Western Sichuan Basin Research Articles

Gradient variations of formation mechanisms and sources of PM1 at the steep slope from western SiChuan Basin to eastern Tibetan Plateau

Vertical distributions of chemical components of particulate matter (PM) are essential for better understanding the climate, environmental and health effects. The steep slope from western SiChuan Basin (SCB) to eastern Tibetan Plateau (TP) provides a good platform for obtaining the gradient variations of PM chemical components. Daytime and nighttime PM1 (particulate matter smaller than 1 μm) samples were collected with the medium-volume sampler at six sites with elevation ranging from 500 m to 3500 m (Chengdu, Sanbacun, Wenchuan, Lixian, Maerkang and Hongyuan). The secondary inorganic ions and carbonaceous aerosols were the largest contributor to PM1 concentrations. The chemical components from the anthropogenic sources existed strong stratification with high concentrations inside the basin, while primary natural ions showed little discrepancy among the sites. The concentrations of primary inorganic ions from anthropogenic sources were much higher at nighttime than daytime, which was contrary to the diurnal cycle of secondary inorganic ions. Spatial heterogeneity of PM chemical components was large between basin and plateau sites, especially for NO3− and NH4+, large depending on season and daylight. The excess NH4+ concentrations existed in spring, summer and fall, while SO42− and NO3− cannot be completely neutralized by NH4+ in winter. The proportion of secondary formation in all sources significantly increased from about 10 % to 30 %–40 % with the increased elevation, while the contribution of motor vehicles declined from western SCB to eastern TP. This study will fill the scarce observations of PM chemical components at the sloped terrain and deepen the understanding of formation mechanism of heavy pollution inside the basin.

Differences in the Virulence Between Local Populations of Puccinia striiformis f. sp. tritici in Southwest China.

The virulence analysis of Puccinia stiiformis f. sp. tritici (Pst), the cause of wheat stripe rust, is essential for predicting and managing the disease epidemic in Southwest China, where the wheat cultivation has significantly reduced in the past few decades due to the impact of this disease. From 2020 to 2021, 196 Pst isolates were collected from Guizhou, Yunnan, and Sichuan. The virulence and race assessments were conducted using Chinese differential genotypes. Additionally, the resistance expression of 102 wheat lines was evaluated in 2021 in two disease nurseries located in Ningnan and Jiangyou. All the 45 Pst isolates from Guizhou and Yunnan belonged to pathogroup Hybrid 46, with 36 identified as race CYR32. Among the 69 isolates from the Liangshan Prefecture, 67 belonged to the Hybrid 46 group, while the remaining two were identified as race CYR34 in the G-22 group. Furthermore, all 79 isolates from the western Sichuan Basin belonged to the G-22 group, with 54 identified as race CYR34. The diversity indices of the Pst populations from Guizhou, Sichuan, and Yunnan exhibited a sequential decline. Virulence variation among the Pst populations from Yunnan, Guizhou, and the Ganzi-Liangshan region was minimal; however, significant virulence differences were observed when these populations were compared to those from the western Sichuan Basin. Results from disease nurseries indicated that Pst virulence was notably stronger in Ningnan compared to that in Jiangyou. The Sichuan Basin exhibits a notable diversity in Pst virulence, coupled with a more frequent genetic exchange occurring between the Liangshan Prefecture and the Yunnan-Guizhou Plateau. This information is essential for developing effective management strategies to mitigate the impact of wheat stripe rust in this region.

Chronology and geofluids characteristics of calcite cement in the Upper Triassic Xujiahe Formation tight sandstone reservoir, Western Sichuan Basin, SW China

Geological fluid activities significantly influence diagenesis and hydrocarbon accumulations in reservoir rocks. However, it remains challenging to precisely determine the timing of these activities. This study aims to investigate the chronology and geofluids characteristics of calcite cement that leads to reservoir tightness in the Upper Triassic Xujiahe Formation in the Western Sichuan Foreland Basin, which is a critical exploration target for hydrocarbon resources. We examine calcite cement as a marker of fluid activities, employing petrological analyses, U-Pb dating, and in-situ carbon-oxygen isotope analysis. These methods are integrated with simulations of regional burial and thermal histories to assess pore evolution and hydrocarbon accumulation mechanisms in these tight sandstone reservoirs. Results reveal two main calcite cementation stages within the second member of the Xujiahe Formation (Xu-2 Member): early cements from the Middle Jurassic around 175 ± 11 Ma and late cements from the Late Jurassic between 161 and 156 Ma. Initially, basin-derived alkaline fluids and near-surface atmospheric freshwater supplied the calcium and carbon for early cements during ongoing basin subsidence. Calcium and carbon for the late calcite cements came from the dissolution of carbonate rock fragments by organic acids. The heavier δ13C is primarily associated with in-situ generation and consumption of carbon within the system, while the δ18Ofluid, which is close to modern seawater values, is mainly related to active water-rock reactions induced by processes such as the dissolution of carbonate rock fragments by organic acids. The closed diagenetic system due to calcite cementation in the Late Jurassic led to reservoir tightening. Therefore, basin-scale chronological constraints on calcite cement are of great significance for quantitatively studying sandstone reservoir densification and exploring the mechanism and control factors of hydrocarbon accumulation.

Distributary channel type and high‐quality reservoirs in tight sandstone—A case study on the outcrops and reservoirs of Xujiahe formation in Western Sichuan Basin

Distributary channel type and high‐quality reservoirs in tight sandstone—A case study on the outcrops and reservoirs of Xujiahe formation in Western Sichuan Basin

Differential controlling on the deep tight sandstone reservoirs: Insight from the second member of lower Triassic Xujiahe Formation in Xinchang area, western Sichuan basin, China

With advancements in deep exploration, the deep tight sandstone gas reservoir has become a significant exploration field. However, it remains challenging to develop on a large scale due to the unclear distribution of relatively high-quality reservoirs. In this paper, the petrology, reservoir properties, diagenesis, and structural fracturing of deep tight sandstone reservoirs are systematically studied, focusing on the second member of the Upper Triassic Xujiahe Formation (T3x2) in the Xinchang area, and the types of relatively high-quality reservoirs and their differential controlling are further clarified. According to the matching relationship between pores and fractures, tight sandstone reservoirs can be classified into four types: extremely tight, fractured, porous, and pore-fractured types. Among these, the porous and pore-fractured types are considered effective reservoirs. The formation of tight sandstone reservoirs is closely related to sedimentary microfacies, grain size, diagenesis and tectonic fracturing, with distinct controlling differences across reservoir types. Overall, sedimentary microfacies provide the foundation, while differential diagenesis and tectonic fracturing are the key factors influencing reservoir quality. Among them, the extremely tight sandstone reservoirs can form in various sedimentary microfacies, particularly in medium to fine, lithic-rich sandstones, where strong compaction and cementation are the main factors for the underdevelopment of reservoir space. In contrast, fractured reservoirs mainly form based on porous reservoirs through the superimposition of tectonic fracturing. The porous reservoirs are typically found in relatively high-energy environments such as distributary channels and mouth bars, with medium to coarse feldspar-rich sandstone. Dissolution and chlorite-liner cementation are the key factors for their pore formation. Similarly, pore-fractured reservoirs originate from porous reservoirs that have been further altered by superimposing tectonic fracturing.

Layered massive fracturing of carrier beds commercially unveils the deep tight sandstone gas reserves in Sichuan Basin

Achieving commercial production from deep tight gas formations poses challenges to existing geological and engineering technologies. Xujiahe tight sandstone gas, deeply buried in the Western Sichuan Basin, is proven with enormous reserves; however, preliminary hydraulic fracturing trials mostly failed to bring up the productivity. From the angle of incorporating geological and engineering knowhow, we first adopted the carrier beds theory to distinguish the favorable zones in Xujiahe Formation, then a dual sweet spot identification criterion was established based upon analytic hierarchy process, which links major geological and engineering parameters together. Provided the identified sweet spot, hydraulic fracture conductivity was then optimized to maximize the productivity. Fracturing operational parameters consisting of fluid volume and pumping, proppant size and addition, and fluid formula were then screened to secure high conductivity. Implementation of this updated geological-engineering technical route in exploiting Xujiahe tight gas reservoirs turns out to be a great success with an average gas rate of 130 Mm3/d/well during the early production stage in 14 wells. Electromagnetic monitoring further demonstrated the effectiveness of precise massive fracturing by visualizing the morphology of fracture clusters. Comparative productivity analyses manifested the breakthrough of this technical route over preceding ones in Xuejiahe gas reservoirs, providing a superior reference for discovering and developing deep tight gas resources around the world.

Characteristics and Control Factors of a High-Quality Deeply Buried Calcareous Sandstone Reservoir, the Fourth Member of the Upper Xujiahe Formation in the Western Sichuan Basin, China

A special type of sandstone in which carbonate rock fragments (CRFs) dominate the composition developed in the Upper Triassic Xujiahe Formation’s fourth member (Xu4) in the western Sichuan Basin, known as calcareous sandstone. Calcareous sandstones are widely distributed in the western Sichuan and is the main production target of tight sandstone gas in the Sichuan Basin. In this study, thin sections, porosity–permeability testing, scanning electron microscopy, and X-ray diffraction are applied to examine the characteristics and control factors for high-quality reservoirs in the calcareous sandstones, with a view to providing guidance for natural gas exploration and development in calcareous sandstones. The results show that the calcareous sandstone belongs to litharenite, with an average framework grain composition of 30% quartz, 1% feldspar, and 69% rock fragments, while the Xu4 sandstone has a high quartz content (average content of 71%). Primary intergranular pores are the main storage space, and the reservoir quality is quite poor. Under the influence of different parent rock properties of sandstones, there are obvious differences in the composition of framework grains between the calcareous sandstone and the ordinary Xu4 sandstone, which in turn affects the reservoir storage space, diagenesis, and reservoir quality. High-energy depositional conditions, low content of late cements, and the development of fractures are the main controlling factors for the formation of high-quality reservoirs in Xu 4 calcareous sandstones.

Development characteristics of natural fractures in tight sandstone reservoirs and their controlling factors: upper Triassic Xujiahe Formation, western Sichuan Basin

Development characteristics of natural fractures in tight sandstone reservoirs and their controlling factors: upper Triassic Xujiahe Formation, western Sichuan Basin

Solid bitumen Rhenium-Osmium (Re–Os) isotope geochronology and existing problems: Sampled of Sinian-Cambrian gas reservoir in Sichuan Basin, China

Understanding the key timings related to petroleum evolution is crucial for optimizing exploration targets and assessing oil/gas resources, attract petroleum geologists’ attention worldwide. Recently, hydrocarbon (oil and bitumen) Re–Os isotope dating has been innovatively applied to constrain the timing related to oil/gas generation, however, the resulting Re–Os isochron ages can be complex and challenging to interpret. This study utilizes various geochemical and geochronological data from Sinian to Cambrian natural gas reservoirs in the Sichuan Basin to reconstruct the hydrocarbon evolution process and discuss the significance of different bitumen Re–Os dating results. The gas accumulation in the Sinian-Cambrian reservoirs experienced four stages of evolution: (1) initial oil generation during the Ordovician to Silurian periods, (2) secondary oil generation during the Triassic period, (3) gas generation through thermal cracking of liquid oil from the Jurassic to Cretaceous periods, and (4) gas reservoir redistribution since the late Cretaceous. The Re–Os dates (ca. 485 Ma) of low maturity and biodegraded bitumen from the western Sichuan Basin record the oil generation during the Ordovician before the Caledonian tectonic event. The Re–Os dates (ca.184–128 Ma) of highly mature bitumen associated with MVT Pb–Zn deposits in northern Sichuan Basin provide insights into both liquid oil-cracking and thermochemical sulfate reduction (TSR) processes. The complex Re–Os dates (ca.414 Ma, ca.154 Ma) of highly mature bitumen from the central Sichuan Basin may represent different periods related to either oil or gas generation. Future studies should explore the genetic type, maturity, thermal cracking, or TSR degrees of bitumen to better understand the significance of Re–Os dates.

Impacts of sedimentary characteristics and diagenesis on reservoir quality of the 4th member of the Upper Triassic Xujiahe formation in the western Sichuan basin, southwest China

The 4th member of Xujiahe Formation (Xu4) in the western Sichuan Depression is a typical porous tight sandstone reservoir. Finding reservoirs with relatively high porosity controlled by sedimentation and diagenesis is crucial for successful tight gas exploration. In this study, the Xu4 sandstone was studied by diverse methods, including outcrop and core observation, logging data interpretation, thin section and cathodoluminescence examination, scanning electron microscopy analysis, and X-ray diffraction, to determine sedimentary and diagenetic characteristics and to discuss their impact on the sandstone reservoir quality. During the depositional period of Xu4, subaqueous distributary channel sandstone of braided river delta front developed extensively under controls of bidirectional provenance (LASP and SASP), mainly consisting of litharenite and sublitharenite, with low feldspar and high rock fragments volume, respectively. Strong compaction and carbonate cementation have largely destroyed primary porosity, while secondary pores constitute the main pore space in the Xu4 sandstone. The sediment provenance, sedimentary microfacies and A/S determine the clastic composition of sandstone, genetic type and thickness of sand bodies, furthermore affect the subsequent diagenetic variations. The larger the thickness of the sand body indicates a higher-energy depositional environment, coarser grain size, better sorting, and lower shaliness content of the sandstone during deposition period. This results in stronger resistance to compaction of the sandstone, which is conducive to the preservation of primary pores and dissolution of organic acids, resulting in better physical properties of sandstone. Among them, the physical properties of the sandstones of the Xu4s and Xu4x are better than those of the Xu4z; moreover, LASP's sandstone is superior to SASP. It is worth noting that subaqueous distributary channel sandstones with thickness exceeding 13 m in Xu4s and Xu4x from LASP represent favorable areas for high-quality reservoirs. The work can be used as a reference for the exploration and evaluation of similar tight sandstone gas in the world.

Differential enrichment mechanism of helium in the Jinqiu gas field of Sichuan Basin, China

Helium is extensively utilized in aerospace, healthcare, electronics, semiconductors, advanced science, and renewable energy sectors, playing a pivotal role in the advancement of human technology. Presently, helium is primarily extracted from natural gas as an associated resource. The cost of extraction decreases with an increase in the helium concentration within the gases. Therefore, understanding the mechanisms involved in helium generation, migration, and accumulation underground is vital. This article delves into the differential enrichment mechanism of helium in the Jinqiu gas field, situated in the Sichuan Basin of China, by analyzing the molecular and isotopic composition characteristics of the major gases and noble gases, employing gas dissolution and diffusion models. Compared with other Jurassic gas reservoirs in the Sichuan Basin, the Jinqiu gas field contains a significantly higher helium concentration, displaying unique enrichment patterns within its area. The majority of hydrocarbon gases originate from the local underlying Triassic Xujiahe Formation source rocks. Specifically, the western section of the Jinqiu field is primarily supplied by the higher maturity Xujiahe Formation source rocks located in the western Sichuan Basin. Notably, there is no significant contribution of mantle-derived volatile components in the Jinqiu field. Helium and argon are primarily radiogenic, whereas neon primarily originates from atmosphere. The good linear correlation between 4He and 20Ne suggests that their transfer is primarily facilitated through groundwater. The solubility calculation reveals that the helium exsolved from the in-situ pore water of the reservoir, due to stratigraphic uplift during the Himalayan movement, comprises only 0.04%–0.11% of the existing reserves. This suggests that gas desolvation plays a relatively minor role in helium enrichment in the Jinqiu field. Additionally, the calculation results of gas diffusion indicate that diffusion's contribution to helium enrichment in the Jinqiu gas field remains minimal. Finally, our analysis of methane reserve changes suggests that the combined effect of differential loss and dilution of hydrocarbon gases is the primary mechanism for helium's differential enrichment in the Jinqiu field. Notably, methane and carbon dioxide depletion may be a significant factor in helium enrichment within the crustal natural gas system.

Application of multicomponent seismic data to tight gas reservoir characterization: A case study in the Sichuan Basin, China

Combining PP and PS waves from multicomponent data is an effective hydrocarbon characterization strategy because these two wave types are sensitive to different subsurface properties. We develop a case study using multicomponent data to characterize tight sandstone gas reservoirs in the Jurassic Shaximiao Formation, the western Sichuan Basin, China. In the study area, with PP data it is difficult to distinguish sandstones, mudstones, and gas sand, whereas recently acquired multicomponent data indicate potential for the joint characterization of lithology, porosity, and gas saturation from the combination of PP and PS reflections. Sandstones and mudstones have small P-wave velocity contrasts and large S-wave velocity contrasts. Thus, PS data provide a more accurate description of sandstones, as a substantial proportion of sandstones are undetectable from PP sections. P impedance is sensitive to sandstone porosity variation, and seismic-predicted porosities based on impedance inversion are in good agreement with log-interpreted porosities. The ratio of P-wave velocity to S-wave velocity is sensitive to gas accumulation, and the ratios derived from joint PP-PS prestack inversion are determined to be better than from PP prestack inversion in terms of their agreements with logs and distinct gas-sand boundaries.

Diverse Behaviors of Feldspar Grains during Sandstone Diagenesis: Example from the Xujiahe Formation in the Western Sichuan Basin, China

Feldspar alteration is among the most important processes in clastic rocks during diagenesis, but uncertainty remains about the factors that control feldspar diagenesis under subsurface conditions. Hence, the Upper Triassic Xujiahe formation of the Western Sichuan Basin were examined by an integrated petrographic, mineralogical and geochemical approach to unravel the causes and effects of feldspar diagenesis, with implication for mass transfer and openness of the geochemical system. The sandstones at various depths demonstrate three distinct, separate diagenetic behaviors of detrital feldspar within a single formation including (1) the complete dissolution of both plagioclase and K-feldspar in the upper member; (2) conservation of abundant detrital feldspar grains with minor albitization or overgrowths within the lower member of depths greater than 5 km; and (3) complete disappearance of K-feldspar within the uppermost horizons of the lower member, while plagioclase have survived in significant amounts. The exceptional disappearance of K-feldspar is the result of selective dissolution of K-feldspar during burial, accompanied by illite cementation and substantial K transfer at a scale of tens of meters. It is apparent that the clay diagenesis in the overlying mudstones, rather than porewater chemistry, is the major control of the reactivity of K-feldspar in adjacent deeply buried sandstones.

Ozone pollution aggravated by mountain-valley breeze over the western Sichuan Basin, Southwest China

The impact of thermally driven mountain-valley breezes (MVB) on the atmospheric environment remains poorly understood, especially in ozone (O3)-polluted regions with complex underlying topography. To address this knowledge gap, we focused on the western Sichuan Basin (SCB), situated immediately east of the Tibetan Plateau (TP), which is considered susceptible to MVB coupled with severe O3 pollution in southwest China. We revealed the MVB driving diurnal O3 variations and meteorological mechanisms using surface observations and ERA5 reanalysis data. Local MVB days accounted for up to 47% of cases in the summers of 2015–2022. Driven by the MVB, the near-surface O3 concentrations increased by 8.8%, with 12.7% and 50.0% deterioration in the O3 light and moderate exceedance rates, respectively, on the western SCB edge. The daytime upslope valley breeze with 20% higher wind speed drove the westward transport of rich O3 and precursors from the upwind-polluted inner SCB towards its western edge, and the O3 photochemical production, followed by intensifying solar radiation and air temperature, gave rise to 14.8% of surface O3 concentrations over the western SCB edge. The nighttime downward mountain breeze with a 20% increase in wind speed could transport the rich O3 in the mountainous area to the basin edge, causing O3 levels to increase by 2.8%. In summary, we quantitatively assessed the impacts of MVB on changes in O3 concentrations and air quality along with its meteorological mechanisms, facilitating a comprehensive understanding of meteorological drivers in the atmospheric environment.

Geomechanical Response Characteristics of Different Sedimentary Hydrodynamic Cycles—Exampled by Xujiahe Formation of Upper Triassic, Western Sichuan Basin

This study delves into the geomechanical responses of different sedimentary hydrodynamic cycles in deep tight sandstone formations. Employing core observation and thin section analysis, we quantitatively identified and characterized bedding planes, sedimentary microfacies, and tectonic fractures. Then, the intricate relationships between various architectural interfaces and geomechanical parameters were elucidated. Subsequently, utilizing finite element numerical simulation software, in situ stress and fracture parameters were derived. By identifying a fracture facies zone correlated with the sedimentary hydrodynamic cycle and production data, our findings unveil several key insights: (1) Geomechanical parameters (Young’s modulus, Poisson’s ratio, brittleness index) exhibited noteworthy variations within the T3x2−5 sand group, indicative of weak elasticity and robust plasticity. (2) The effective distance, influenced by diverse reservoir architecture interfaces, displayed variability, with each transition between peak-valley-peak or valley-peak-valley pinpointed as a distinct sedimentary hydrodynamic cycle. (3) In environments characterized by strong sedimentary hydrodynamics (between two level 3 architecture interfaces), fractures with larger strike angles and lower dip angles were observed to be more prevalent. (4) Three significant fracture faces—level I, level II, and level III—were discerned within the study area. Notably, reservoirs associated with level III exhibited characteristics suggestive of medium porosity and permeability, indicative of a gas layer. By thoroughly understanding the geomechanical response characteristics of formations such as the Xujiahe Formation, it is possible to guide the exploration and development of energy resources such as oil and natural gas. This helps to improve the efficiency and safety of resource extraction, promoting the sustainable utilization of energy.

Modelling a foreland fold-and-thrust petroleum system: a Permian example in the NW Sichuan Basin, Southwest China

ABSTRACT Foreland thrust belts are globally significant areas for oil and gas exploration. These belts undergo unique tectonic activity, resulting in distinct variations in the accumulation of oil and gas. Studying the genetic evolution of deep oil and gas in these thrust belts is a crucial means of revealing the patterns of deep oil and gas accumulation. This study utilized a novel approach that combines reservoir geochemistry and basin simulation to determine the migration process of oil and gas in the foreland thrust fault belt of northwestern Sichuan Province. To achieve this goal, we established a thermal evolution model of lower Palaeozoic source rocks through organic chemical analysis. The balanced profile technique was used to determine the paleotectonic evolution pattern of the critical period in this area. Additionally, the mixed flow migration algorithm was employed to simulate the evolution of the Palaeozoic petroleum system. Through basin petroleum system modelling (BPSM), it was discovered that the bitumen found in the thrust nappe belt originated from the lower Cambrian strata. Furthermore, the gas reservoir in the hidden fault belt primarily migrated from the lower Cambrian strata and was also partially mixed with thermogenic gas from Permian source rocks. After establishing the genetic models of various tectonic belts in the study area, it became clear that there are notable differences in hydrocarbon accumulation within these belts. The BPSM results revealed that the oil present in the thrust nappe belt was generated in Lower Cambrian source rocks through early Indosinian hydrocarbon expulsion and continuous charging. The gas found in the thrust nappe belt was produced through the process of oil cracking in the deep reservoir located in the hidden front belt during the late Indosinian. The depth of the thrust nappe belt has helped to preserve hydrocarbon reservoirs. The natural gas present in the hidden fault belt primarily originates from lower Cambrian source rocks, which are combined with gas generated by oil cracking in lower Permian source rocks after the Yanshanian. Therefore, hydrocarbons accumulate deeply in the footwall of the thrust fault belt. The BPSM offers a fresh approach to hydrocarbon genetic models of thrust fault zones in the western Sichuan Basin, and it is beneficial for accurately evaluating the potential for deep hydrocarbon resources in comparable geological settings.

Angle-dependent seismic attenuation-based gas-bearing detection of a sandstone river channel reservoir in the western Sichuan Basin

Abstract The shallow river channel sandstone reservoirs of Jurassic in the western Sichuan Basin are rich in natural gas. The gas-bearing sweet-spot has a ‘sausage-like’ distribution feature, with complex gas water distribution. Analyses of seismic data at different angles show that gas-bearing reservoir formation has different seismic attenuation features from the water-bearing formations, and such differences have a certain correlation with gas production. Accordingly, a gas-bearing detection technology based on angle-dependent seismic attenuation feature is proposed. First, the matching-pursuit time-frequency analysis method is used to extract high-resolution time-frequency spectra from seismic data at different incidence angles. Then, the angle-dependent seismic attenuation attribute is estimated using the extracted time-frequency spectra. Finally, the attribute is combined with the inverted impedance for gas-bearing detection. With the advantages of lower uncertainty and being less affected by reservoir porosity, the application results of the developed method have a high coincidence rate with the drilled wells, and the drilling wells deployed based on the detection results have achieved high production.

Progressive development of stylolites in cryptocrystalline quartz

Stylolites are common dissolution structures in carbonate rocks in the upper crust and document a coupled physical-chemical feedback during pressure solution in rocks. By applying high-resolution electron-based analysis we investigate a rare example of stylolites in cryptocrystalline quartz (novaculite) from western Sichuan Basin, China. The investigated stylolites exhibit an increased amplitude from the tip toward the center, which correlates with the maturity of stylolites and thus provides an excellent opportunity to infer the dynamic processes operating during stylolite formation. Microstructurally, stylolites are characterized by a concentration of biotite and hematite, dissolving irregular quartz grains, and the occurrence of dissolution porosity. By investigating the progressive co-variations between stylolite amplitude and microstructure characteristics from the stylolite tip toward the center, we demonstrate, for the first time, the evidence of stylolite growth through anticracks. It is inferred that stylolites in cryptocrystalline quartz originate from the propagation of mode I micro-cracks around pre-existing biotite, followed by pore-scale dissolution. Both processes, micro-cracking and dissolution, are interacting and co-evolving via a self-organization fashion during shortening. Our study contributes to the ongoing debate about whether stylolites are conduits or barriers for fluid flow.

Discovery of Pseudomorph Scapolite and Diagenetic Indication from the Permian Volcaniclastic Rocks in Western Sichuan (SW China)

Volcaniclastic rocks are important unconventional oil and gas reservoirs from which it is difficult to determine the protolith due to strong metasomatic alteration. Intensive alteration has occurred in much of western China, but few robustly documented examples are known from which to assess the alteration processes. Further recognition from the petrological and mineralogical record is essential to quantify the diagenetic environment, the degree of alteration, and its impacts. Permian volcanic rocks are widely developed in the western Sichuan Basin (southwestern China), with a thickness of more than 200 m. The thickness of volcaniclastic rocks in the Permian Emeishan Basalt Formation is up to 140 m, with a 5600~6000 m burial depth. In this study, we demonstrate an approach to recognizing hydrothermal alteration by the occurrence of scapolite megacryst mineral pseudomorphs (SMMPs) in Permian volcaniclastic rocks in the Sichuan Basin (southwestern China). The results show that SMMPs in the Permian volcaniclastic rocks in the western Sichuan Basin mainly occur in the lower part of the Permian basalts as intragranular minerals and rock inclusions. Scapolite is transformed into quartz and albite, and only the pseudomorph is preserved, indicating secondary hydrothermal fluid metasomatic alteration. Scapolite is formed after microcrystalline titanite and is the product of the high-temperature pneumatolytic metasomatism of plagioclase from the mafic protolith during the post-magmatic stage. The mixing of meteoric water and barium-rich hydrothermal fluid leads to the precipitation of barite; additionally, the pores are filled with barite and halite after the alteration of scapolite. The silicification and hydrothermal dissolution of scapolite and the albitization of sodium-rich matrix minerals increase the pore volume, which is conducive to the later recharge by hydrothermal fluids. The discovery of SMMPs can serve as an indicator of the high-temperature pneumatolytic metasomatism and mixing of meteoric water and deep hydrothermal fluid.

Roles of the Tibetan Plateau and Yunnan‐Guizhou Plateau in the Regional Extreme Precipitation Over Sichuan Basin in Summer: A Case Study

AbstractThe roles of the Tibetan Plateau (TP) and Yunnan‐Guizhou Plateau (YGP) in regional precipitation extremes over the Sichuan Basin (SCB) in summer under specific circulation background remain unclear. This study quantifies the impact of TP or YGP on the regional extreme precipitation event (REPE) which occurred during 04:00 Beijing time (BJT) on 11 August to 03:00 BJT on 13 August 2020 with a rainfall center located in the western SCB based on numerical experiments. Results show that the accumulated precipitation regionally averaged over the western SCB during the REPE can be reduced by 84% (51%) when the TP (YGP) is absent, and the decreased precipitation is mainly caused by the reduced stratus precipitation. Mechanism analysis indicates that relative to the control experiment including the TP and YGP, the absence of TP or YGP leads to reduced temperature over the SCB induced by anomalous cold advection from the upstream regions where the terrains are removed, and further results in much more stable stratification and thereafter weakens the ascending motions and reduces the stratus precipitation over the western SCB. On the other hand, the absence of TP or YGP induces an anomalous anti‐cyclone at 850 hPa over SCB, which further weakens both the uplift effect of terrain on airflow along the windward slope and water vapor transport over the western SCB and thereafter reduces the precipitation over this region. This study deepens the understanding of the topographic effect on regional extreme precipitation over SCB in summer from the thermodynamic and dynamic processes.