Search

Topography is a critical factor that determines the characteristics of regional soil formation. Small-scale topographic changes are referred to microtopographies. In hilly mountainous regions, the redistribution of water and soil materials caused by microtopography is the main factor affecting the spatial heterogeneity of soil and the utilization of land resources. In this study, the influence of microtopography on pedogenesis was investigated using soil samples formed from mudstones with lacustrine facies deposition in the middle of the Sichuan Basin. Soil profiles were sampled along the slopes at the summit, shoulder, backslope, footslope, and toeslope positions. The morphological, physicochemical, and geochemical attributes of profiles were analyzed. The results showed that from the summit to the toeslope, soil thickness increased significantly and profile configuration changed from A–C to A–B–C. The total contents of Ca and Na decreased at the summit, backslope, and footslope, while the total contents of Al, Fe and Mg showed an opposite trend. On the summit and shoulder of the hillslope, weathered materials were transported away by gravity and surface erosion, exposing new rocks. As a result, soil development in these areas was relatively weak. In flat areas such as the footslope and toeslope with sufficient water conditions, the addition of weathered components and the prolonged contact between water, soil, and sediment led to further chemical weathering, resulting in highly developed characteristics. Microtopography may influence physicochemical properties, chemical weathering, and redistribution of water and materials, causing variations in pedogenic characteristics at different slope positions. [ABSTRACT FROM AUTHOR]

The northeastern Sichuan area in the northern Yangtze margin has unique Ediacaran geological records, especially the Doushantuo Formation (DST), and become a hot research area in recent years. However, the Cryogenian-Ediacaran (C-E) boundary has not been precisely identified, which restricts the in-depth study of geological information during this crucial transitional period and is unfavorable for a systematic and complete understanding of the Yangtze Block and even the global paleogeographic pattern. This study conducted stratigraphy, sedimentology, and chronostratigraphy to establish the stratigraphic framework and sedimentary evolution of the C-E transition strata in northeastern Sichuan. The results showed that the Ediacaran sediments, without the cap dolomite, unconformably overlaid the Cryogenian sediments in the studied area. The Member II of the DST, characterized by 50e160 m of red-green sandstone (approximately equivalent to the original Chengkou "Guanyinya Formation"), directly overlaid the Cryogenian sediments and displayed a 623 ± 2.3 Ma maximum depositional age from the detrital zircon UePb dating. Regional stratigraphic correlations indicate that the C-E transition strata in northeastern Sichuan had a consistent lithological association and sedimentary sequence characteristics but differed from the Three Gorges. Typically, the upper Nantuo massive glacial diamictites transition to the icebergs rafted lonestone-bearing mudstones at the top, then change upward to DST barrier coast sandstones. The proposed DST of the northeastern Sichuan Basin was divided into three lithostratigraphic members without the regional Member I cap dolomite: (i) Member II purple-red, gray-green sandstone strata, (ii) Member III black mudstone strata, and (iii) Member IV P-Mn bearing strata. During the C-E transition, the study area experienced (i) the global deglaciation stage in the terminal Marinoan glaciation and (ii) the filling-leveling up stage with clastic rocks in the early Ediacaran. Overall, the early Ediacaran of northeastern Sichuan succeeded the paleogeographic features of the late Cryogenian. [ABSTRACT FROM AUTHOR]

Red-layer mudstone is a typical expandable rock, that is, it easily expands and deforms in water. This study focused on the dynamic quantitative analysis of the mudstone hygroscopic process and development of strain caused by the hydration expansion of clay, by combining a self-designed simple self-imbibition device with the digital image correlation method. The mudstone samples were self-imbibed in deionized water and salt solutions of different concentrations under the same atmospheric conditions (25°C and 50% humidity). The effects of the salt type and salt concentration on the water absorption quality, expansion rate, crack initiation and expansion, matrix deformation, and water migration of mudstone over time were analyzed. The change in the water absorption capacity of mudstone with time was determined from the water absorption characteristic curve. The location and strain development of the local high-strain area during the water–rock interaction was captured from the full-field strain cloud image. The migration of water on the rock surface at different times was observed using camera photographs. Finally, water absorption was combined with strain, water migration, and crack propagation. The effect of saltwater on the hygroscopic expansion of mudstone was evaluated. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Railway Science & Engineering is the property of Journal of Railway Science & Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The sedimentary environment and organic matter enrichment relationship of the Upper Ordovician Wufeng Formation black shale, Guanyinqiao mudstone, and Lower Silurian Longmaxi Formation black shale in the Sichuan Basin of Weiyuan are analyzed using geochemical methods such as organic carbon, sulfur, major elements, and trace elements. The experimental results illustrate that the upper section of the Wufeng Formation and the lower section of the Longmaxi Formation are organic matter enrichment layers. The presence of P indicates a high productivity level in the Sichuan Basin from the Late Ordovician to the Early Silurian. In addition, indicators such as V/Cr, Ni/Co, and S/C suggest that the Wufeng Formation was deposited under anoxic reductive conditions, that the ice age Guanyinqiao segment was in an oxygen-rich to oxygen-poor environment, and that the Longmaxi Formation was in a sulfidic environment. Mo/TOC indicates that the Wufeng Formation shale was controlled by a restricted basin and that the Guanyinqiao segment and the Longmaxi Formation were in a medium-to-weak retention environment. The weak correlation of TOC with P/Al and Al indicates that the level of primary productivity and terrigenous detritus had a minor effect on the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Conversely, the positive correlation of TOC with V/Cr and Ni/Co illustrates that the anoxic reductive sedimentary environment is the main factor affecting the organic matter enrichment of the Wufeng and Longmaxi Formation black shale. Based on these studies, the development model of organic-rich shales of the Ordovician–Silurian in Weiyuan, Sichuan Basin is proposed. This paper may provide a reference for shale gas exploration in the Wufeng–Longmaxi Formation and a sedimentary response to the major geological events of Ordovician–Silurian. [ABSTRACT FROM AUTHOR]

The Triassic and Jurassic tight sandstone gas in the western Sichuan depression, Sichuan Basin has attracted much attention in recent years, and the Upper Triassic coal-bearing Xujiahe Formation is believed to be the major source rock. However, there are relatively few studies on the carbon isotopic heterogeneity of methane generated from coal-measure source rocks and the origin of the natural gases in Xinchang Gas Field is still controversial. In this study, one coal-measure mudstone sample and one coal sample of the Xujiahe Formation in western Sichuan Basin were selected for gold tube pyrolysis experiment to determine their gas generation characteristics. Geological extrapolation of gas generation and methane carbon isotope fractionation parameters reveals that the main gas generation stage of Xujiahe Formation ranges from Late Jurassic to Cretaceous in the Xinchang Gas Field. The natural gas in the 5th member of Xujiahe Formation in Xinchang Gas Field is mainly derived from the 5th member of Xujiahe Formation itself, i.e., self-generation and self-reservoir, however, the gas in the Jurassic gas pools is mainly derived from the source rocks of the 3rd member of Xujiahe Formation rather than the 5th member of Xujiahe Formation. [ABSTRACT FROM AUTHOR]

In recent years, new breakthroughs have been made in the field of shale oil and gas exploration in the Lower Jurassic Lianggaoshan Formation in Sichuan Basin. At present, there is a lack of systematic studies on reservoir properties and sedimentary facies of the Lianggaoshan Formation shale. Therefore, in this study, taking the Lianggaoshan Formation in Sichuan Basin as an example, the sedimentary facies types of shale reservoirs and their control over shale oil and gas are systematically studied, based on a large number of outcrops, experimental testing, logging, and seismic interpretation methods. The results show that five sedimentary microfacies are developed in the Lianggaoshan Formation in the study area, namely, semi-deep lake mud, shallow lake mud, wave-influenced shallow lake mud, delta-influenced shallow lake mud, and underwater interbranch bay microfacies. The stratum thickness of the Lianggaoshan Formation is in the range of 26–315 m, and mainly distributed in the eastern region, but rapidly thinned in the northwestern region. The sedimentary sequence framework of the Lianggaoshan Formation has been constructed. Moreover, the lithology of the Lianggaoshan Formation shale has been divided into three types, including shale, massive mudstone and silty mudstone. The brittleness index and total organic carbon (TOC) value of three types of shale show a negative correlation. Silty mudstone has the highest brittleness, while that of black shale is the lowest. For porosity and permeability, massive mudstone is better than silty mudstone, and silty mudstone is better than black shale. There are many kinds of matrix pores in the Lianggaoshan Formation shale, and the development degree of inorganic pores is higher than that of organic pores. Finally, based on the analysis of oil-bearing, pore types, physical properties and productivity, it is considered that black shale facies is the most favorable lithofacies type. The deep–semi-deep lacustrine facies belt obviously controls the shale oil enrichment of the Lianggaoshan Formation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The pore-microfracture structure, water-rock interaction and capillary water absorption mechanism of red-layer mudstone were comprehensively studied. The mineral composition was determined using X-ray diffraction, and the pore-microfracture structure was observed using scanning electron microscopy. Mercury intrusion porosimetry and nitrogen physisorption were used to obtain pore throat size distribution information. The effects of mineral composition and pore structure on adsorption and desorption behaviour were investigated using water vapour physisorption. The wettability of the mudstone surface was characterised by contact angle measurement, atomic force microscopy. The capillary water absorption coefficient and capillary water absorption height of mudstone were quantitatively studied using capillary water absorption experiments. The results showed that the clay mineral content of mudstone was 46.8%, and the pores were mainly micropores and transition pores, which were very dense. The water vapour physical adsorption isotherm of mudstone showed that the average water-holding rate of mudstone was 31.47%. In the process of desorption of mudstone, the "ink-bottle" effect limited the desorption of water under medium and high relative humidity. The macroscopic contact angle of the mudstone surface was less than 29.79°, indicating mudstone's strong hydrophilic property. The driving force of capillary water absorption of mudstone should consider the capillary force, osmotic pressure and gravity. The hydration fracture of the mudstone was the dominant channel of its capillary water absorption. Understanding the relationship between the water absorption mechanism of mudstone capillarity and pore-microfractures is necessary for revealing the disaster mechanism of the red-layer mudstone. [ABSTRACT FROM AUTHOR]

Water has a significant effect on the mechanical properties of rocks, especially those containing expansive clay minerals. The red-layer mudstone contains various clay minerals, which easily collapse in water. To further study the effect of water on the mechanical properties of red-layer mudstone, uniaxial and triaxial tests were first conducted on samples with different moisture contents, prepared using the gaseous moisture absorption method in the laboratory. The effect of water on the fracture mode, mechanical behaviour and strength of the red-layer mudstone was subsequently studied at a heterogeneous particle scale using the three-dimensional numerical simulation program 3DEC. The results demonstrated that the strengths of the mudstone samples (uniaxial compressive and triaxial compressive strengths) decreased with an increase in the moisture content. As the moisture content increased, the elastic modulus and residual strength of mudstone decreased. The Mohr–Coulomb strength envelope under different moisture contents was constructed, and it was found that with an increase in the moisture content, the cohesion of the mudstone decreased significantly and the internal friction angle remained almost unchanged. This numerical model accurately demonstrates the damage evolution (tensile and shear cracks) at a heterogeneous particle scale. Simultaneously, the macroscopic mechanical behaviour, strength and failure patterns observed in the laboratory were reproduced. Finally, this study reveals the weakening mechanism of water on the mechanical properties of red-layer mudstone at macroscopic and Ümesoscopic scales. Highlights: Numerical simulation and laboratory tests were used to study the variation of mechanical properties of red-layer mudstone with moisture contents and the weakening mechanism of moisture on its mechanical properties Mudstone samples with different moisture contents were prepared by gaseous hygroscopic method, which overcame the challenge of mudstone disintegrating in water The discrete element method (3DEC) was used to randomly generate heterogeneous mudstone samples with different clay mineral grains. An empirical formula was established to predict the mechanical properties of red-layer mudstones at different moisture content. The effects of different moisture contents on mesoscopic mechanical parameters and joint displacements of contacts between red-layer mudstone grains were analysed. [ABSTRACT FROM AUTHOR]

The slope stability of Ya'an expressway in Sichuan dominated by mudstone strata, is influenced greatly by both the mechanical properties and stressstrain relationship of mudstone. In this paper, the mechanical properties of the Ya'an mudstone samples under triaxial compression conditions were studied, based on an established constitutive model under the framework of breakage mechanics to simulate the mechanical properties of mudstone. Firstly, triaxial compression tests and SEM tests at the confining pressures of 0.5 MPa, 1.0 MPa, and 2.0 MPa were carried out on the mudstone samples, and it was found that the mudstone sample undergoes strain softening and dilatancy followed by the volumetric compaction. Then, based on analysis on the breakage mechanism of the above test results, we idealized the mudstone sample as a binary medium material consisting of the bonded elements and frictional elements, of which the bonded elements are composed of solid matrix and pores, and the frictional elements are composed of broken aggregates. During the loading process, the cementation between clay minerals and non-clay minerals in the mudstone sample is first destroyed, leading to the formation of micro-cracks within the particle aggregate, that is, the bonded elements are gradually damaged during the loading process and gradually turned into the frictional elements, and the two jointly bear the external load. The bonded elements are composed of mudstone matrix and pores, which have the cementitious characteristics of mudstone, and the frictional elements are composed of the broken aggregate with the frictional characteristics of the broken particles. Based on the homogenization theory, the constitutive model for the mudstone is established, and the determining method for model parameters is also given. Finally, the results of the triaxial compression tests of the mudstone samples are predicted by the constitutive model proposed here, which can reflect the main mechanical properties of the mudstone samples. [ABSTRACT FROM AUTHOR]

This study investigates the provenance and tectonic background of the lower Silurian Longmaxi Formation black shale of the Yanyuan Basin in the western Kangdian ancient land and provides guidance for shale gas exploration and development in the area. The mineral petrological and geochemical characteristics of the Longmaxi Formation black shale in this area have been studied in detail. The study area is mainly a passive continental margin environment, but also has the attributes of an active continental margin and island arc environment due to the collision between the western oceanic crust and the Yangtze continental crust. The source rocks are mainly felsic igneous rocks, with a small contribution of intermediate–basic rocks. It is inferred that the Kangdian ancient land in the eastern part of the area could be the main provenance area, but with the contribution of sediments derived from oceanic island arc located in the west. During the whole Longmaxi period, the tectonic movement was intense, the climate was cold, the degree of chemical weathering was low, and it was a highly volatile geological sedimentary basin. Therefore, the highly siliceous organic-rich mudstone, which was different from the Sichuan Basin, was deposited. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

"Sweet sections" in giant shale oil provinces are preferential fields that primarily support China to increase the reserves and production of continental shale oil. Based on the study of the geological conditions of shale oil in the continental basins in China, it was found that the shale stratum in major oil generation windows generally has higher degrees of oil and gas accumulation, and mostly contains oil. Hydrocarbon generation and reservoir capacities are the two key parameters for evaluating and optimizing favorable shale oil provinces. The evaluation index (volume of shale stratum multiplied by total organic carbon (TOC) multiplied by total porosity) for the giant continental shale oil provinces is also proposed. It is optimized that the Upper Triassic Chang 7 Member in the south-central Ordos Basin, Lower Cretaceous Qing 1 Member in the Gulong-Changling Sag in the Songliao Basin, Middle — Lower Permian in the Junggar Basin, Da'anzhai Member of the Ziliujing Formation of Lower Jurassic in the central and northern Sichuan Basin, and Paleogene oil-rich sag in the Bohai Bay Basin are the five giant continental shale oil provinces. The word "geological sweet sections" in continental shale oil provinces of China refers to favorable shale intervals which are relatively rich in oil, with superior physical properties, and more easily modified and developed commercially under applicable economic and technological conditions. After evaluation, there are mainly two types of "geological sweet sections" of giant continental shale oil developed onshore in China. One type of "geological sweet sections" is generally mudstone with optimal physical properties or a thin tight reservoir, to which the shale oil migrates a short distance. They are medium-to-high-mature zones with a thin sandy shale stratum in the Chang 7 Member in the Ordos Basin, mixed shale stratum in the medium-mature Lucaogou Formation in the Jimsar Sag, and multi-layered mixed Paleogene shale stratum in the Bohai Bay Basin. The other type of "geological sweet sections" is generally shale oil residing in various shale reservoir spaces. This type was developed in the Qing 1 Member in the Gulong Sag and Da'anzhai Member in the north-central Sichuan Basin. Free shale oil mainly occurs in shale, sandy-carbonate lamina, micro-lamella structure, and micro-fractures. Layers with lamina, lamination, and micro-fractures are generally shale oil "geological sweet sections." Starting from field tests and the construction of the "geological sweet sections" in giant continental shale oil provinces, the shale oil industry has been rapidly developing and will become an important supplement to domestic oil production in China. [ABSTRACT FROM AUTHOR]

Previously conducted studies have established that gaseous water sorption of mudstone is widespread in nature. The deterioration of its uniaxial compression properties during gaseous water sorption can cause engineering problems. However, related studies were currently in the initial stage of this research direction. On the one hand, there were few studies on the deterioration characteristics of the uniaxial compression properties of mudstone in this process. The results might not be applicable to all projects. On the other hand, its microstructure changes in this process were unclear. Therefore, to obtain the deterioration characteristics of uniaxial compressive performance during gaseous water sorption for offering scientific reference to the geotechnical engineering of mudstone in the central Sichuan region of China, red-bed mudstone was used as a research material. A swelling test and uniaxial compression tests were carried out. To clarify microstructure changes for advancing the depth of research on the effects of gaseous water on mudstone, scanning electron microscopy (SEM) tests were performed. As a result of this study, formulas were first established that could correctly characterize the deterioration of uniaxial compressive strength (UCS) and elastic modulus when the moisture absorption rate increased. Secondly, the dependence was obtained, which was the relationship between both the UCS and elastic modulus and moisture absorption time. Finally, microstructure changes were revealed during gaseous water sorption. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Exploring the development of multiscale behaviors of red-bed mudstone is beneficial for in-depth evaluation of swelling capacity and its progressive failure mechanism. In this paper, a macro-micro study was performed on rock samples in central Sichuan, China. The influence of environmental factors such as water content, degree of soaking, and gaseous moisture absorption was discussed. Spatial characterizations were finally described from pore structure and distribution. Experimental results suggested that deformation curve under water immersion had three stages of rapid growth, deceleration growth, and gradual stabilization. It can enter stable deformation within 60~80h and has short-term swelling effects, while gaseous moisture absorption presented long-term performance during 0~700 h. The difference in deformation between partial and complete immersion may be the release of swelling potential caused by humidity gradient and capillary suction. The strain-time curves during water immersion all tended to short-term swelling. Moreover, repeated drying and wetting promoted the time of shrinkage adjustment in the second cycle to be significantly shortened, resulting in a large amount of irreversible plastic deformation. Although initial water content influenced the final stable strain, it only changed the degree of deformation at initial stage. For the microscale structures, the closer to the end, the greater the porosity and the smaller the middle. The average porosity after 0, 1, and 2 dry and wet cycles reached 0.227%, 4.027%, and 6.121%, respectively. Geometric parameters such as volume and surface area of the pores enlarged with the increase of the number of cycles. Small pores gradually evolved into microcracks and large fissures. Besides, the pores developed from "tree-like" to "net-like" structures with continuous cycles. [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology & Engineering Geology / Shuiwendizhi Gongchengdizhi is the property of Hydrogeology & Engineering Geology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In addition to marine and marine-continental transitional strata, the continental ones are also widely distributed in various oil and gas-bearing basins in China. The continental shale generally provides favorable material bases for hydrocarbon generation, such as wide distribution, large thickness, multiple series of strata, high TOC content, nice organic matter type, and moderate thermal evolution. Part of such shale contains shale oil, but the pore space characteristics for the occurrence of this oil are not thoroughly studied. In order to accurately and quantitatively characterize the pore space where the continental shale oil in different types of lithofacies occurs, we sampled the rock cores from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Area. The TOC content and mineral composition were analyzed, and we also carried out experiments on CO2 and N2 adsorptions, high-pressure mercury injection, and wash oil. Results show significant differences in pore space characteristics for the occurrence of shale oil in different types of lithofacies. In organic-rich mixed and clayey mudstones with the highest TOC content, the free shale oil, occupying the largest reservoir space, mainly occurs in macropores and mesopores, and the adsorbed shale oil, occupying the largest reservoir space, mainly occurs in mesopores. In the organic-bearing clayey mudstone, which has a higher TOC content, the free shale oil takes a larger reservoir space and mainly occurs in macropores, followed by mesopores, and the absorbed one, occupying a larger reservoir space, mostly occurs in micropores and then the mesopores. The organic-bearing mixed mudstone has a moderate TOC content, in which the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, followed by macropores, and the absorbed one, which takes a larger reservoir space, all occurs in mesopores. In the fine sandstone, the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, while the absorbed one occupies a smaller reservoir space and all occurs in mesopores. [ABSTRACT FROM AUTHOR]

This study investigates the degradation mechanism of red mudstone under the influence of water vapor cycling from the macro-micro perspectives. Experiments are conducted on mudstone samples collected from Ziyang City, Sichuan Province, with a relative humidity change (ΔRH) of 70 %. Subsequently, the damage and deformation characteristics of rocks during uniaxial loading processes are further investigated using DIC-3D and AE techniques. The results reveal that water vapor cycling leads to an increase in the irreversible cumulative volume strain, and caused the damage variable D to increase exponentially with irreversible cumulative strain. The 3D-total fracture rate, 3D-connected fracture rate, and 3D-fractal dimension increase in a negative exponential manner from initial values of 0.011, 0, and 2.160–0.034, 0.780, and 2.510, respectively. The fracture morphology of mudstone during uniaxial loading transforms from global shear failure to locally dispersed cracks with the increase in the number of water vapor cycles, causing a linear increase in the proportion of shear fractures from 18.07 % to 29.04 %. Moreover, water vapor cycles decrease the normalized initial stress and normalized damage stress from 0.513 and 0.896–0.392 and 0.791, prolong the compaction process of rocks, and shorten the linear elastic deformation stage. • The degradation mechanism of red mudstone under water vapor circulation conditions were studied. • The damage and deformation of the red mudstone during uniaxial compression underwater vapor cycling were studied based on DIC and AE. • The degradation law of red mudstone under water vapor circulation conditions was obtained. [ABSTRACT FROM AUTHOR]

Great progress has been made in middle Permian exploration in Northwest Sichuan in recent years, but there are still many questions in understanding the hydrocarbon accumulation conditions. Due to the abundance of source rocks and the multi-term tectonic movements in this area, the hydrocarbon accumulation model is relatively complex, which has become the main problem to be solved urgently in oil and gas exploration. Based on the different tectonic backgrounds of the middle Permian in northwest Sichuan Basin, the thrust nappe belt, the hidden front belt, and the depression belt are taken as the research units to comb and compare the geologic conditions of the middle Permian reservoir. The evaluation of source rocks and the comparison of hydrocarbon sources suggest that the middle Permian hydrocarbon mainly comes from the bottom of the lower Cambrian and middle Permian, and the foreland orogeny promoted the thermal evolution of Paleozoic source rocks in northwest Sichuan to high maturity and over maturity stage. Based on a large number of reservoir physical properties data, the middle Permian reservoir has the characteristics of low porosity and low permeability, among which the thrust nappe belt and the hidden front belt have relatively high porosity and relatively developed fractures. The thick mudstone of Longtan formation constitutes the regional caprock in the study area and the preservation condition is good as a whole. However, the thrusting faults destroyed the sealing ability of the caprock in the nappe thrust belt. Typical reservoir profiles revealed that the trap types were different in the study area. The thrust fault traps are mainly developed in the thrust nappe belt, while the fault anticline traps are developed in the hidden front belt, and the structural lithological traps are developed in the depression belt. The different structural belts in northwest Sichuan have different oil and gas accumulation models. This paper built three hydrocarbon accumulation models by the analysis of reservoir formation conditions. The comprehensive analysis supposed the hidden front belt is close to the lower Cambrian source rock, and the reservoir heterogeneity is weak, faults connected source rock is developed, so it is a favorable oil and gas accumulation area in the middle Permian. [ABSTRACT FROM AUTHOR]

The swelling and softening of mudstones caused by water often lead to serious geological disasters. In petroleum exploration and development, the hydration expansion of mudstone can also lead to serious wellbore instability hazards. The swelling of smectite clays is the main reason for this hydration expansion in mudstones. It is generally recognized that illite and illite–smectite mixed layer clays are both poor swellable and has a low effect on rock strength of smectite-poor mudstone. Furthermore, weakening law and degree of smectite-poor mudstone in water is not clear. This study conducted swelling tests and rock mechanics experiments on a smectite-poor mudstone from the Sichuan basin. The results show that both water content and swelling capacity tend to remain steady after soaking the samples for 24 h. The mudstone water content reached 5.36%, and the swelling rate was 2% after soaking in water for 72 h, and the rock had a lower unconfined compressive strength (UCS) value of 1.41 MPa. Adding KCl inhibited these processes, and elevated KCl concentrations drastically reduced the mudstone water content and swelling rate. Compared to water, the water content and swelling rate of mudstone soaked in a 6% KCl solution both decreased by 50%. The UCS and the elastic modulus also increased for a half-dry mudstone, although the inhibitor was important. At elevated KCl inhibitor concentrations, the UCS and elastic modulus of the hydrated mudstone were lower than those of the dry mudstone. The findings of this study can help for better understanding of the application of KCl inhibitor in water-based fluid for drilling engineering. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The formation of large igneous provinces (LIPs) seems to require a series of conditions conducive to massive dolomitization; however, the specific mechanisms remain unclear. Based on petrographic, geochemical and fluid-inclusion microthermometry analyses, this study identifies the types of dolomites in the Maokou Formation (Middle Permian) in the eastern Sichuan Basin. It analyses the related dolomitization fluids and discusses the influencing mechanisms of the Emeishan large igneous provinces (ELIP) on dolomitization at different stages. Three types of dolomite were identified: (1) fabric-retentive, very finely crystalline, planar-s to nonplanar-a dolomite (MD1); (2) fabric-retentive, fine to medium crystalline, planar-s to nonplanar-a dolomite (MD2); and (3) medium to coarsely crystalline, nonplanar (saddle) dolomite (SD). MD1 exhibited dark-red luminescence under cathodoluminescence and rare earth element patterns that are similar to those of lime mudstone, indicating that its dolomitization fluid was medium-salinity Permian seawater. MD2 displayed orange-red luminescence under cathodoluminescence, with a high Ba content (mean: 397.5 ppm) and positive Eu anomalies (mean: 2.34), indicating that its dolomitization fluid was a mixture of shallowly buried seawater and a small amount of external hydrothermal fluid. SD showed red luminescence under cathodoluminescence, with the highest Ba content (mean: 9820.4 ppm) and significant positive Eu anomalies (mean: 3.165), indicating that its dolomitization fluid was mainly ELIP-related hydrothermal fluid with a small amount of residual seawater. Before the ELIP activity, no dolomite occurred in the study area. In the initial stage of ELIP activity, upwelling of the Emeishan mantle plume led to a shift in sedimentary patterns, promoting the occurrence of reflux dolomitization and the formation of MD1. In the intermediate stage of ELIP activity, the activation of basement faults promoted the occurrence of thermal convection dolomitization, resulting in MD2 formation. In the peak stage of ELIP activity, large-scale eruption of Emeishan basalt led to the development of hydrothermal dolomitization, forming SD. This study provides an approach to the investigation of the influences of LIPs on dolomitization processes. • Three types of dolomite are developed in the Maokou Formation in eastern Sichuan. • Each type of dolomite show different origin and nature of dolomitized fluids. • The ELIP control the formation and distribution of dolomite of Maokou formation in Sichuan Basin. [ABSTRACT FROM AUTHOR]

The construction of a high-speed railway (HSR) in Southwest China is being hindered by a severe shortage of high-quality subgrade materials. However, red mudstone is widely distributed in the Sichuan Basin of China. The ability to use weathered red mudstone (WRM) to fill subgrade beds by controlling its critical stress and cumulative strain would enable substantial savings in project investments and mitigate damage to the ecological environment. To better understand the dynamic behaviour of WRM, both monotonic and cyclic triaxial tests were performed. The evolution of the cumulative strain vs. increased loading cycles was measured. The influences of confining pressure and loading cycles on the dynamic modulus, damping ratio, critical cyclic stress ratio (CSR), and dynamic stress level (DSL) were investigated. The relationship between the CSR and loading cycles under different failure strain criteria (0.1%-1.0%) was analysed. The prediction model of cumulative strain was also evaluated. The results indicated that the shear strength of WRM sufficiently meets the static strength requirements of subgrade. The critical dynamic stress of WRM can thus satisfy the dynamic stress-bearing requirement of the HSR subgrade. The critical CSR decreases and displays a power function with increasing confining pressure. As the confining pressure increases, the DSL remains relatively stable, ranging between 0.153 and 0.163. Furthermore, the relationship between the dynamic strength and loading cycles required to cause failure was established. Finally, a newly developed model for determining cumulative strain was established. A prediction exercise showed that the model is in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Quartz is critical to reservoir properties (e.g., rock mechanical behavior and porosity) of organic mudstones, exerting an important effect on shale gas production. However, the types and origins of quartz in deeply-buried Upper Permian mudstones, a potential target of shale gas exploration in South China, remain poorly understood. A suite of deeply buried (5636–5645 m) mudstone samples of the Upper Permian Dalong Formation (DL) in the Sichuan Basin (South China) was selected to characterize quartz minerals using petrological and geochemical approaches, including SEM-CL observation. The studied mudstone samples are relatively enriched in organic matter (OM) (total organic carbon (TOC) = 1.88–18.46 wt%) and have been thermally over-matured. The DL organic mudstones accumulated in a failed rift basin, far removed from provenance areas, are chiefly composed of intrabasinal quartz and carbonate (mostly calcite). Quartz is represented by six major forms, including detrital quartz grains, siliceous skeletal fragments, silica nanospheres, microquartz aggregates, grain-replacive quartz, and matrix-dispersed microquartz, of which the latter five are of authigenic origin. Detrital quartz grains are not common in the DL mudstones. Silica reprecipitated as authigenic quartz was largely sourced from dissolved siliceous skeletal fragments (mostly radiolarians), and some silica appeared to have been released from the alteration of volcanic debris (mainly plagioclase) to form matrix-dispersed microquartz grains. Authigenic quartz influenced pore development and preservation of shale reservoirs in two ways. A rigid framework formed from numerous authigenic quartz grains (e.g., microquartz aggregates, silica nanospheres, and siliceous skeletal fragments) shields organic pores during mechanical compaction, whereas authigenic quartz precipitation (e.g., cavity-filling quartz, grain-replacive quartz) reduces porosity by filling voids. A moderate abundance of authigenic quartz (Si xs = ∼20%) is most favorable for development and preservation of pores (especially organic pores) in deep mudstone. Organic pores, widely developed in the studied DL deep mudstones, seem to be largely preserved in the samples with TOC values as much as 18.46 wt%. In summary, the DL mudstone deposits enriched in OM and contained moderate contents of authigenic quartz are especially favorable targets of deep shale gas exploration. • Six major types of quartz have been identified in the DL Shale. • Alteration of volcanic materials supply additional silica sources to silica diagenesis. • Authigenic quartz exerts dual roles in pore development and preservation. • Preserved organic pores show great potentials for deep shale gas exploration. [ABSTRACT FROM AUTHOR]

Geology, stratigraphy, drilling, logging, biomarker studies and apatite fission tracks were used to study Sinian- Ordovician mudstone and reservoir bitumen extracted from exploration wells located in central and southeastern Sichuan. This analysis confirms the source rocks were located in a reducing sedimentary environment, with organic matter such as bacteria and algae. The bitumen found in Sinian-Ordovician reservoirs in central Sichuan and Sinian reservoirs in southeastern Sichuan were mostly derived from Cambrian mudstones. The reservoir bitumen found in Cambrian rocks in southeastern Sichuan are from a mixed Cambrian-Silurian source. The apatite fission track combined with the Easy% Ro model were used to reconstruct the hydrocarbon-generation history of Cambrian source rocks in the Dingshan 1 well in southeastern Sichuan. The comparison between the Gaoke 1 well located in central Sichuan and the Dingshan 1 well demonstrates the maturation time and oil and gas generation stage from Cambrian source rocks in southeastern Sichuan is younger than the source rocks from central Sichuan. The source rock formations underwent tectonic uplift and reburial for secondary hydrocarbon-generation. The research methods provided can assist in oil exploration of deep and old hydrocarbon-bearing strata with a multi-stage evolution and tectonically superimposed features in the Chinese Qinghai-Tibet plateau basin, or similar basins worldwide. [ABSTRACT FROM AUTHOR]

The organic matter (OM) enrichment patterns and mechanisms in lacustrine deep-water shale successions were controlled by complex geological and biological processes, rather than any single factor. The integration of sedimentology, petrography, geochemistry, and sequence stratigraphy in the study of shale successions is vital to the development of robust paleoenvironmental interpretations and OM enrichment model. Based on a systematic sedimentological, petrographic, and geochemical analysis of three continuous cored wells of the Lower Jurassic Dongyuemiao Formation, Sichuan Basin, within a sequence stratigraphic framework, this study reports the complex geological responses in detrital input, sedimentation rate, benthic redox conditions, primary water column productivity, and OM enrichment pattern that caused by lake-level fluctuation. The results indicate that none of the proxies for detrital input, redox conditions, or productivity exhibit a significant correlation with total organic carbon (TOC) content in sediments. The OM enrichment was regulated by a complex interplay of these three factors, which is a function of relative lake-level fluctuation. In general, detrital input was decreased during lake-level transgression; whereas increased during lake-level regression. Reducing conditions and primary productivity were generally enhanced during lake-level transgression; whereas diminished during lake-level regression. Coupled relationships between lake-level fluctuation, bulk TOC content, and OM types were observed. The bulk TOC content and liptinite proportion in sediments increased gradually during the lake-level transgression. In contrast, the bulk TOC content and liptinite proportion in sediments gradually decreased during the lake-level regression. This general principle applies to both second-order and third-order lake-level fluctuation. Our study demonstrates that the development of a reliable OM enrichment model in lacustrine shale successions requires a comprehensive sequence stratigraphic framework. • Relative lake-level variation influences primary productivity, detrital input, and benthic redox conditions. • Coupled lake-level variation and organic matter (OM) enrichment patterns were observed. • The OM enrichment model in the lacustrine Dongyuemiao Formation was established. • The establishment of a reliable OM enrichment model requires a comprehensive sequence stratigraphic framework. [ABSTRACT FROM AUTHOR]

In the deep and ultra-deep carbonate reservoirs, the porosity will be reduced due to the compaction, pressure solution, and cementation which are considered as the crucial factors for the mechanism of high-quality carbonate reservoirs. However, it is still a challenge to quantify the influence of stylolites on the Longwangmiao Formation carbonate reservoirs. To better understand the controls on the formation and distribution of stylolites, the Longwangmiao Formation is selected as an example to evaluate the effect of stylolites on the carbonate reservoirs. The mineralogy of the Longwangmiao Formation reservoirs is mainly dominated by dolomite (79.6%) with minor calcite (11.4%), and terrigenous minerals (clay and quartz) (5.6%). The rocks with dolomite content of more than 80%, calcite content of less than 30%, and terrigenous minerals of less than 1% have lower stylolite density and amplitude. The content of dolomite can affect the occurrence of stylolite indicating that the higher abundance of dolomite has a lower density and amplitude of stylolite. Based on the multiple techniques, four types of carbonate lithofacies were recognized, including the grainstone (dolo-grainstone, grainstone), packstone (dolo-packstone, packstone), wackestone (dolo-wackestone, wackestone), and mudstone (dolo-mudstone, mudstone). The distribution of stylolite also shows the lithofacies-specific behavior in the Longwangmiao Formation carbonate reservoirs. The lithofacies of grainstone shows the lowest density and amplitude of stylolite, while the occurrence of stylolite in the mudstone shows the highest abundance. Vertically, the stylolite density and amplitude in the lower part of the fourth-order sequences is higher than this in the upper part. This study suggested that the dolomitized lithofacies with relatively high grain (50–70%) and cement (∼25%) contents can resist the pressure solution and can preserve the porosity of carbonate reservoirs. [Display omitted] • Taking deep to ultra-deep strata as the research object. • The influencing factors of stylolite are analyzed quantitatively. • Pure dolostones have low stylolite density and amplitude. • Mud-supported rocks' stylolites are obviously different from grain-supported rocks. • Dolo-grainstones facilitate the formation of good reservoirs. [ABSTRACT FROM AUTHOR]

The mechanism of fault-controlled hydrothermal dolomitization has long been controversial due to its complicated tectonic and diagenetic conditions, especially for the source of dolomitizing fluids and its hydrological driving models. In the southeastern Sichuan Basin, fracture-related dolomite bodies are well developed in a Middle Permian carbonate succession, providing an example of dolomitization and its relationship with regional tectonic activities. Previously, many of the dolomites in the Sichuan Basin have been interpreted as of hydrothermal origin but are usually poorly defined since it is hard to determine the nature and timing of dolomitization. In the current study, new petrographic, geochemical, and chronological data of Middle Permian dolomites have been integrated and allowed the classification of those dolomites into two categories (replacive dolomites of D1–D2 and fracture-/vug-filling dolomites of D3–D5). Calcite (C1–C3) and other non‑carbonate cements have also been found. The near-micritic dolomite (D1) was formed at the near-surface to early burial stage, likely from modified seawaters, which was reflected by its similar δ18O fluid estimates of the parent dolomitizing fluids and rare earth element patterns to matrix lime mudstone, as well as the fabric retentive texture. The fabric destructive (D2) and fracture-/vug-filling dolomites (D3–D5) are characterized by the high estimates of δ18O fluid of their parent dolomitizing fluids, high 87Sr/86Sr ratios, high Fe and Mn concentrations, positive Eu anomalies, high T h values, and high salinity estimates, indicating a hydrothermal origin. However, the limestone-like δ13C VPDB and strongly negative Ce anomalies of D2–D5 suggest that these dolomites may have originally evolved from seawater-derived fluids which circulated in the basal clastic rocks and flowed upwards by thermal convection through fault systems. New U–Pb ages bracketed a time range of these dolomites from 251 ± 11 Ma to 264 ± 10 Ma, coinciding with the emplacement of the Emeishan Large Igneous Province (~257–260 Ma). This may imply that the contemporary basement faulting and associated abnormal thermal event in southeastern Sichuan Basin still remotely echoed with the activities of the Emeishan Large Igneous Province, although the study area was far away from its eruption center. It facilitated the fluid circulation through fault systems and underlying sandstone aquifers and thus drove fault-controlled hydrothermal dolomitization at shallow burial backgrounds. • The fault-related dolomites formed at shallow burial depths from hydrothermal fluids. • New U–Pb dating results bracket a dolomitizing time range of 251 ± 11 Ma to 264 ± 10 Ma. • The dolomitizing period was coincident with the emplacement of ELIP. • Basement faulting and thermal convection facilitated hydrothermal dolomitization. [ABSTRACT FROM AUTHOR]

The self-sealing of caprocks plays a vital role in CO 2 geological sequestration. However, the mechanisms by which distinctive mineral combinations and types of caprock bring about self-sealing are unclear. Studies of the self-sealing of caprocks with different mineral combinations and rock types promise to effectively identify the self-sealing capabilities of cap rocks and evaluate the long-term safety of CO 2 geological sequestration. This study focuses mainly on the mechanisms of self-sealing in three caprocks, specifically the shale of the Qingshankou Formation in the Songliao Basin, the calcareous mudstone of the Da'anzhai Formation in the Sichuan Basin, and the shale of the Yanchang Formation in the Ordos Basin. This study applies the key technology of CO 2 –fluid–rock numerical simulation and petrological and analytical methods to systematically investigate the self-sealing capability of these caprocks, which have different mineral compositions, and identify the key minerals that trigger self-sealing. The results show that the key minerals that can trigger self-sealing after CO 2 injection are carbonates such as calcite, siderite, and dolomite and clays such as kaolinite. The CO 2 –fluid–rock interactions in feldspar- and illite-rich shale are relatively mild, which is conducive to the safe geological sequestration of CO 2. In contrast, the CO 2 –fluid–rock interactions in carbonate-rich shale are violent and fluctuate greatly, which is not conducive to the short-term safe sequestration of CO 2 , but they are favorable for the safe sequestration of CO 2 in the medium to long term. After CO 2 injection, the main minerals precipitated in a carbonate-rich shale are K-feldspar and small amounts of ankerite and dawsonite. The main minerals precipitated in illite-rich shale are quartz and small amounts of ankerite and dawsonite. The results of this study show that it is possible to effectively evaluate the sealing capability of caprocks, thus providing a basis for the safe sequestration of CO 2. • Key precipitated minerals that trigger self-sealing are k-feldspar, dawsonite and ankerite. • Key dissolution minerals that affect self-sealing are albite, calcite, dolomite and siderite. • Feldspar- and illite-rich shales react mildly with injected CO 2 , enabling the safe geological sequestration of CO 2. [ABSTRACT FROM AUTHOR]

The lime mudstone in the oolites of the Upper Triassic Maantang formation has not been examined in detail by many geologists though the intraclast-bearing lime mudstone is obviously different from the underlying and overlying gray oolitic layers in northwestern Sichuan Basin, China. This study presents an interpretation of four newly measured stratigraphic sections for investigating the distribution and thickness of the lime mudstone. The distribution of the lime mudstone is discontinuous, and the deposits vary in thickness. The lime mudstone occurs as lentoids or bands of approximately 0.2-2 m in thickness and ranges in length from about 5 m to more than 30 m in length in the outcrops. Similar complexes occur in other places in the world although there are regional and site-specific variances. However, the phenomenon in the Maantang mudstone formation seems most like those of the modern Bahama bank. The lime mudstone of Maantang is assumed to have formed by sequential historic storms of great strength, much as did the similar Bahama deposits. [ABSTRACT FROM AUTHOR]

Pressure solution, a common and important diagenetic process in carbonates, involves physical reduction of the volume in rocks and chemical dissolution-precipitation process in pores, and thus leads to the significant heterogeneity of reservoirs. Stylolite is a typical product of pressure solution. The amplitude and residual seam thickness of stylolite can effectively characterize the intensity of pressure solution. Quantitative characterization of the density, morphology, and distribution of stylolites in the Feixianguan Formation limestones in the northeastern Sichuan Basin were performed to evaluate the intensity of pressure solution. Revealing the impact of pressure solution on the evolution of porosity and permeability is essential when looking for high-quality reservoirs by integrating quantitative statistics from cores and thin sections with geochemical data. The following four types of stylolites were observed in the Feixianguan limestones according to pure geometry, known as wave-like, rectangular, sharp-peak, and seismogram types. From matrix supported limestone to grain supported limestone, the type of stylolites gradually change from wave-like to rectangular, sharp-peak and seismogram types, accompanied by increased surface roughness and amplitudes, and significantly decreased residual seam thicknesses. The stylolite density tends to decrease from the lower to upper part of the sedimentary cycle. The dissolved calcium carbonates released via pressure solution precipitated as cement in adjacent pore spaces, forming a relatively less porous light gray band on each side of the stylolite. We implement two quantitative methods show that the intensity of pressure solution in grainstone (90.42 mm/m) is significantly smaller than that in mudstone (235.78 mm/m) suggesting that the grainstones are more resistant to chemical compaction. Moreover, pressure solution caused a decrease in the porosity of limestone by approximately 11.31%. The porosity of grainstones varies widely and decreases drastically as clay content increases, suggesting that the clean particles formed in high-energy environments are favorable for the formation of dominant reservoirs. A large number of pores, calcite cements, and bitumen within stylolites indicates that stylolites could act as fluid pathways, contributing about an order of magnitude to the reservoir permeability. Concludingly, we highlight quantitatively differences in pressure solution intensity on a millimeter scale. • Quantitative characterization of pressure-solution intensity was established. • Clean particles are more resistant to pressure solution. • The intensity of pressure solution in various limestone was compared. • Porosity loss caused by pressure solution was estimated. • Stylolites have different effects on porosity and permeability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Different cycles of the Longwangmiao Formation have different petrological and petrophysical properties. • Spatial variation of the depositional cycles was mainly controlled by paleobathymetry. • The grain-riched cycles were more than other cycles to develop in the upper parts of the sequences. • High dolomite and oolitic content, strong dissolution of grainstone facilitate the formation of good reservoirs. The carbonate shoals are generally considered as the critical reservoirs in the carbonate sediments. However, whether the quality of reservoir directly related to the development of carbonate shoal is continuously debated. To further explore the control of carbonate shoal on the quality of reservoirs, the Longwangmiao Formation is selected as an example to examine this hypothesis. Based on the multiple techniques, four types of deposition environments were recognized, including the shoal, tidal flat, mixed tidal flat and intershoal. Moreover, three types of depositional stacking pattern (e.g., type A, B and C) were identified. Type A is characterized by continuously carbonate shoal deposits. Type B has two different sub-types which consist of basal shoal deposits that grade upward into tidal flat or mixed tidal flat deposits. For type C, one of the sub-types is dominated by the mudstone and upward evolve to the grainstone in the top. In contrast, the other sub-type consists of basal intershoal deposits that grade upward first into shoal deposits, and then into tidal flat deposits. The spatial variation of depositional cycles was mainly related to paleobathymetry and relative sea-level changes. In terms of physical properties, type A is favorable for the formation of the reservoir as the result of the high primary porosity and experienced meteoric dissolution and dolomitization. Inversely, the content of grain-riched sediments is less in type C and leads to low porosity. This study confirms that type A cycles are the key factor in the formation of carbonate reservoirs. [ABSTRACT FROM AUTHOR]

The quality of hydrocarbon source rocks is affected by the sedimentary paleoenvironment. A paleoenvironment with anoxia and a high paleoproductivity is beneficial to source rocks. The paleoenvironment of the Lower Ordovician Meitan Formation of the Sichuan Basin and its adjacent areas is lacking, restricting the oil and gas exploration of the Ordovician in the Sichuan Basin and its adjacent areas. In this paper, the content of major and trace elements of 50 samples was tested to clarify the paleoenvironment of the Meitan Formation. The paleoclimate, paleosalinity, paleoredox, and paleoproductivity during the deposition of the Meitan Formation were analyzed. The control effect of the paleoenvironment on the development of source rocks was clarified, and the favorable paleoenvironment for source rock development was pointed out. The results show that the paleoenvironment of the Meitan Formation has the following characteristics: humidity, brackish water, oxygen depletion, anoxia environment, and high paleoproductivity. These characteristics are conducive to the development of poor and moderate source rocks. The source rocks of the Meitan Formation were developed in the north, west, and south of the Sichuan Basin and its adjacent areas. The organic matter of the source rocks is mainly typed II1 kerogen, and the quality is evaluated as poor-medium source rocks having the potential of generating oil and gas. This study can provide fundamental parameters for the further exploration of Ordovician petroleum. [ABSTRACT FROM AUTHOR]

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

Increasing cases have indicated that oceanic anoxic events (OAEs) appear to have records in terrestrial sediments, which provide valuably high-resolution materials for the debated and challenging issues of OAEs. However, the current works are preliminary and need more geological analyses to test. Here we present a case study in the Lower Jurassic Da'anzhai Member of the Toarcian (ca. 180 Ma) Sichuan Basin, SW China. The petrology and sedimentary facies of the Da'anzhai Member were used to reconstruct the relative water-level fluctuations in the Sichuan Basin, which are revealed to be consistent with coeval changes in global sea levels. The Da'anzhai Member was deposited during a period of lake shallowing with enhanced salinities corroborated by the Sr/Ba and S/TOC ratios, when sea level was generally high after global transgression. During the lake shallowing, there were four complete secondary lacustrine deepening–shallowing cycles over shorter time-scales. The total organic carbon contents and organic carbon isotopic stratigraphy show there was a basin wide terrestrial response to the T-OAE as recorded by the Da'anzhai Member. The enrichment factors of Mo relative to PAAS (Mo EF) and C org /P values imply a dysoxic redox condition. The terrestrial response of OAEs is likely caused by changes in hydrological conditions, which were related to the global warming triggered by the Karoo–Ferrar Large Igneous Province. The associated organic carbon accumulation mechanism is a combined effect of elevated primary productivity and oxygen-depleted preservation conditions. Our data suggest that OAEs do have terrestrial response and induce organic carbon accumulation, but the mechanisms are still not much clear and need further studies. • Water-level and redox fluctuations in a Sichuan Basin lacustrine system were revealed. • The fluctuations are coincident with the Toarcian oceanic anoxic event. • Relative water-level fluctuations are consistent with coeval changes in global sea levels. • The response is most likely caused by changes in hydrological conditions. • Organic matter accumulated under elevated primary productivity and dysoxic conditions. [ABSTRACT FROM AUTHOR]

Fractures, which are related to tectonic activity and lithology, have a significant impact on the storage and production of oil and gas in shales. To analyze the impact of lithological factors on fracture development in shales, we selected the shale formation from the Da'anzhai member of the lower Jurassic shales in a weak tectonic deformation zone in the Sichuan Basin. We defined a lithology combination index (LCI), that is, an attribute quantity value of some length artificially defined by exploring the lithology combination. LCI contains information on shale content at a certain depth, the number of layers in a fixed length (lithology window), and the shale content in the lithology window. Fracture porosity is the percentage of pore volume to the apparent volume of the rock. In the experiment, fracture porosity was obtained using 50 samples from six wells, by observing rock slices under a microscope. The relationship between LCI and fracture porosity was analyzed based on machine learning, regression analysis, and weighting methods. The results show that LCI is able to represent the impact of multiple lithological factors (i.e., shale content at a certain depth, the number of layers in the lithology window, and the shale content in the lithology window). The LCI within a thickness of 2 m for the lithology window demonstrates a good linear relationship with fracture porosity. We therefore suggest LCI be used for fracture predictions of shale formations from weak tectonic deformation zones. Our proposed LCI and fracture prediction methods also provide implications for sandstone, mudstone, or carbonate formations under similar processes. [ABSTRACT FROM AUTHOR]

The elastic properties of shale and mudstone from unconventional subsurface reservoirs are of great significance for the evaluation of rock fracturing, especially for hydraulic fracturing and borehole stability. In this study, mineral compositions of the rock cuttings over 100 m in length from the well XK-1 (Wufeng and Longmaxi Formations in the Sichuan Basin, China) were accurately measured by automated mineralogy and a new multi-mineral equal probability contact Hertz-Mindlin model that considers three or more granular medium contacts is proposed to estimate the elastic properties of the Wufeng and Longmaxi Formations. The results show that elastic parameters obtained from the new multi-mineral equal probability contact Hertz-Mindlin model are consistent with those from sonic logging. Thus, this model can be used to estimate the elastic properties of Wufeng and Longmaxi shales. When applied to mudstone and sandy mudstone of Longmaxi Formation, the clay content plays an important role to improve estimates of the elastic parameters. The new rock physics model is fast, cost-effective, and provides an alternative in case there are no rock blocks available for conventional uniaxial or triaxial tests in the laboratory, or wireline logging tools are not best options. • A new rock physics model is established to estimate the elastic properties of mudrock; • The clay content plays an important role to improve estimates of the elastic parameters; • The new rock physics model is fast, cost-effective, and provides an alternative to conventional methods. [ABSTRACT FROM AUTHOR]