Eastern Sichuan Research Articles

Triassic in the eastern Sichuan basin ray slope and badong group strata division and correlation

Triassic strata are widely developed in the eastern part of Sichuan Basin. The Mingyuexia anticline and Huangnitang anticline located in the eastern Sichuan structural belt develop Triassic Jialingjiang Formation to Xujiahe Formation. The middle Triassic Leikoupo Formation and Badong Formation on both sides of the anticline are synchronous and heterogeneous strata. They are distinguished regionally according to their different sedimentary environments and lithologic characteristics, and the phase transition boundary between Leikoupo Formation and Badong Formation is determined.

Natural Gas Characteristics and Gas-Source Comparisons of the Lower Triassic Jialingjiang Formation, Eastern Sichuan Basin

Natural Gas Characteristics and Gas-Source Comparisons of the Lower Triassic Jialingjiang Formation, Eastern Sichuan Basin

A target-oriented integrated workflow for seismic delineation of thin tight reservoirs in the eastern Sichuan Basin, China

Complex lithology association along vertical direction results in thin layers with different interbed patterns of the Lower Triassic Jialingjiang Formation (T 1 j), Sichuan Basin. The prediction of thin tight reservoirs in the Jialingjiang Formation is important for further oil and gas exploration. However, seismic data in the eastern Sichuan Basin has poor lateral continuity of seismic events because of strong interference noises, which makes it difficult to pick horizons. Moreover, low dominant frequency (<25 Hz), low seismic resolution, and weak response characteristics of strong transverse heterogeneity on logging parameters make it challenging for the thin reservoirs prediction. To tackle these issues, in this study, we propose a target-oriented workflow by integrating seismic data processing with interpretation to delineate the thin reservoirs and corresponding hydrocarbon production capacity. In the processing workflow, we first remove the noise interferences using the structure-oriented filtering (SOF), which can improve the effective information of seismic events and also preserve the features of fault structures. Second, we propose a novel method based on spectral shaping and sparse inversion to enhance the seismic resolution. Tests on filed data demonstrate that the effective information and resolution of processed seismic data are significantly enhanced. Then, in the seismic interpretation, fluid mobility and spectral decomposition are first used to delineate the spatial distribution of favorable reservoirs. Furthermore, multi-attribute integrated interpretation involving impedance analysis, reservoir thickness estimation and porosity inversion is used to evaluate the storage capacity and productivity of the thin reservoirs. The estimated impedance, reservoir thickness and porosity show good agreement with well information, demonstrating the feasibility of the proposed method for accurately predicting thin reservoirs in this area. Multi-attribute integrated interpretation revealed that the favorable thin reservoirs are mainly developed from the northeast to the southwest in the study area, which can provide effective guidelines for future production well drilling and distribution delineations of a favorable reservoir in the Eastern Sichuan Basin. • Structure-oriented filtering (SOF) can remove noises and enhance the effective continuities and structural features. • A new resolution enhancement (RE) method improves the seismic resolution significantly and reduces the artifacts. • The target-oriented processing workflow can markedly improve the quality of the seismic data. • Multi-attribute analyses indicating the favorable thin reservoirs is developed for the target zone.

Diagenetic evolution and its impact on reservoir porosity in Lower Triassic Jialingjiang Formation in eastern Sichuan Basin, SW China: Evidence from petrography, trace elements, and Isotopic Studies

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13CVPDB values of −3.4–+7.3 ‰ and δ18OVPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm)N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34SV-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13CVPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18OVPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34SV-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity.

3‐D Crustal Azimuthal Anisotropy Reveals Multi‐Stage Deformation Processes of the Sichuan Basin and Its Adjacent Area, SW China

AbstractThe collision of the South China Block and the North China Block and the eastward extrusion of the Tibetan Plateau materials have caused complicated tectonic deformation and strong seismicity around the Sichuan Basin (SCB). In order to reveal regional stress distribution and deformation mechanisms, we invert for a 3‐D high‐resolution shear wave velocity and azimuthal anisotropy model from ambient noise tomography with data from 483 broadband seismographs deployed around the SCB. Our results reveal strong lateral contrasts of azimuthal anisotropy in the crust. The western Sichuan depression shows weak deformation. The central Sichuan uplift, used to be a stable microcontinent, shows relatively strong and coherent fossil anisotropy. In contrast, the eastern Sichuan folds shows strong anisotropy, indicating intense regional deformation. The fast axis direction is bifurcated when reaching the Longmenshan fault, which indicates the obstruction of the SCB to the crustal low velocity materials beneath the Songpan‐Ganzi Block. In the east, the rigid SCB and strike‐slip faults may transform the compressive stress to shear deformation in adjacent areas and cause a ring‐shaped anisotropy pattern below 15 km depth. Furthermore, anisotropy patterns display strong contrast at shallow depths in the Weiyuan and Changning regions, which may facilitate the accumulation of strain and more likely induce earthquakes during the shale gas exploration stage. Our new model provides important constraints for understanding the multi‐stage deformation processes in and around the SCB.

Controls of interlayers on the development and distribution of natural fractures in lacustrine shale reservoirs: A case study of the Da'anzhai member in the Fuling area in the eastern Sichuan Basin

Lacustrine shale reservoirs with interbedded organic-rich shales and thinly layered tight shell limestones are mainly developed in the Da'anzhai Member of the Lower Jurassic Ziliujing Formation in the Fuling area. These reservoirs generally have low porosities and permeabilities. Natural fractures provide effective spaces for these reservoirs and significantly improve fluid flow capability. However, little research has been conducted on the genesis and distribution of fractures in the Da'anzhai Shale Member. This study aims to understand the features, distribution, and factors that influence natural fractures of these shale reservoirs in the study area. Based on field outcrop observations, core descriptions, and thin section analysis, four types of natural fractures are developed in the study area: tensile fractures, shear fractures, interlayer bedding fractures, and diagenetic shrinkage fractures. Most low-angle and horizontal natural fractures have lengths of 5 cm–10 cm, and the effective fracture density is 0.90 pieces/m. The lithology ( Y ), thickness of the rock layer ( H ), regional tectonic stress ( F ), and fault scale ( S ) have a significant impact on the development of natural fractures. Most natural fractures develop when there is a large number of laminations in thin rock layers. The large regional tectonic stress in the Late Himalayan, which had a large fracture density index, leads to the formation of natural fractures in numerical simulations. Large faults would have co-developed with more fractures than small faults due to the high energy of large faults. The relationship between the effective permeability and the four parameters (i.e., Y , H , F , and S ) was established. Based on this relationship, it is concluded that H is the most critical factor influencing the effective permeability of natural fractures in the study area. As a result, the middle to upper section in the 2nd subsection of the Da'anzhai Member, in which the lithofacies is thinly laminated shales with shells, can be considered to contain favourable shale gas layers. • Genesis and distribution of natural fractures in Da'anzhai Shale reservoir. • The thickness and frequency of interlayers and lamination mainly control on natural fracture intensity and penetration. • The thickness of interlayers is the most critical factor influencing the effective permeability of natural fractures. • The lithofacies of thin-shell lamination shales can be considered as favor shale gas layers.

Petrographic Characterization of Major Diagenetic Events in the Wufeng-Longmaxi Formations, Eastern Sichuan Basin, South China: Implications for Porosity Generation and Modification

Petrographic Characterization of Major Diagenetic Events in the Wufeng-Longmaxi Formations, Eastern Sichuan Basin, South China: Implications for Porosity Generation and Modification

Microscopic pore structure characteristics of tight limestone reservoirs: New insights from Section 1 of the Permian Maokou Formation, Southeastern Sichuan Basin, China

Recently, industrial gas flows have been produced from the vertical well acidification tests of Wells JS1 and YH1 in Section 1 of the Maokou Formation in the southern region of eastern Sichuan Province, China. This result proves that Section 1 contains a group of excellent source rocks and gas-bearing reservoirs. In this study, the pore structure characteristics and primary causes of the nanopore structure were explored using scanning electron microscopy, X-ray diffraction of total rocks, total organic carbon, liquid nitrogen adsorption, and other experimental methods to determine the pore development features of nodular limestone reservoirs in Section 1. The results substantiate the presence of complex pore structures in the nodular limestone reservoirs in Section 1. Nanopores are dominant and mainly comprise micropores and mesopores approximately 4 nm in diameter. The pore forms are particularly schistose clay silt pores consisting of rigid particles and flask-shaped pores. N 2 adsorption analysis showed that the specific surface area of the carbonate reservoir was 4.8842–8.1594 m 2 /g (mean = 6.0439 m 2 /g), and the pore volume was 0.008422–0.015098 cm 3 /g (mean = 0.043212 cm 3 /g). Mesopores were positively related to the total pore volume and total specific surface area, whereas macropore content had poorer correlations. Mesopores contribute more to the total pore volume and total specific surface area than macropores do. Spaces among the clay mineral layers (especially talcum gaps) predominantly contributed to the carbonate pore volume and specific surface area of the reservoir, and were positively related to the clay mineral content. The rocks have highly developed calcite pores, and their pore volumes and specific surface areas are negatively related to calcite pore content. Moreover, low organic content and low pore development degrees were observed. The organic content was weakly correlated with the pore volume and specific surface area, suggesting a low effect on reservoir performance. This assessment of the pore structure characteristics of Section 1 of the Maokou Formation yielded essential geological data that could inform the formulation of exploitation measures. • Identify the pore development features of nodular limestone reservoirs in Section 1 of the Permian Maokou Formation. • Explore the main factors that influence the pore structure. • Discussion on the relationship between pore structure and oil and gas resources.

Impact of climate change on wheat security through an alternate host of stripe rust

AbstractIn the 21st century, stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is still the most devastating disease of wheat globally. Despite the critical roles of the alternate host plants, the Berberis species, in the sexual reproduction and spread of Pst, the climate change impacts on the redistribution of Berberis plants, and their potential risk of incurring wheat stripe rust are scarcely discussed in the literature. This study evaluated the potential risk of Pst in wheat cultivation in Sichuan Province, China, for the first time, by analyzing the dynamics of Berberis plants and the change in Pst oversummering range under climate change scenarios. Ensembles of small models (ESMs) in combination with four algorithms were used to project the future (i.e., 2041–2060, 2061–2080, and 2081–2100) distributions of 54 Berberis species in Sichuan Province based on two shared socioeconomic pathways (SSP) scenarios. The species richness of Berberis was predicted to increase in northwestern Sichuan but decreased in southern and eastern Sichuan under both SSP245 and SSP585 scenarios. The mean temperature of the driest quarter was found to be the determinant of the distributional shifts for the majority of Berberis plants. Our overlapping assessments indicated that the risk of wheat stripe rust would be reduced in southwestern Sichuan. In contrast, it would be aggravated in the northwest due to the predicted shifts of Berberis plants northwards and stronger shrinkage of Pst oversummering range occurring in the south than in the north. In the “hotspots” of the stripe rust pathogen, risk assessment in advance could provide a scientific basis for decision‐makers and local communities in effectively controlling wheat stripe rust, thus further sustaining food security through adapting to future climate change.

Benthic foraminifera distribution and sedimentary environmental evolution of a carbonate platform: A case study of the Guadalupian (middle Permian) in eastern Sichuan Basin

Benthic foraminifera are significant indicators of habitat changes and are useful for paleoenvironmental reconstruction. During the Permian period, the variation in species, individual sizes, and morphological characteristics of the different assemblages, such as the fusulinids, is a clue to paleoecological and paleoenvironmental interpretations. The eastern part of the Sichuan Basin, located near the equator in the middle Permian, Guadalupian period, was precipitated by thick beds of marine carbonate rocks with numerous benthic foraminifera. In this study, we record the species and calculate the frequency of benthic foraminifera from 445 thin sections of Guadalupian (Maokou Formation) marine carbonate rocks. Seven types of benthic foraminiferal biofacies are recognized and associated with five sedimentary microfacies in the Erya and Huilongchang sections, eastern Sichuan area. During the early Guadalupian period, small uniserial nodosariids and Endothyrida dominated the benthic foraminiferal assemblages; in the middle Guadalupian period, species of Miliolida, Ammodiscidae, and uniserial nodosariids were common; however, the species of Fusulinida, such as Schwagerina sp. and Verbeekina sp., were abundant in the late Guadalupian period. The associated sedimentary microfacies indicates a shallow open platform to marginal platform. In our study area, the early Guadalupian was deposited in an open platform to grain shoals. This was followed by gradual shallowing as the sedimentary environment shifted to marginal shallow shoals from the middle to late Guadalupian period until the paleoweathering abruptly terminated the deposition at the end of the Guadalupian period.

The source and tectonic setting of the Changhsingian K-bentonites in the Huaying Mountain region, South China

Numerous volcanic ash (or K-bentonite) beds are ubiquitous in the Permian-Triassic boundary (PTB) successions in South China and are conjectured to have been a critical factor in global events, such as the Permian-Triassic mass extinction. However, the precise location and tectonic setting of related volcanism remain debated. In this study, we present whole-rock geochemistry, zircon U Pb ages, trace element compositions and in situ Hf isotope analyses of K-bentonites found in the Late Permian Changxing Formation in the Huaying Mountain region, eastern Sichuan Province, northern South China. The two samples yield U Pb zircon ages of 253.7 ± 1.4 Ma (QJP-38-R-1 below) and 253.0 ± 2.0 Ma (QJP-38-R-2 above). Zircons with ages of 240–280 Ma from sample QJP-38-R-1 have negative ε Hf ( t ) values from −16.65 to −1.65 and two-stage model (T DM c ) ages from 1.38 to 2.34 Ga, whereas those from QJP-38-R-2 have mainly negative ε Hf ( t ) values from −7.28 to 0.13 and T DM c ages from 1.28 to 1.74 Ga. The low ε Hf ( t ) values and T DM c ages suggest that the original magmas mainly involved Paleoproterozoic-Mesoproterozoic crustal components and a few mafic mantle materials. Moreover, zircon trace element characteristics show that the two samples were sourced from a subduction-driven magmatic arc. The zircons from QJP-38-R-1 are primarily derived from S-type granites that formed in continental granitic rocks related to a continental arc tectonic setting, whereas the zircons from QJP-38-R-2 are mainly derived from S-type granites in oceanic granitic rocks related to an island arc tectonic setting. The K-bentonites near the PTB in South China may have originated from felsic volcanism as a result of the convergence of the Paleo-Tethys Ocean. The integration of zircon Hf isotope and trace element compositions indicates that the PTB K-bentonites in South China may have been derived from a series of volcanic eruptions occurring within a short interval at the continental margins around South China. Volcanic eruptions took place at the Ailaoshan-Sanjiang continental margins in or near southwestern South China and may be the major source of K-bentonites. Volcanic eruptions took place at the East Kunlun or West Qinling continental margins and may have provided a small number of original materials. This result indicates that these K-bentonites may have multiple sources. • Newly found PTB K-bentonites were reported weighted mean 206 Pb/ 238 U ages of 253.7 ± 1.4 Ma and 253.0 ± 2.0 Ma. • The PTB K-bentonites may be sourced from felsic volcanism as a result of the convergence of the Paleo-Tethys Ocean. • The PTB K-bentonites in South China may have multiple sources.

Controlling factors of lamellation fractures in marine shales: A case study of the Fuling Area in Eastern Sichuan Basin, China

The lamellation fractures in marine shales are important to the accumulation and preservation of shale gas, whose characteristics and controlling factors are still unclear. This paper focus on the characteristics descriptions and controlling factor analyses based on core observations, thin sections, and scanning electron microscope (SEM) experiments. The marine shales of Paleozoic Wufeng Formation and Longmaxi Formation in the Fuling Area of Eastern Sichuan Basin are taken as an instance. The lamellation fractures are occurring along the direction of the lamellations, partially of them curved, bifurcated, and converged. The apertures of fractures typically range from 1 to 500 μm. Only a few (<1%) lamellation fractures are filled with quartz, calcite, pyrite, and bitumen. After a series of analyses, developments of lamellation fractures are controlled by total organic carbon (TOC), brittle minerals, laminae, and pyrite. (1) The organic matters control the formation and evolution of lamellation fractures by generating gas and resulting in overpressure. An increase in the TOC content of 1% can increase the lamellation fracture density by 0.26 cm-1. (2) Brittle minerals, especially authigenic quartz, is also an important controlling factor to the development of lamellation fractures. When BI increases by 10%, the lamellation fracture density increases by 19%. (3) Besides, siliceous and carbonate laminae in the shale are more suitable for lamellation fractures to develop. When the laminae density is smaller than 4 cm−1, it is easier to develop lamellation fractures with higher laminae density. However, too densely-distributed laminae will suppress the formation of lamellation fractures. (4) Higher pyrite content is good for the development of lamellation fractures. Lamellation fracture density will increase by 9%, with pyrite content increased by 1%. For the shale at the bottom of the Wufeng Formation - Longmaxi Formation, lamellation fractures develop extensively due to the high TOC and pyrite content, great brittleness, and moderate laminae density. The density of lamellation fractures decreases from the bottom to the top.

Reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin and its periphery and inspirations to petroleum exploration, SW China

Based on a large number of field outcrops and cores taken systematically from boreholes, by microscopic observation, physical property analysis, mineralogy analysis, geochemical analysis etc., reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin (“Mao 1 Member” for short) are analyzed. (1) Rhythmic limestone-marl reservoirs of this member mostly exist in marl layers are a set of tight carbonate fracture-pore type reservoir with low porosity and low permeability, with multiple types of storage space, mainly secondary dissolution pores and fissures of clay minerals. (2) The clay minerals are mainly diagenetic clay minerals, such as sepiolite, talc and their intermediate products, aliettite, with hardly terrigenous clay minerals, and the reservoir in different regions have significant differences in the types of clay minerals. (3) The formation of high quality tight carbonate reservoir with limestone-marl interbeds is related to the differential diagenesis in the early seawater burial stage and the exposure karstification in the early diagenetic stage. It is inferred through the study that the inner ramp of southwestern Sichuan Basin is more likely to have sweet spots with high production, while the outer ramp in eastern Sichuan Basin is more likely to have large scale contiguous reservoir with low production.

Origin and evolution of Sr-enriched hydrothermal brines: Insights from vein mineralization of Lower Triassic marine carbonates in the Eastern Sichuan Basin, South China

This paper focuses on celestite-carbonate-cemented veins that developed in the Jialingjiang Formation of the Lower Triassic carbonates from the Libixia anticline in eastern Sichuan Basin, China. Comparative petrography and geochemistry of the host rock and vein minerals provide clues to the nature, origin, and evolution of the vein-cementing fluids. The host rocks are composed mainly of micrite dolomite which yields δ18O, δ13C and 87Sr/86Sr ratios close to the values that characterize contemporaneous seawater. Two generations of vein-mineralization were recognized based on textural relationships, fluid inclusion data and isotope geochemistry. The early generation is dominated by celestite and saddle dolomite, formed from hypersaline, Sr-enriched, hydrothermal fluids. The later generation is represented by calcites, precipitated from less saline fluids at lower temperatures. The 87Sr/86Sr ratios of vein dolomite and celestite vary from 0.7076 to 0.7081, significantly lower than those of host carbonate counterparts (0.7078 to 0.7081; average 0.7080). This isotopic discrepancy suggests that the majority of Sr was sourced from deeper sequences of the lower Triassic carbonates, probably associated with thermochemical sulfate reduction for Sr accumulation. It is argued that the fracture systems associated with the Huayingshan fault provided the heat source and served as conduits for migration of hydrothermal fluids through several hundred meters, and the final mineralization occurred as a result of mixture of Sr-enriched brines with sulfate-bearing solutions. The large celestite ore bodies in the studied area are proposed to form related partially to these hydrothermal fluids.

Simulation of potential suitable distribution of Alnus cremastogyne Burk. In China under climate change scenarios

Alnus cremastogyne is a broad-leaved tree species with fast-growing and promising nitrogen-fixing capacity. Its plantation is under steady growing due to the commercial and restoration importance in China. However, little is known about the effects of projected climate change on its adaptability and future distribution. In the present study, we simulated the ecological suitability of A. cremastogyne under three climate change scenarios (CCS) (i.e., SSP1-2.6, SSP2-4.5 and SSP5-8.5) in 2050 s and 2070 s using geographic information technology (GIS) and maximum entropy model (MaxEnt). The results demonstrated that under current climate situation, the highly suitable areas were mainly located in central and eastern Sichuan, most of Hunan, central and northern Jiangxi, central and eastern Guizhou, most of Chongqing and southeast Hubei, with a total area of 52.1 × 104 km2. The areas of the moderately and poorly suitable areas were 65.52 × 104 km2 and 92.99 × 104 km2, respectively. Under the future CCS, both the highly suitable area and the total suitable area of A. cremastogyne showed increasing trends, 1.22-fold and 1.57-fold higher for highly suitable area by 2050, 1.27-fold and 1.41-fold by 2070, respectively. While the areas of the poorly suitable areas would decline. Moreover, a northwestward migration of the geometric center of the total suitable area was projected. Annual precipitation (5.9–1314.5 mm), the minimum temperature of coldest month (-0.2–16.8 ℃) and the mean temperature of coldest quarter (0.7–11.7 ℃) were the three most important environmental variables determining the distribution of A. cremastogyne. Our results highlight the plasticity of A. cremastogyne to climate change and also the feasibility of intercropping with this species to improve soil nitrogen availability.

Geological Characteristics and Exploration Prospect of Black Shale in the Dongyuemiao Member of Lower Jurassic, the Eastern Sichuan Basin, China

China has yielded huge commercial production from the marine organic-rich shale but shows a slow exploration process in the lacustrine organic-rich shale. Multiple lacustrine shales in the Lower Jurassic of the eastern Sichuan Basin, rich in organic matters, are potential targets for shale hydrocarbon exploration and development. An investigation of the Dongyuemiao member, Lower Jurassic, was firstly conducted utilizing the macroscopic and microscopic analyses on outcrops and drilling cores to reveal the characteristics of sedimentary subfacies, mineral compositions, organic matter content and types, thermal maturity level, and reservoir quality. The dark shales in the Dongyuemiao member can be grouped into four general categories: shore, shallow, semi-deep, and deep lacustrine shales. The semi-deep and deep lacustrine shales generally have higher values in thickness (&amp;gt;20 m), average total organic carbon (TOC) content (&amp;gt;1.5 wt.%), and average porosity (&amp;gt;2%) relative to shore and shallow lacustrine shales. All four categories of shales primarily consist of type II kerogen and have thermal maturity levels exceeding the vitrinite reflectance value of 0.9–1.0% (or the Tmax of ∼440°C). Thermally powered pore generation generally promoted the pore system as indicated by the positive correlation between porosity and Tmax. Notably, the semi-deep lacustrine shale in the vicinity of the Qiyueshan Fault Zone shows abnormally high porosity and low oil saturation index (OSI) at Tmax&amp;gt;∼465°C potentially due to the promoted hydrocarbon expulsion through multiscale fractures. Except for the vicinity of the Qiyueshan Fault Zone, the semi-deep and deep lacustrine shales generally show the better exploration prospect relative to the shore and shallow lacustrine shales. Additionally, the high content of clay minerals (&amp;gt;40 wt%) reduced the brittleness of the semi-deep and deep lacustrine shales which may challenge the artificial hydraulic fracturing.

Late Cenozoic erosion pattern of the eastern margin of the Sichuan Basin: Implications for the drainage evolution of the Yangtze River

Evolution of the drainage network of the Yangtze River plays an important role in landscape evolution across East Asia during the Cenozoic. The mountains on the eastern margin of the Sichuan Basin form a drainage divide between the tributary rivers of the modern Upper and Middle Yangtze, and the erosion history of these mountains has major implications for the evolution of the Yangtze River. Linear inversion of long profiles of two Yangtze tributaries draining the area allows us to estimate their incision processes, and reveals contrasting erosion patterns between the west and east sides of the mountain belt. Along the Wu River, which drains into the Sichuan Basin, higher incision rates are focused on lower channels near the river's outlet on the Upper Yangtze. In contrast, within the catchment of the Yuan River, which drains into the Jianghan Basin of the Middle Yangtze, the inverted fluvial erosion rate is distributed relatively uniform in space. We calibrate the inferred incision history using previously published cosmogenic 10Be-derived basin-averaged erosion rates, and the results show that the contrasting erosion patterns between the two rivers emerged since the early Miocene (~21–16 Ma). At this time, the incision rates of the lower Wu River started to increase from ~0.04 km/Ma towards the Quaternary average at ~0.07 km/Ma, while the rates of the Yuan River remained low (<0.04 km/Ma). By comparing our results with erosion histories of the eastern Sichuan Basin and Three Gorges, we suggest that during the early Miocene, connection between the Sichuan and Jianghan Basins through the Three Gorges led to additional lowering of the local base level in the Sichuan Basin, which triggered an acceleration in incision rates of the Upper Yangtze tributaries draining into the basin.

Gas source in lower Permian Maokou Formation and gas accumulation in the syncline area of eastern Sichuan basin

In this paper, the original nature of organic matters is introduced for the first time to reevaluate the lower Permian (P1) source rocks. The carbonate rocks in Maokou Formation (P1m) whose high organic carbon values were caused by external asphalt have poor hydrocarbon generation ability, but the argillaceous rocks in P1m have some, especially the recent discovered black shale in Gufeng deep-water facies in the north have good hydrocarbon generation ability. The black shale is 5–20 m thick and the measured average value of organic carbon can reach up to 7.24 wt%, which can greatly increase the hydrocarbon generation capacity in the north; The studies of natural gas sources of the lower Permian in eastern Sichuan basin show that the contribution of gas source is mainly Silurian. In the north, the lower Permian source rock is supplemented, while in the south, it's greatly affected by the coal-measure source rocks of Longtan Formation (P2l); Take the Wutan-1 well in Tanmuchang as an example to dissect the reservoir in syncline area. The Wutan-1 well has a tested daily production of more than 80 × 104 m3, and the paleotectonic evolution history shows that it was in the high position of the structure during the generation and expulsion peak of hydrocarbon. The fracture-vuggy lithologic reservoir of Wutan-1 well is favorable for hydrocarbon capture and later preservation. According to the above, the Tanmuchang syncline has a good spatio-temporal matching relationship, which is the key to hydrocarbon accumulation and exploration. This knowledge has expanded the exploration area of the Lower Permian Maokou Formation in eastern Sichuan, and more importantly, it brings much enlightenment to the exploration of the syncline areas in Sichuan basin and other basins at home and abroad.

A study of the evaluation of geo-hazards development degree based on time-space coupling

The Geo-hazards comprehensive evaluation index system was established in the early stage of the team, and the number, the number density, the volume and the volume density, which reflect the spatial dimension, are used to evaluate the development degree of geo-hazards, excluding the evaluation factors reflecting the time dimension. In this paper, disaster years reflecting the time dimension are introduced to enrich the development degree evaluation system of geo-hazards. The median logarithmic method and probability density distribution are respectively used to processing data of disaster years and volume, and the entropy weight is used to determine the weight of evaluation factors. The evaluation models are established and the evaluation results are divided into four levels by the slope method, reflecting the development status of disasters. The geo-hazards occurred in China (excluding Hong Kong, Macao and Taiwan) from 2011 to 2020 are taken as examples, the development degree of geo-hazards (landslide, collapse and debris flow) is evaluated and divided according to this method. The results show that the geo-hazards high development areas cover 323 counties in total, involving 19 provinces, mainly occurring in most of Hunan, western and eastern Hubei, southeastern Jiangxi, eastern Guangdong, central and western Fujian, eastern Sichuan, northern Chongqing and southern Gansu. The sub-high development areas occupy 566 counties in total, involving 25 provinces, mainly occurring in most of Sichuan, western and southern Yunnan, southeastern Hunan, northern Hubei, most of Jiangxi and Guangdong, Guangxi, Fujian and Zhejiang, southwestern Guizhou, southern Shaanxi and eastern Jilin. The moderately development areas cover 623 counties in total, involving 30 provinces, mainly occurring in most of Xinjiang, Tibet, Gansu, Hebei, Shanxi and Henan. The low development areas occupy 1336 counties in total, involving 30 provinces, mainly occurring in Qinghai, most of Tibet, northern Gansu, Inner Mongolia, Heilongjiang, western Jilin and Liaoning, eastern and southern Hebei, Shandong, eastern Henan, central and northern Anhui and Jiangsu. When compared with the national disaster prevention deployment situation or the Geo-hazards distribution, the evaluation results are in good agreement with the reality. In addition, through field investigation, the evaluation results are more scientific and reasonable than before.

Study on the Sensitivity of Initial Perturbations to the Development of a Vortex Observed in Southwest China

AbstractIn this study, numerical experiments on the sensitivity of initial perturbations to the development of southwest vortices (SWVs), which are frequently observed in Southwest China, are performed with weather research and forecasting model. Regarding the development of SWVs as a strong nonlinear process, conditional nonlinear optimal perturbation (CNOP) with an adjoint‐free algorithm is used. The sensitivity denotes how and where the perturbations of model variables at the initial time can influence non‐moving SWV enhancement. The CNOPs leading to the strengthening of the SWV observed at 5UTC July 17, 2010 are obtained. Three sensitive areas are identified based on the distribution of the most significant CNOP norms. Two of them are located on the Eastern Tibetan Plateau (TP), and the third one is found in the eastern Sichuan Basin (SCB) near the SWV center. Sensitivity experiments indicate that the strengthening of the SWV is mainly associated with perturbations at the middle level at approximately 500–450 hPa in the eastern part of the TP and below 450–400 hPa in the eastern SCB near the center of the SWV. Furthermore, the potential vorticity (PV) budget, divergence, vorticity, vertical velocity, diabatic heating, and southerly wind are also analyzed in different simulations. The results show that at the initial stage of SWV formation, the diabatic heating term is not the main cause of PV budget differences between simulations; it is the dynamical effect that is the main contributing factor. However, when it comes to the intensification stage, the diabatic heating caused by the release of latent heat of condensation becomes important.