Northeast Sichuan Basin Research Articles

Microbially induced fabrics in the Upper Ediacaran coated grain dolostone, Northeast Sichuan Basin, SW China: Implications for reconstruction of the paleoceanic environment

In the Upper Ediacaran Dengying Formation of the Sichuan Basin, the presence of abundant, well-preserved microbial carbonates provides a unique opportunity to study Precambrian paleoceanography and microbial carbonate origins. Particularly, the underexplored coated grain dolostones of this formation, characterized by distinct microbially induced fabrics and intragranular dissolution, offer crucial insights for understanding late Ediacaran microbial mineralization and diagenetic sequences. Utilizing selected typical samples, we conducted detailed petrographic and geochemical analyses, revealing a dynamic interplay between microbial processes and sedimentary dynamics. We observe the coexistence of constructive micrite envelopes with microbially induced fabrics such as clots and laminations, highlighting microbial biomineralization's key role in fabric formation. The sedimentary dynamics critically determines the formation processes of the coated grains: low-energy settings foster grain agglomeration and consolidation through clot precipitation between grains, while high-energy settings favor smaller grains binding to microbial mats. Geochemically, micrite envelopes play an essential role in preserving distinct rare earth element (REE) signatures. The weak negative Ce anomalies and positive Eu anomalies within these envelopes point to a suboxic to anoxic depositional environment, directly indicative of the microenvironmental conditions conducive to microbial mineralization processes. Furthermore, our study sheds light on the structural evolution of coated grains with hollow nuclei, proposing that their internal pore formations are influenced by both mineral instability and selective dissolution by meteoric freshwater. These findings not only provide fresh insights into complex diagenetic processes in the Dengying Formation but also substantially advance our understanding of early microbial life and environmental adaptations during the Precambrian.

Natural Fractures and Their Contribution to Natural Gas Migration and Accumulation in Marine Carbonate Reservoirs: Lower Triassic Feixianguan Formation, Northeast Sichuan Basin, China

The Lower Triassic carbonate succession of the Feixianguan Formation represents a primary focus for gas exploration in the northwestern Sichuan Basin. This study area includes the massive Puguang gas field and other nearby gas fields of considerable size. These carbonate reservoirs display significant heterogeneity, which is primarily influenced by the presence of natural fractures. Extensive documentation of fracture types, characteristics, effectiveness, and their role in enhancing reservoir properties was conducted by examining and analyzing various data sources, including cores, thin sections, image logs, and experimental measurements. Shear fractures primarily characterize the Feixianguan Formation carbonate reservoir, although tensile and diagenetic fractures are also present, albeit in fewer numbers. Tectonic fractures are the dominant type, particularly unfilled ones with dip angles greater than 60° in the NEE–SWW direction. These fractures are mainly filled with calcite. The tectonic fractures were formed in three stages: Late Indosinian-Early Yanshanian, Late Yanshanian-Early Himalayan, and Late Himalayan. These fractures intersect with the in situ stress direction at a small angle in the NE–SW, NEE–SWW, and near E–W directions, contributing to their effectiveness. Compared with the total fracture density, the effective fracture density is the factor in controlling gas production. An increase in the proportion of effective fractures tends to result in a rise in gas productivity. Additionally, the orientation of effective fractures also influences natural gas production. Fractures striking in the E–W and NE–SW directions, which are particularly effective, are associated with high natural gas production.

A dataset of gas reservoir microscopic images in Xujiahe Formation in northeast Sichuan Basin

The gas reservoir in Xujiahe Formation in northeast Sichuan Basin holds promising potential for exploration and development. In this paper, we collected and sorted the microscopic image data of the clastic reservoir of Xujiahe Formation gas reservoir in the study area, so as to further study the petrological characteristics, diagenesis, reservoir space and physical properties of the region. The reservoir rocks are mainly lithic sandstone, lithic arkose sandstone and quartz sandstone. There are intergranular solution pores, residual intergranular solution pores, intragranular solution pores, clay mineral intergranular (solution) pores and various microfractures in the reservoir space. According to the observation of the cast thin section, the pores are relatively undeveloped, while some thin sections (micro-cracks) are relatively developed. Based on thin section identification and observation, the sandstone reservoirs of Xujiahe Formation in the study area have undergone processes such as compaction, cementation, dissolution and tectonic fracture. This dataset can provide a direct source for the comparative analysis and reference of sandstone image data. Moreover, it can serve as a benchmark for the intelligent identification of lithology.

Characteristics and Evolution of Tectonic Fractures in the Jurassic Lianggaoshan Formation Shale in the Northeast Sichuan Basin

The features and formation stages of natural fractures have significant influences on the fracturing of shale reservoirs and the accumulation of oil and gas. The characteristics and evolution of tectonic fractures in the Lianggaoshan Formation in Northeast Sichuan were investigated based on outcrops, drill cores, geochemical data, and acoustic emission test results. Our results demonstrated that the fracture types of the Lianggaoshan Formation were mainly low-degree bedding-slip fractures, followed by high-degree through-strata shear fractures and vertical tensile fractures. The influences of strike-slip faults on the fractures were stronger than those of thrust faults; fractures in thrust faults were concentrated in the hanging wall. The densities of tensile and shear fractures were inversely proportional to the formation thickness, while the density of interlayer slip fractures was independent of the formation thickness. The density of tectonic fractures was proportional to the quartz content. The fractures of the Lianggaoshan Formation were generated in three stages during uplift: (1) Late Yanshan–Early Himalayan tectonic movement (72~55 Ma), (2) Middle Himalayan tectonic movement (48~32 Ma), (3) Late Himalayan tectonic movement (15 Ma~4 Ma). Fractures greatly improve the oil and gas storage capacity and increase the contents of free and total hydrocarbons. At the same time, they also reduce the breakdown pressure of strata. This study facilitated the prediction of the fracture distribution and oil and gas reservoirs in the Lianggaoshan Formation and provided references for the selection of favourable areas for shale oil and the evaluation of desert sections in the study area.

Influencing Factors of the Brittleness of Continental Shales Containing Shell Limestone Interlayer

Brittleness is important in the evaluation of the fracturing ability of shale reservoir and has a significant impact on shale gas exploration and development. This paper discusses the characteristics and controlling factors of brittleness of continental shale in the Da’anzhai Member of the Ziliujing Formation of Lower Jurassic age in the northeast Sichuan Basin. Continental shale lithofacies and their associations were grouped into four main rock types: clayey shale, silty shale, shell calcareous clayey shale, and silty clayey shale, characterized by the high clay content and local enrichment of carbonate minerals as a whole. Compared with the marine shale, the continental shale contained a low content of siliceous minerals, a high content of carbonate minerals, and a large number of shell limestone interlayers. Carbonate minerals play an important role in controlling the brittleness of continental shale. The shale interlayers were mainly shell limestone interlayers with a thickness of several centimeters and a large number of shell laminates with thicknesses of several millimeters were also observed. The shell laminates were mainly filled with calcite. Due to the dissolution process, a large number of bedding joints and corrosion joints were formed in the calcite shell layers. In the interlayers with a high shell content, a large number of microfractures developed. The energy consumption required for maintaining fracture expansion was lower after fracturing; the fractures greatly improved the reservoir’s brittleness.

Risk Assessment of Flood Disaster in Sichuan Province Based on GIS

In the context of global climate change, flood disaster has become one of the significant disasters endangering human life. Sichuan Province has more annual precipitation and complex landforms, which suffer from yearly floods. To reveal the spatial distribution and spatiotemporal dynamic change characteristics, we developed a risk assessment model based on geographic information systems and natural disaster risk assessment theory. Considering both natural and human factors, precipitation, terrain, climate type, population, and GDP are selected as evaluation indexes. We Used the expert score-AHP method to obtain the annual flood disaster risk regionalization from 2017 to 2021. In the end, the results were compared with the entropy weight method in the objective weighting method. The results showed that the comprehensive flood disaster risk in Sichuan Province is higher in the east and lower in the west. The flood disaster risk in the eastern hill area, Panxi area, and northeast Sichuan Basin is generally higher. In contrast, that in the west Sichuan Plateau is lower. Based on the evaluation results, it is found that the risk assessment of flood disasters based on expert scoring-AHP method is more in line with the actual situation.

Oscillations of a fluvial‐lacustrine system and its ecological response prior to the end‐Triassic: Evidence from the eastern Tethys region

The end‐Triassic mass extinction is considered one of the “Big Five” extinction events in the Phanerozoic. However, whether the terrestrial ecosystem began to deteriorate or even collapse prior to the Triassic–Jurassic (Tr‐J) transition remains controversial. Compared with the documented data from the western Tethyan region, evidence from the eastern Tethyan realm is limited. We undertake a fitting analysis of the sedimentary system, floral community successions and major geological events of the Xujiahe Formation as reflected by the Qilixia Section, Xuanhan area, northeast Sichuan Basin, China. Our results reveal an oscillating fluvial‐lacustrine depositional system during the Late Triassic, with the dominant sedimentary processes mainly controlled by the Indosinian Movement. Beside the sedimentary influence on the Xujiahe Flora, climate changes played a more important role. Fluctuating conditions to cooler and dryer climates at this time promoted diversification of gymnosperms under an overall warm and humid climate setting in the Late Triassic in the Xuanhan area. Superimposed on this oscillating long‐term climate state, ecosystem destabilization occurred over 1 million years prior to the Tr–J interval in the Xuanhan study area, possibly in response to the intensified storm and wildfire activity and the following environmental changes. Although the Xujiahe Flora always recovered from the interruption of the tectonic movement, it ultimately collapsed under extreme climatic events and ecological pressures induced by the Late Triassic Central Atlantic Magmatic Province event.

Characteristics and origin of Lower Triassic Feixianguan Formation dolostones on the west side of Kaijiang–Liangping Trough, northeastern Sichuan Basin, China

High-quality reservoirs of the Feixianguan Formation of the northeast Sichuan Basin contain abundant natural gas and are dominated by dolostones. Their origin is controversial, which impedes the exploration of these high-quality carbonate reservoirs. Petrographic observation and geochemical analysis were performed on various carbonates to trace the source of dolomitizing fluids and clarify the formation mechanism of the Feixianguan dolostones. Three types of dolostones were distinguished, including fine dolomite, fine-medium dolomite, and medium dolomite according to the grain size of dolomites. Fine dolomite was characterized by crystal sizes of 5–60 μm and high total rare earth elements (ΣREEs) of 1.87–2.45 ppm. The thin layers of fine dolomite mainly occur at the top of the depositional cycles and have the highest δ18O values and the lowest SmSN/YbSN values of 0.65–0.87 of the three types, suggesting that they were formed by evaporation pumping dolomitization from the restricted evaporated seawater. Fine-medium dolomite was characterized by the inheritance of the original particle texture with crystal sizes of 60–120 μm. Medium dolomite was characterized by crystal sizes of 100–200 μm and the highest porosity among the three types of dolostones. The similar REE patterns to seawater and the same 87Sr/86Sr ratios as Feixianguan seawater for fine-medium and medium dolomites indicate that the dolomitizing fluids originated from coeval seawater with the Feixianguan limestone. Both the high concentrations of Na and low concentrations of Fe and Mn imply the shallow burial or near surface condition of dolomitization. According to higher δ13C and δ18O values than coeval limestones, and the descending tendency of δ18O values and Na concentrations from the top to bottom of the sedimentary cycles, fine-medium and medium dolomites were formed by reflux dolomitization of evaporated Feixianguan seawater restricted by beach bridges. However, slightly brighter cathodoluminescence of medium dolomite than fine-medium dolomite and the decreasing of δ18O values as the crystal size increasing imply that medium dolomites were slightly recrystallized from fine-medium dolomites. Moreover, a small number of coarse dolomites in pores are derived from exogenous non-seawater fluids in the burial environment.

The Fossil Taphonomic Patterns and Age of the Newly Discovered Large Dinosaur Fossil Groups in the Yunyang Area, Northeast Sichuan Basin, China

The Fossil Taphonomic Patterns and Age of the Newly Discovered Large Dinosaur Fossil Groups in the Yunyang Area, Northeast Sichuan Basin, China

The genesis of authigenic minerals and the porosity evolution of various lithologies and their implications for identifying high-quality reservoirs in the fourth member of Xujiahe Formation (Northeast-Sichuan Basin, China)

With petroleum exploration extending to deeper reservoirs, it becomes more and more difficult to find relatively high-quality reservoirs in heterogeneous sandstones. This research integrates fluid inclusion analysis, petrographic, mineralogical, and geochemical data to determine the origin of authigenic minerals, establish the porosity evolution model and discuss diagenetic effects on reservoir quality of the fourth member of the Upper Triassic Xujiahe Formation. The main rock types of studied sandstones are identified by thin-section, SEM and XRD analysis-feldspathic litharenites, litharenites, calcareous sandstone and chlorite-rich sandstone. Of the four types of sandstone, reservoir porosity of the chlorite-rich sandstone is generally the highest. Target sandstones have experienced complicated diagenetic histories including mechanical compaction, three stages of carbonate cementation, the dissolution of K-feldspar or rock fragments, three kinds of authigenic chlorite precipitation, and two stages of quartz precipitation. Two reaction pathways between K-feldspar and kaolinite were determined and limited dissolution was generated in the closed diagenetic system. Diagenetic sequence and porosity evolution suggest that the critical factors in reducing reservoir quality can be attributed to early intense compaction and widespread calcite cementation. The genesis of high-quality reservoirs among the various lithologies is different. In feldspathic litharenites and litharenites, the key factors for the development of relatively high-quality reservoirs is the acidic dissolution of K-feldspar and less cement being present in the rock. Because chlorite helps retain primary pores by inhibiting quartz overgrowths, almost all chlorite-rich sandstone are relatively high-quality reservoirs. In general, the reservoir quality of calcareous sandstone is poor unless fractures exist that can improve permeability. Determining the porosity evolution of various lithologies is an effective method to predict relatively high-quality reservoirs, which can provide a useful reference for deep petroleum exploration in other areas.

The Influence of Pressure Solution on the Quality of Limestone Reservoirs in the Lower Triassic Feixianguan Formation, Northeast Sichuan Basin: A Quantitative Characterization of Stylolites

The Influence of Pressure Solution on the Quality of Limestone Reservoirs in the Lower Triassic Feixianguan Formation, Northeast Sichuan Basin: A Quantitative Characterization of Stylolites

Paleoclimate Characteristics of the Middle Jurassic Xintiangou Stage in Yunyang Area, Northeast Sichuan Basin, China

Paleoclimate Characteristics of the Middle Jurassic Xintiangou Stage in Yunyang Area, Northeast Sichuan Basin, China

In situ stress characteristics of the NE Sichuan basin based on acoustic emission test and imaging logging

This study presents the distribution rule of in situ stress in the northeast Sichuan basin and its relationship with fracture. Sixty-seven sets of core samples of 21 Wells from the terrigenous clastic rock formation (Shaximiao, Qianfoya, Xujiahe) and marine carbonate formation (Jialingjiang, Leikoupo, Feixianguan) in the northeast Sichuan basin were tested by acoustic emission experiment. The in situ stress variation with the depth was established and the corresponding regression analysis was done. The horizontal principal stress direction of terrigenous clastic rock formation and marine carbonate rock formation was obtained by combining the dual diameter data of 6 wells and the imaging logging data of 3 wells. The results show that the vertical stress in the northeast of Sichuan basin has a linear relationship with the depth, and there is little difference between the vertical stress and the overburden weight of rocks. The maximum and minimum horizontal principal stress and horizontal shear stress increase with the burial depth. The divergence degree of horizontal shear stress with depth greater than 3000 m is greater than that of the stratum smaller than 3000 m. The horizontal stress plays a dominant role in the northeast Sichuan basin. With the increase in depth, the influence of tectonic stress field decreases and the vertical stress increases. Impacted by Dabashan and Qinling plate tectonic movement, the direction of in situ stress in marine carbonate strata is nearly east–west. The direction of maximum horizontal principal stress in terrigenous clastic rock formation is basically northwest–southeast. The imaging logging data show that the fracture direction is consistent with the horizontal principal stress direction, and the present in situ stress direction is favorable to the secondary reconstruction of natural fractures, and the fractures keep good opening. The distribution law of in situ stress in northeast Sichuan basin shows σH > σV > σh, indicating that the fault activity in this area is dominated by strike-slip type, the tectonic stress field is dominated by horizontal tectonic stress, in addition that the stress state is conducive to reverse fault activity.

Formation of an up-flow hot spring of SO4-Ca type in the sandstone aquiclude near an anticline

The formation of springs is vital for understanding the subsurface transport of substances and energy in groundwater circulation. The Xiannvshan hot spring in the sandstone aquiclude near the Tongluoxia anticline in northeast Sichuan Basin of China is an up-flow spring discharging thermal groundwater from the underlying carbonate aquifer of the basin-anticline controlled type. This rarely-encountered up-flow spring neither issues directly from an aquifer, nor is controlled by a fault. The spring water is a very hard (hardness of 1050.069-1835.697 mg/L) and brackish hot spring water (TDS of 1650-2802 mg/L) of SO4-Ca type and low temperature (34-38°C) issuing from a neutral and reduction subsurface environment, and is characterized by high ratios of γNa/γCl (average 1.354) and low γCa/γSO4 (average 0.779), and high concentrations of Sr (7.62-17.183 mg/L) and F (1.87-8.15 mg/L). The ΣMREE of the hot water samples are greater than ΣLREE, and positive Eu and negative Ce anomalies exist. The dissolved constituents in the hot water are derived from the dissolution of calcite, dolomite, anhydrite, gypsum, polyhalite, halite, albite and K-feldspar and small amount of sylvite, epsomite, thenardite, langbeinite, celestite, barite, strontianite, magnesite, and fluorite in the carbonate aquifer, and the dissolution of sulphates prevails in the groundwater system. Hydrochemical similarity is found among the water samples from the two springs’ vents and the flowing borehole near the vents. Faster groundwater circulation caused by the flowing hot water borehole encourages the dissolution of minerals in the carbonate aquifer and slightly increases the TDS and temperature of the hot water from the flowing borehole.

Facies-dependent early diagenesis and hydrothermal reworking in dolostones: a case study on upper Permian and lower Triassic in northeast Sichuan Basin

Facies-dependent early diagenesis and hydrothermal reworking in dolostones: a case study on upper Permian and lower Triassic in northeast Sichuan Basin

Evidence of sulfur isotope about the sedimentary environment of new type of polyhalite potassium ore in the northeast Sichuan Basin

Evidence of sulfur isotope about the sedimentary environment of new type of polyhalite potassium ore in the northeast Sichuan Basin

Evaluating the mechanical properties of carbonate and evaporite caprocks in the Sichuan Basin

Caprocks are nearly impermeable rocks that inhibit the migration of hydrocarbons. Caprock efficiency at sealing hydrocarbon reservoirs is contingent upon maintaining its integrity, or the absence of features that enhance fluid flow such as fractures/faults. Caprock lithology is an important factor in sealing efficiency, as it controls caprock mechanical behavior and brittleness. Evaluating caprock failure due to chemo-thermo- mechanical forces requires an understanding of the deformation and failure properties consistent with effective caprocks. Core samples of caprocks from the northeast Sichuan Basin – primarily evaporites, limestones and dolostones – were tested under laboratory conditions to determine the relationships between lithology and geomechanical properties of different caprocks. The results of hardness, ultrasonic velocity, and axial deformation tests of the caprock samples indicate that caprock brittleness is strongly correlated with high values of hardness, peak and yield strength, Young’s modulus and coefficient of internal friction. Lithologic heterogeneity in caprocks is found to enhance sample failure in most cases, though the inclusion of minor phyllosilicate and evaporite minerals may conversely decrease caprock brittleness. Evaporites and dolostones consistently exhibit the least and most brittle characteristics, respectively, though individual mechanical properties can indicate contradictory caprock behavior. These results suggest evaluation of the geomechanical-risk to caprocks can thus be enhanced by combining mechanical properties to map trends in caprock effectiveness. When combined with accurate in- situ conditions, the mechanical properties of a caprock allow for a quantitative interpretation of caprock integrity.

Characteristics and evaluation of tight gas reservoir in Shaxi miao Formation, Northeastern Sichuan Basin

The reservoir of Shaximiao Formation is mainly composed of channel coarse sandstone and fine sandstone. The rocks are dense, the lithology is single and the thickness is l arge. The physical properties of Shaximiao Formation are not clear for the northeast Sichuan area. Based on field and core physical property analysis of Shaximiao Formation in Northeast Sichuan Basin, the characteristics of porosity and permeability of Shaximiao Formation are defined, the porosity-permeability relation chart is established, and on this basis, The tight reservoir of Shaximiao Formation was evaluated, the result show that the favorable reservoir is mainly located in the intersection area of channel sand, the reservoirs are mainly of III type, followed by the reservoirs of II type, and the I type are the least.

Assessments of future climate extremes in China by using high-resolution PRECIS 2.0 simulations

Based on bias-corrected future projections from the Providing REgional Climates for Impacts Studies (PRECIS) regional climate model under both RCP4.5 and 8.5 scenarios over China, extreme climatic events at the middle and end of the twenty-first century are investigated in this paper. The model performance of PRECIS is validated using comparisons with observations and HadGEM2-ES projections, and the bias correction adds fidelity to the projections of basic climate variables and extreme climate events. In the future, our single-realisation estimates show that the number of frost days is projected to decrease and days with tropical nights are projected to increase. Including northeast China, Sichuan Basin, middle and lower reaches of Yangtze River and south China, the number of consecutive dry days will increase from our single-realisation estimates, which will exhibit a spatial distribution almost similar to that of consecutive wet days in the coming decades. Although precipitation indices associated with duration will increase, a simple precipitation index depicting the intensity of extreme events will decrease over east and south China in the future from our single-realisation projections. Daily rainfall above 50 mm, which is usually regarded as a rainstorm event, is predicted to increase under the RCP4.5 scenario over most of China from our single-realisation estimates; while the same change pattern will occur in southernmost China, similarly spatially distributed for the RCP8.5 scenario, the number of rainstorm will first increase in the middle of the twenty-first century and then slow at the end of the twenty-first century in the Yangtze River region. In conclusion, our single-realisation estimates indicate that the persistence of extreme precipitation will increase with time, but the change of extreme precipitation intensity is not significant in the future. The comparison of climatic extreme events under the RCP4.5 scenario with those under the RCP8.5 scenario shows that extreme climate events will be enhanced under the higher emissions scenario; hence, reductions in greenhouse gas emissions will help alleviate climate change effects in the future.

Impact of coexisting clay minerals and organic matter on pore growth in the Lower Jurassic Da’anzhai lacustrine shale reservoir in the Northeast Sichuan Basin, West China

Understanding pore growth is of great significance to investigating reservoir performance in shale-gas systems. However, different from the marine shale reservoir, the lacustrine shale reservoir is commonly rich in clay minerals, resulting in a complicated and poorly understood pore system. We have investigated the impact of coexisting clay mineral and organic matter on pore growth in the Lower Jurassic Da’anzhai Shale in the Northeast Sichuan Basin, West China, through performing total organic carbon (TOC) analysis, X-ray diffraction, field-emission scanning electron microscopy, focused ion beam-scanning electron microscopy (FIB-SEM), [Formula: see text] and [Formula: see text] adsorption experiment, and high-pressure mercury intrusion porosimetry. Our results indicate that the Da’anzhai Shale is dominated by clay-mineral-hosted pores, which are commonly filled or partly filled by pyrobitumen. Controlled by organic maceral, organic pores are poorly and heterogeneously developed in pyrobitumen, and minor or even no organic pores grow in vitrinite. Mesopore and macropore are popular in the Da’anzhai Shale reservoir with complex shapes, e.g., slit- or plate-like shapes combined with “ink-bottle” shapes, confirming a pore system dominated by clay-mineral-hosted pores. The weak positive correlation between the clay mineral content and the meso/macropore volume confirms that the clay mineral is a positive contributor to the storage space, and the weak negative correlation between the TOC and the mesopore volume suggests that infilling of pyrobitumen decreases the pore volume significantly. Similar correlations occur between specific surface area and clay mineral/TOC. FIB-SEM observation confirms that the pore system, e.g., the pore size, pore shape, and pore volume, is controlled by the coexisting clay mineral and pyrobitumen filling in a later stage. The calculated plane porosity of the initial inorganic pore and the unfilled inorganic pore in the Da’anzhai Shale is in the range of 3.66%–10.95% and 0.79%–1.46%, respectively, suggesting that 76.66% of inorganic pores is inactive due to pyrobitumen filling. All of this evidence suggests that pore growth in the Da’anzhai Shale is positively contributed by clay minerals, but it is negatively contributed by pyrobitumen filling. Further discussion suggests that pyrobitumen infilling between clay minerals in the Da’anzhai lacustrine shale can decrease the original pore volume significantly, which work together to govern the pore system in shale reservoirs.