Southwestern Sichuan Basin Research Articles

Application of the wavelet transform and Hilbert–Huang transform in stratigraphic sequence division of Jurassic Shaximiao Formation in Southwest Sichuan Basin

Abstract In the southwestern Sichuan Basin, the Jurassic Shaximiao Formation encompasses a multitude of working areas, displaying intricate sedimentary traits. Traditional methods of stratigraphic division based on sequence suffer from inherent subjectivity and limitations. This study employs a combined mathematical approach to use the wavelet transform (WT) and the Hilbert–Huang transform (HHT). It decomposes the natural gamma ray (GR) logging curve into energy spectrum plots and wavelet coefficients at different scales, high and low frequency signals at different frequencies, and a set of intrinsic mode function components and residual functions. The study conducted a detailed stratigraphic division of the Jurassic Shaximiao Formation in the southwestern Sichuan Basin using these methods. The WT offers greater resolution for the periodic changes in the base level, whereas the HHT demonstrates a superior correlation with the positions of stratigraphic interfaces. The combined utilization of the continuous wavelet transform, the discrete wavelet transform, and HHT methods has demonstrated encouraging outcomes in the stratigraphic division of the Jurassic Shaximiao Formation. These methods have been shown to enhance the accuracy of stratigraphic division and to reduce the influence of subjective factors. This study presents new insights and approaches for geological data processing, offering significant theoretical and practical implications and novel technical means for oil and gas exploration and development.

Episodic fluid charging and mixing triggers calcite-bearing agates in the Permian Emeishan Basalt, SW China

ABSTRACT Agate, a distinctive form of banded chalcedony, derives its characteristic appearance from the deposition of silica and the presence of impurities. A significant variety of agate-bearing calcite was identified in the Permian Emeishan Basalt located in the southwestern Sichuan Basin. This calcite-bearing agate provides valuable insights into the complex interplay and evolution of fluids and minerals. This study aims to provide a detailed description and analysis of the microscopic textural and geochemical characteristics of the agate, while elucidating the sequence of mineral deposition, fluid properties, and the genetic mechanism underlying its formation. The findings reveal that the agate primarily consists of lamellar calcite and banded chalcedony, exhibiting a variety of textures, thicknesses, and colours. Through the analysis of the transformation and precipitation of epidote and allanite, and the identification of four types of calcites and five micro-silica textures, it has been confirmed that the early stage of fluid in geodes involved the mixing of meteoric water with post-magmatic hydrothermal fluid. The middle and late stages involve the mixing of magmatic-hydrothermal fluid with organic-acid fluid. The charging of silica-rich hydrothermal fluids and the generation of hydrocarbons from organic matter cause fluctuations in SiO2 concentration, pH, and crystallization rates, contributing to the diversity of silica minerals, such as chalcedony and quartz. The interlayered growth observed between chalcedony and calcite is attributed to secondary metasomatism, which occurs after the dehydration and transformation of chalcedony due to increasing temperatures. Notably, the internal texture of the agate is somewhat influenced by pressure variations caused by the episodic charging of the fluid.

Phylogeographic Structure and Population Dynamics of Baoxing Osmanthus (Osmanthus serrulatus), an Endemic Species from the Southwest Sichuan Basin, China.

The mountainous regions of southwest China are recognized as pivotal centers for the origin and evolution of Osmanthus species. Baoxing Osmanthus (Osmanthus serrulatus Rehder), a rare and endemic species known for its spring blooms, is sparsely distributed within the high altitude evergreen broad-leaved forests surrounding the southwestern Sichuan Basin. However, persistent anthropogenic disturbances and habitat fragmentation have precipitated a significant decline in its natural population size, leading to the erosion of genetic resources. To assess the genetic status of O. serrulatus and formulate effective conservation strategies, we conducted sampling across ten wild populations, totaling 148 individuals in their natural habitats. We employed two cpDNA fragments (matK and trnS-trnG) to elucidate the phylogeographic structure and historical population dynamics. The results revealed low species-level genetic diversity, alongside pronounced regional differentiation among populations (FST = 0.812, p < 0.05) and a notable phylogeographic structure (NST = 0.698 > GST = 0.396, p < 0.05). Notably, genetic variation was predominantly observed among populations (81.23%), with no evidence of recent demographic expansion across the O. serrulatus distribution range. Furthermore, divergence dating indicated a timeline of approximately 4.85 Mya, corresponding to the late Miocene to early Pleistocene. This temporal correlation coincided with localized uplift events in the southwestern mountains and heightened Asian monsoons, suggesting pivotal roles for these factors in shaping the current phylogeographic pattern of O. serrulatus. These findings support the effective conservation of O. serrulatus germplasm and offer insights into the impact of Quaternary climate oscillations on companion species within evergreen broad-leaved forests. They also enhance our understanding of the origin and evolution of these forests in the southwestern mountains, aiding biodiversity conservation efforts in the region.

Study on the multifractal characterization and seepage of the shale matrix: A case study of the Longmaxi Formation in southwestern Sichuan Basin, China

The pore structure and heterogeneity of shale significantly impact the occurrence and permeability of shale gas. Studying these aspects is beneficial for exploring and developing shale gas. In this study, organic-rich shale from the Longmaxi Formation in the Luzhou-Changning area was selected as the research subject. Experiments, including spontaneous imbibition, pressure saturation, and nuclear magnetic resonance, were conducted to characterize the pore structure and heterogeneity of the shale in this area. Using multifractal theory, we analyzed and discussed the multifractal characteristics of pores with different connectivities in relation to permeability. The results indicate that the T2 cutoff value (T2f) of the shale is greater than the T2c value, suggesting that the pore size of connected pores exceeds that of pores retaining bound water. The sizes of connected pores primarily range from 1 to 30 nm, predominantly between 1 and 10 nm. Significant differences were observed in the multifractal spectrum functions under dry, spontaneous imbibition, and water-saturated conditions, reflecting varying degrees of heterogeneity. The multifractal parameters differ, with higher ΔD values under water-saturated conditions, indicating that pores with weaker connectivity enhance the overall heterogeneity of the shale when saturated. Conversely, lower ΔD values under dry conditions suggest less heterogeneity in pores containing bound water. Total organic carbon (TOC) and clay content were identified as the main factors influencing the development of connected pores. Based on these findings, a discriminant 3D diagram correlating permeability with porosity, tortuosity, and multifractal parameters (D0-Dmax) of strongly connected pores was established for the Luzhou-Changning area. This diagram serves as an effective tool for evaluating the flow capacity of shale reservoirs and offers guidance for identifying sweet spots within the reservoir.

Determination of multistage oil charge processes in the Ediacaran Dengying gas reservoirs of the southwestern Sichuan basin, SW China

The Sichuan Basin has experienced multi periods of tectonic uplift event, accompanied with multiple hydrocarbon accumulation and phase transitions processes in the deep and ultra-deep reservoirs. The formation of paleo-oil reservoir plays an important role in the distribution of present-day gas fields, yet many records of early oil reservoirs have been destroyed. A series of methods including, petrography, cathodoluminescence, fluid inclusion, in-situ U–Pb geochronology, and basin modeling, were used to determine the multistage oil charge process in the Ediacaran Dengying Formation of the Weiyuan area, SW Sichuan Basin. The lace-like dolomite lamination and three stages of dolomite cement were petrographically, and geochronologically distinguished in the Dengying Formation, dated at c. 542 Ma, 486–482 Ma, 410 Ma, and 270 Ma, respectively. Three stages of bitumen in the Dengying reservoir indicated three episodes of oil charge. The first and second oil-charging events occurred at 500–486 Ma and 410–400 Ma, by combining the modeled timing of oil generation with mineral ages of the two generations before and after the solid bitumen. The modification of the Weiyuan paleo-oil pool occurred during the Caledonian tectonic uplift, after the second oil-charging event. The timing of the third oil charge was at 270–230 Ma, according to the Th value of aqueous inclusions coeval with secondary bitumen inclusions in the stage 3 dolomite cement (CD-3), close to the timing of main oil generation for the Lower Cambrian source rocks. The first oil charge may be a contribution from the Doushantuo source rock, and the Lower Cambrian source rocks provide the major contribution for the third oil charge. This research reveals the timing of multistage oil charging events in the southwest Sichuan basin, and provides a further method for determining the timing of oil charge in multi-cyclonic composite basins.

Episodic hydrothermal alteration on Middle Permian carbonate reservoirs and its geological significance in southwestern Sichuan Basin, SW China

To analyze the episodic alteration of Middle Permian carbonate reservoirs by complex hydrothermal fluid in southwestern Sichuan Basin, petrology, geochemistry, fluid inclusion and U-Pb dating researches are conducted. The fractures and vugs of Middle Permian Qixia–Maokou formations are filled with multi-stage medium−coarse saddle dolomites and associated hydrothermal minerals, which indicates that the early limestone/dolomite episodic alteration was caused by the large-scale, high-temperature, deep magnesium-rich brine along flowing channels such as basement faults or associated fractures under the tectonic compression and napping during the Indosinian. The time of magnesium-rich hydrothermal activity was from the Middle Triassic to the Late Triassic. The siliceous and calcite fillings were triggered by hydrothermal alteration in the Middle and Late Yanshanian Movement and Himalayan Movement. Hydrothermal dolomitization is controlled by fault, hydrothermal property, flowing channel and surrounding rock lithology, which occur as equilibrium effect of porosity and permeability. The thick massive grainstone/dolomites were mainly altered by modification such as hydrothermal dolomitization/recrystallization, brecciation and fracture-vugs filling. Early thin–medium packstones were mainly altered by dissolution and infilling of fracturing, bedding dolomitization, dissolution and associated mineral fillings. The dissolved vugs and fractures are the main reservoir space under hydrothermal conditions, and the connection of dissolved vugs and network fractures is favorable for forming high-quality dolomite reservoir. Hydrothermal dolomite reservoirs are developed within a range of 1 km near faults, with a thickness of 30–60 m. Hydrothermal dolomite reservoirs with local connected pore/vugs and fractures have exploration potential.

Depositional model of the Member Deng-2 marginal microbial mound-bank complex of the Dengying Formation in the southwestern Sichuan Basin, SW China: Implications for the Ediacaran microbial mound construction and hydrocarbon exploration

Recent advances in hydrocarbon exploration have been made in the Member Deng-2 marginal microbial mound-bank complex reservoirs of the Dengying Formation in the western Sichuan Basin, SW China, where the depositional process is regarded confusing. The microfacies, construction types, and depositional model of the Member Deng-2 marginal microbial mound-bank complex have been investigated using unmanned aerial vehicle photography, outcrop section investigation, thin section identification, and seismic reflections in the southwestern Sichuan Basin. The microbialite lithologic textures in this region include thrombolite, dendrolite, stromatolite, fenestral stromatolite, spongiostromata stone, oncolite, aggregated grainstone, and botryoidal grapestone. Based on the comprehensive analysis of “depositional fabrics–lithology–microfacies”, an association between a fore mound, mound framework, and back mound subfacies has been proposed based on water depth, current direction, energy level and lithologic assemblages. The microfacies of the mound base, mound core, mound flank, mound cap, and mound flat could be recognized among the mound framework subfacies. Two construction types of marginal microbial mound-bank complex have been determined based on deposition location, mound scale, migration direction, and sedimentary facies association. Type Jinkouhe microbial mound constructions (TJMMCs) develop along the windward margin owing to their proximity to the seaward subfacies fore mound, with a northeastwardly migrated microbial mound on top of the mud mound, exhibiting the characteristics of large-sized mounds and small-sized banks in the surrounding area. Type E’bian microbial mound constructions (TEMMCs) primarily occur on the leeward margin, resulting from the presence of onshore back mound subfacies, with the smaller southwestward migrated microbial mounds existing on a thicker microbial flat. The platform margin microbial mound depositional model can be correlated with certain lateral comparison profile and seismic reflection structures in the 2D seismic section, which can provide references for future worldwide exploration. Microbial mounds with larger buildups and thicker vertical reservoirs are typically targeted on the windward margin, while small-sized microbial mounds and flats with better lateral connections are typically focused on the leeward margin.

Characteristics and Controlling Factors of Pores in Different Shale Lithofacies Reservoirs of Lower Cambrian Qiongzhusi Formation, Southwestern Sichuan Basin, China

The shale reservoirs of the Lower Cambrian Qiongzhusi Formation are widely distributed in the Sichuan Basin and have abundant gas resources. However, the shale lithofacies of the Qiongzhusi Formation are complex due to frequent sea level changes. The reservoir pore structure characteristics and gas content of different shale lithofacies vary significantly, which makes identifying the ‘sweet spot’ a challenging task. In this study, core observation and X-ray diffraction (XRD) were used to analyze the lithofacies types and characteristics of the study area. The pore types of different shale lithofacies were observed using field emission-scanning electron microscopy. Pore structures were studied using low-temperature gas (including N2 and CO2) physisorption, and the pore volume (PV), specific surface area (SSA) and pore structure were systematically characterized. The primary factors influencing pore formation in different types of shale lithofacies were analyzed by combining geochemical experiments and mineral contents. The results indicate that the lithofacies of the Qiongzhusi Formation shale in the study area can be classified into five categories according to mineral compositions: Siliceous argillaceous shale (CM-1), Argillaceous siliceous mixed shale (M-2), Argillaceous siliceous shale (S-3), Siliceous rock (S) and Calcareous siliceous shale (S-2). Pores are abundant in S-3 shale, M-2 shale and CM-1 shale. The S-3 shale is more enriched in organic pores and clay mineral pores compared to other lithofacies shales, and the pore morphology is mainly wedge-shaped and plate-like. M-2 shale and CM-1 shale are rich in clay minerals and mainly develop clay mineral pores and are mainly wedge-shaped and plate-like. The S shale and S-2 shale mainly develop interparticle pores and clay mineral pores, which are mainly slit-like. The results show that TOC, pyrite content, quartz and feldspar mineral content, clay mineral type and content affect the pore structure in the study area. Quartz and feldspar content have a negative effect on micropore and mesopore volumes. TOCs have a weak positive correlation with micropore volume and micropore SSA. Clay mineral content has significant positive effects on the PV and SSA of micropores and mesopores, indicating that clay mineral content is the main factor affecting the pore structure of shale.

Seismogenic structures and earthquake mechanisms in the Changning area, China: Insights from seismicity and tomography

The Changning area, located in southwestern Sichuan Basin, China, was previously known to be a seismically inactive region. However, in recent years, the region has experienced an increase in the rate of seismic activity. To gain a better understanding of the seismogenic structures and earthquake mechanisms in the region, we deployed 298 short-period seismic stations and collected 47 days of continuous waveform data following the 2019 Ms 6.0 Changning earthquake. Using a machine-learning-based earthquake location workflow (LOC-FLOW; Zhang et al., 2022), we detected and located over 60,000 seismic events. These events were then used for three-dimensional Vp and Vs inversion using a double-difference tomography method (tomoDD). The results show that the earthquake distribution has distinct zoning characteristics with different mechanisms. In the Changning-Shuanghe anticline, most earthquakes are distributed in areas with large velocity and Vp/Vs gradients, typically occurring in regions with high velocity and low Vp/Vs, where tectonic stresses are prone to accumulate. Long-term water injections may promote fault activation in the southeast of Changning anticline. Earthquakes around the salt mining well were found to occur in sedimentary cover, indicating that the basement fault has not been activated but rather acts as a fluid-flowing path. In the Changning shale gas block, earthquakes showed different triggering mechanisms, accompanied by different velocity characteristics. The spatial variations in earthquake locations and velocity characteristics can be explained by the terrain at shallow depths, syncline structures, and the distribution of fluids and gas reservoirs. Our study provides new insights into the detailed seismogenic structures and earthquake mechanisms in the Changning area. These findings can be used to improve earthquake hazard assessment in the region.

Reservoir characterization of the shallow to deep Longmaxi formation in the Weiyuan Block, southwestern Sichuan Basin

Marine shale gas exploration targets are reaching into shallow to deep or ultradeep burial depths. Rock microstructure and reservoir quality emerge as the main risk considerations for a profitable reservoir at these large depths. Shallow to deep marine shales occur in the Silurian Longmaxi Formation of the Weiyuan Block, which is located on the southwestern margin of the Sichuan Basin. However, few details of the characteristics of the Longmaxi shales in this Block have been reported. In this study, five wells approximately 3500 m deep were drilled. Field emission scanning electron microscopy, low-pressure gas adsorption and core plug porosity-permeability measurements are conducted on 6 shallow (2651–2940 m) and 11 deep (3539–3575 m) Longmaxi samples to obtain the organic geochemical characteristics, mineral constitutions, pore structures and petrophysical properties, and they are the major controls on reservoir quality. The results show similar mineralogical and organic geochemical characteristics in all samples from the various depths. Both shallow and deep shales are mainly composed of quartz, carbonate and clays and have a total organic carbon (TOC) content more than 2 wt.% and a mean S1 + S2 value more than 52 mg/g. Source rock quality criteria using the TOC and S1 + S2 suggest most shale samples fall excellent source rocks. The samples are mostly siliceous rocks that contain organic pores, intraparticle dissolution pores, interparticle quartz pores and interparticle clay pores and they play a positive role in improving reservoir quality. Pore surface area and pore volume increase with increasing TOC, indicating that the porous organic fraction is a major control on pore structure and porosity. We suggested that siliceous deep Longmaxi formation in Weiyuan Block belongs to a high-quality shale with an average TOC value of 3.9 wt.% and well-developed pore networks, which should be the important target for deep shale gas exploration.

Comprehensive Evaluation of the Hydrocarbon Preservation Conditions in the Complex Tectonic Area of the Southwestern Sichuan Basin, China

There are various styles of faults, folds, and unconformity structures in the southwestern Sichuan Basin, which have an important impact on hydrocarbon preservation. Based on the data of regional geological, seismic, formation water characteristics, caprock characteristics, and production, the hydrocarbon preservation conditions of the key horizons (Cambrian, Upper Permian, and Triassic) were analyzed. Southwestern Sichuan Basin margin structure has two obvious tectonic layers (the upper and lower). The upper tectonic layer is dominated by Middle Triassic detachment faults and derived thrust faults, while the lower tectonic layer has concealed structures composed of basement detachment faults and derived thrust faults. There are mainly NE, near-SN, NW, and near-EW faults. The outcrops are in the Middle-Cenozoic strata, changing from old to new from outside the Basin to inside the Basin. There are three main sets of high-quality caprocks, i.e., the mudstone of the bottom of the Xujiahe Formation, the gypsum-salt rocks of the Middle-Lower Triassic, and the Lower Cambrian mudstone. Based on the faults, outcrops, formation water type, mineralization, and caprock characteristics, the hydrocarbon preservation evaluation system was established, which divides into the areas of Class I, II, and III. The distribution range of the favorable areas of different horizons has good inheritance, and the hydrocarbon preservation conditions gradually improve from outside the basin to inside the basin. Class I favorable areas are the most widely distributed and are mainly located within the basin, with the characteristics of a low degree of fault development, relatively new outcropping strata, CaCl2 formation water type, and large caprock thickness.

Pulsed counterclockwise rotation of the southwestern Sichuan Basin in response to the India-Asia convergence during 128-42 Ma

Deciphering the building history of the eastern Tibetan Plateau since the India-Asia convergence is crucial to understand geodynamic models of plateau formation. The Sichuan Basin adjacent to the eastern Tibetan Plateau margin potentially provides valuable records of when and how the Tibetan Plateau expanded eastward. In this study, we reconstruct ∼128–42 Ma rotation history of the southwestern Sichuan Basin using detailed paleomagnetic declination data from the Early Cretaceous to Eocene sedimentary sequence of ∼2400 m thick. A total of 78 sampling sites (774 samples) yield primary remanent magnetization confirmed by positive paleomagnetic tests. Our results show that the southwestern Sichuan Basin experienced a long-term clockwise rotation during 128–42 Ma which was interrupted by three transient and rapid counterclockwise rotation pulses. The earlier two pulses (86–82 Ma and 66–60 Ma) coincide with two India Plate acceleration episodes which are probably induced by the Marion and/or Réunion mantle plume upwelling, whereas the later pulse (53–46 Ma) temporally corresponds to the sudden slowdown of the India Plate caused by the India-Asia collision. Our findings indicate that the eastern Tibetan Plateau experienced prolonged and episodic building processes since the Late Cretaceous, long predating the traditional view of the late Miocene. The Tibetan Plateau was built synchronously from the Himalayas to its eastern margin. Lateral extrusion, block rotation, and strike-slip deformation collectively contribute to the mountain building in the eastern Tibetan Plateau region.

Carbonate U-Pb Geochronology and Clumped Isotope Constraints on the Origin of Hydrothermal Dolomites: A Case Study in the Middle Permian Qixia Formation, Sichuan Basin, South China

Reservoirs in the dolomites of the Middle Permian Qixia Formation in the Sichuan Basin are currently important oil and gas exploration objects in China. However, the questions concerning the sources of the dolomitized fluids and the control factors of the Qixia hydrothermal dolomites remain unclear. In this study, the original hydrothermal dolomites (the replacement dolomites (RDs) and saddle dolomites (SDs)) from the Qixia Formation in the southwestern Sichuan Basin (the PR1 well and Baoxing section) were mainly examined using novel in situ carbonate U-Pb dating with clumped isotopes (∆47). Our results show that the U-Pb ages of the latest SDs from the PR1 well (located in the middle zone of the Emeishan large igneous province (ELIP) and distanced from the Orogenic Belt of Longmenshan) are 257.9–251.0 Ma, coincident with the period of main activity of the ELIP. Combined with the previous U-Pb dating, we propose that the high-temperature T∆47 (82.2–108.4 °C and 127.5–205.9 °C) recorded for SDs from the PR1 well and Baoxing section may have responded to ELIP activity and Longmenshan orogeny activity, respectively. In addition, in the entire southwestern Sichuan Basin, the RDs and SDs yield similar δ13C and δ18O values, indicating that the dolomites were formed by hydrothermal fluids of similar sources, with marine hydrothermal fluids being a highly possible source. Finally, this study proposes a new hydrothermal dolomite genesis model for the Qixia Formation, emphasizing that the formation of hydrothermal dolomites mainly depends on the proximity to tectonic thermal events in space and time.

Detailed Imaging of a Seismogenic Fault That Potentially Induced the Two 2019 Weiyuan Moderate Earthquakes in the Sichuan Basin, China

Abstract Induced earthquakes are becoming a significant issue. Several destructive induced earthquakes (Ms ≥5.0) have occurred in shale gas fields located at the southwestern Sichuan basin, China, in the recent years. Specifically, two moderate earthquakes with magnitudes of Ms 5.4 (occurred on 8 September 2019) and 5.2 (occurred on 18 December 2019) exceeded the largest earthquake ever recorded in the Weiyuan. It remains unclear which seismogenic faults were responsible for the two moderate events due to insufficient knowledge of the subsurface structure and contradictory hypocenter depths obtained from different seismological methods. In this study, mainshock locations, focal mechanisms, and precise relocations of microseismicity were used to map the 3D geometry of the seismogenic fault. The result shows that the two moderate earthquakes occurred on the same fault. High-resolution seismic reflection profiles indicate several preexisting faults at various depths and a potential seismogenic fault. No large-scale faults are identified in the sedimentary cover above the Cambrian at the shallow centroid depth of these earthquakes. However, the seismic reflectors show phase discontinuous feature, indicating the existence of preexisting faults in the basement. A preexisting fault in the basement is consistent with a seismogenic fault that may have induced the two moderate earthquakes. Our study suggests that poroelastic stress propagation is likely responsible for the basement fault reactivation. The rupture on a stressed basement fault was triggered and produced two moderate earthquakes in a short period during hydraulic fracturing. The previous strategy of avoiding direct injection of fluids into preexisting faults to reduce seismic risk in operation region is not enough to avoid the possibility of the coupled poroelastic stress triggering deep fault rupture. We need to reevaluate the potential seismic risk of shale gas development in Weiyuan, the southern Sichuan basin shale gas field considering the preexisting faults in sedimentary and in basement and the effect of the fully coupled poroelastic stress field.

Prediction of the Diurnal Variation of Summertime Precipitation Over the Sichuan Basin by a Regional Model

AbstractThe nocturnal precipitation over the Sichuan Basin (SCB) exhibits obvious spatial variation, most of the existing numerical research of precipitation event over the SCB is based on case studies. In this study, the long‐term climatological evaluation of the prediction of diurnal spatio‐temporal evolution of precipitation and relevant physical mechanism over the SCB by 9 and 3 km operational numerical weather prediction model is carried out. It is found that the SWC‐WARMS model with 9 km grid spacing overestimates the summer precipitation, especially over the western slopes of the SCB. The northeastward timing delay of the precipitation peak over the SCB is well reproduced in the model, with predicted precipitation peaking at midnight over the southwestern SCB and in the early morning over the northeastern SCB. However, the predicted diurnal peak of precipitation appears 2–3 hr earlier than the observed peak. Increasing horizontal resolution to 3 km reduces the wet bias and improves the diurnal phase of precipitation. Compared to reanalysis, the predicted easterly anomalous wind appears earlier and the anomalous wind speed is stronger. Moreover, the predicted atmosphere is too unstable, which is associated with “warm below and cold above” vertical biases in temperature and wet biases in specific humidity. Thus, model precipitation peaks too early in the day over the SCB, and is overestimated. The weakening of the predicted southwesterly flow in the middle troposphere weakens the northeastward propagation of precipitation after midnight over the SCB. The most significant improvements are found for the diurnal variation of low‐level anomalous wind speed in the 3 km resolution model, contributing to a more realistic representation of precipitation over the SCB.

Development Characteristics and Distribution Patterns of Fractures in the Wufeng-Longmaxi Formation Shale in the Southwestern Sichuan Basin, China

Abstract As the important storage space and main seepage channel of the shale gas reservoirs, fractures control the migration, enrichment, and preservation of shale gas. Therefore, studying the development characteristics of fractures within shale is the key to the exploration and development of shale gas reservoirs. Based on core observation, microscopic thin section examination, field-emission scanning electron microscopy (FE-SEM) analysis, and three-dimensional seismic attribute analysis, this paper studies the development characteristics and distribution patterns of fractures in the Wufeng-Longmaxi Formation shale in the southwestern Sichuan Basin. The results show the following: (1) Both macrofractures and microfractures are developed in the Wufeng-Longmaxi Formation shale in the study area. On the macroscale, horizontal bedding fractures, low-angle slip fractures, vertical shear fractures, and high-angle fractures are mainly developed; on the microscale, intergranular fractures, intragranular fractures, abnormally high-pressure fractures, hydrocarbon generation shrinkage fractures, bedding fractures, etc. are mainly developed. These fractures of different scales work together to create an intricate fracture system within the shale. (2) Based on the 3D seismic attribute analysis, the distribution patterns of fractures was studied in the research area. It was found that nearly east-west-trending and nearly south-north-trending large faults were mainly developed in the Wufeng Formation and the Long-11 submember, with accompanying small faults and secondary fractures. From the bottom boundary of the Wufeng Formation to the top boundary of the Long-11 submember, the fault development shape is unchanged, and the secondary fractures are developed gradually densely, showing the characteristics of a zonal distribution in the entire study area. From the bottom boundary of the Wufeng Formation to the top boundary of the Long-11 submember, the development of microfractures shows variation in different zones. Microfractures are mainly developed around large faults, and the development range of microfractures decreases from bottom to top, but the development of microfractures is gradually intensive. The microfractures are densely distributed from the central to the southern part of the study area (gradually from convergence to divergence), are sparsely distributed in the western part of the study area, and are densely and widely distributed in the northern, northeastern, and northwestern parts of the study area. The research results can provide some guidance for the prediction of shale gas sweet spots in this area, which is beneficial to the further exploration and development of marine shale gas in this area.

Tectonic Features of the Wufeng–Longmaxi Formation in the Mugan Area, Southwestern Sichuan Basin, China, and Implications for Shale Gas Preservation

Abstract Shale gas resources in mainland China and its commercial exploitation has been widely focused on the Wufeng–Longmaxi Formation organic-matter-rich shale in the Sichuan Basin. However, whether southwestern margin of the Sichuan Basin can produce high-quality shale gas has not been well resolved, which might be related to the poor understanding of the relationship between Cenozoic tectonic deformation and shale gas preservation. To answer the aforementioned scientific question, we conducted a detailed work in the Mugan area to show geologic structures and gas contents in the area through seismic profiles and geochemistry analysis. Specifically, the stable Mugan syncline shows a high gas content (&amp;gt;2.6 m3/t measured at three boreholes D1, D2, and D3), whereas its periphery presents a poor gas content (about 0.6 m3/t measured at two boreholes X1 and Y1). Moreover, oblique fracture density and dissolved pores are much higher at boreholes X1 and Y1 than that at the other three boreholes. We propose an opposite-verging thrust fault model to explain the different gas contents and tectonic features in the Mugan area, which might indicate that regions in the southwestern Sichuan Basin with similar tectonic and stratigraphic characteristics as those in the Mugan syncline are likely to produce high-yield shale gas. This finding provides new insights into the exploration theory of shale gas in the Tibetan Plateau.

Comprehensive Physical Properties and Exploration Potential of the Permian Igneous Rocks in the Southwestern Sichuan Basin

The Permian igneous rocks in the Sichuan Basin represent a major breakthrough, opening up a new prospect for oil and gas exploration, and igneous reservoirs have become a new field of oil and gas exploration. Gravity-magnetic-electric exploration is an effective means of identifying igneous rocks and helps in reducing the multiplicity of the prediction results. However, the lithology of igneous rocks is quite different, and the exploration theory and evaluation techniques need urgently to be improved. In order to deeply study the response characteristics of the gravity-magnetic-electric and physical properties of the Permian igneous rocks in the Sichuan Basin and their relationships with the reservoir parameters, physical property testing was carried out on outcrop samples of the Permian igneous rocks in southwestern Sichuan. The comprehensive physical properties of the samples with different lithologies, including basalt, tuff, and volcanic breccia, were analyzed and studied. Based on the geological characteristics of the igneous rocks, such as the mineral composition, microstructure, and reservoir properties, a multi-parameter intersection relationship model for the resistivity, polarizability, density, magnetic susceptibility, and their relationships with the reservoir parameters was established, and effective parameters favorable for igneous rock identification and reservoir evaluation were identified. The results of this study provide a physical basis and technical support for non-seismic exploration of igneous oil and gas reservoirs in the Sichuan Basin.

Maturity evolution of Permian source rocks in the Sichuan Basin, southwestern China: The role of the Emeishan mantle plume

The Permian source rocks in the Sichuan Basin, Southwestern China, are hypothesized to have undergone thermal over-maturation associated with the Emeishan mantle plume. In this study, by comparing the maturity evolution of Permian source rocks, we identified three maturity evolutionary patterns, including (1) Middle Permian maturing abrupt jump in the areas where the Permian magmatic intrusions, (2) Mesozoic continuous heating in the southwestern Sichuan Basin, and (3) Middle-Late Triassic heating cessation common in the Sichuan Basin. Furthermore, we assess the evolution model of the Emeishan large igneous province and investigate the thermal effect of the mantle plume on hydrocarbon maturation of Permian source rocks. During the mid-Guadalupian, the mantle upflow reached the base of the northeastern Sichuan Basin and little eruptions and intrusions formed along the basement faults. As the South China block kept moving northward, the major eruption and crustal magma underplating occurred at the end-Guadalupian near the southwest margin of the Sichuan Basin. The mantle plume caused the Middle-Lower Permian source rocks in the intrusion area to mature rapidly and achieve peak maturity (Requ > 3%) during the Middle Permian. Except for the areas with magma intrusion, the mantle plume had no obvious impact on the maturity evolution of the Permian source rocks and the maturity evolution is controlled by Mesozoic sedimentation and uplift. In addition to providing new insights into the thermal effects of the Emeishan Permian mantle plume, this study has great implications for gas exploration in the Sichuan Basin.

Identification of deep-buried potassium-rich brines in evaporate basin with aid of geophysical logging methods in co-exploration of oil and potash: a case study of Southwestern Sichuan Basin

Identification of deep-buried potassium-rich brines in evaporate basin with aid of geophysical logging methods in co-exploration of oil and potash: a case study of Southwestern Sichuan Basin