–space relation between the biota crisis and the Emeishan volcanism confirms that volcanic eruptions may have triggered the biota crisis event in South China. Intensive volcanism could result in detrimental environmental and ecological stresses, habitat loss, organic material splitting, or the emission of light carbon and thermal fluid (or aerosol), implying that the losses of the shallow-marine invertebrates either occurred geologically instantaneously or in a series of closely spaced crises coinciding with the initial phase of ELIP formation. These findings in South China may reveal the causal relation between mass extinctions and LIPs in a global context. [ABSTRACT FROM AUTHOR]

Shale has been considered as good gas reservoir due to its abundant interior nanoscale pores. Thus, the study of the pore structure of shale is of great significance for the evaluation and development of shale oil and gas. To date, the most widely used approaches for studying the shale pore structure include image analysis, radiation and fluid invasion methods. The detailed pore structures can be studied intuitively by image analysis and radiation methods, but the results obtained are quite sensitive to sample preparation, equipment performance and experimental operation. In contrast, the fluid invasion method can be used to obtain information on pore size distribution and pore structure, but the relative simple parameters derived cannot be used to evaluate the pore structure of shale comprehensively and quantitatively. To characterize the nanoscale pore structure of shale reservoir more effectively and expand the current research techniques, we proposed a new method based on gas adsorption experimental data and the method of moments to describe the pore structure parameters of shale reservoir. Combined with the geological mixture empirical distribution and the method of moments estimation principle, the new method calculates the characteristic parameters of shale, including the mean pore size (), standard deviation (σ), skewness (Sk) and variation coefficient (c). These values are found by reconstructing the grouping intervals of observation values and optimizing algorithms for eigenvalues. This approach assures a more effective description of the characteristics of nanoscale pore structures. Finally, the new method has been applied to analyze the Yanchang shale in the Ordos Basin (China) and Longmaxi shale from the Sichuan Basin (China). The results obtained well reveal the pore characteristics of shale, indicating the feasibility of this new method in the study of the pore structure of shale reservoir. [ABSTRACT FROM AUTHOR]

Recently, a series of high-magnitude earthquakes, including the 2008 Wenchuan earthquake (M 8.0), have occurred in the Longmenshan tectonic region of southwestern China. Many researchers are currently examining the geological aspects of the activity, but little attention has been given to the huge travertine deposits along large fault zones. Carbon isotopes were measured in 106 samples, and the results indicate that the travertine originated from 13 C-enriched CO 2 sourced from deep reservoirs and can be characterized by notable positive δ 13 C values. CO 2 is thought to have originated from isotopic equilibrium fractionation under thermal metamorphism temperature conditions (within 350–850 °C). The re-equilibrium fractionation between the deep CO 2 and the dissolved carbonate composition during travertine formation is due to the declining temperature of thermo-genetic CO 2 , which in turn leads to higher δ 13 C carb values. Based on oil drilling data and geochemical thermometry modeling, the depth of the 13 C-enriched CO 2 source is inferred to be below 4643 m, which coincides with the measured focal depth of the Wenchuan earthquake and subsequent aftershocks. The variation of tectonic activity intensity and the quantity of CO 2 from deep formations are mainly responsible for the δ 13 C fluctuations observed in the travertine. The δ 13 C values in travertine deposits have increased dramatically, which implies the rapid upwelling of the CO 2 -bearing fluid, the enhancement of thermal metamorphism and the continuous increase of the internal pressure in deep reservoirs. These findings may contribute to a better understanding of the deep seismic activity and high-frequency aftershocks in the Longmenshan region. [ABSTRACT FROM AUTHOR]

In this paper, the genesis of dolomite in Longwangmiao Formation is analysed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ13C value, and higher δ18O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca2+ was consumed, resulting in an increase in the Mg2+/Ca2+ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata. • The highlights of this article are as follows: • This paper provides a case study of terrigenous clastic dolomite. • This paper analyses the genesis of the dolomites of Longwangmiao Formation and establishes a dolomitization model. • We analysed a large number of samples and provided basic geochemical data for Longwangmiao Formation. [ABSTRACT FROM AUTHOR]

• Mid-Permian strata in South China are characterized by high positive δ13C carb values. • Decarbonation during tectono-thermal events is the mechanism for 13C enrichment. • Emplacement causing regression and doming was responsible for δ13C carb fluctuations. During the Permian, Emeishan mantle plume activity created a large igneous province and deeply impacted carbon isotope records in marine carbonate strata in South China. To evaluate the isotopic variations caused by this catastrophic event, 140 carbonate samples from four sections were collected and analyzed. The δ13C carb values in the upper section of the Qixia Formation gradually increased, stabilized at ~4.0‰ in the lower-middle sections of the Maokou Formation, and then abruptly decreased in the upper section of the Maokou Formation. This isotopic evolution trend is likely related to mantle plume geochemical processes occurring during these respective stages. Large amounts of upwelling heat from the emplacement of the mantle plume triggered large-scale decarbonation reactions in the continental crust resulting in CO 2 production enriched in 13C. Substantial amounts of 13C-enriched CO 2 directly affected the isotope compositions of marine carbonates during this time. Temporal and spatial fluctuations in carbon isotopes were also controlled by terrestrial materials as a result of regression and doming caused by plume emplacement. The abrupt decrease in δ13C carb in the upper section of the Maokou Formation reflects the onset of volcanism, resulting in the breakdown of organic materials. These findings emphasize the contribution of a carbon reservoir (with a thermal decarbonation origin) in the upper crust, particularly against the Permian tectono-thermal background. This mechanism, which is similar to biological isotope fractionation in the carbon cycle, should not be overlooked. [ABSTRACT FROM AUTHOR]