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In this paper, taking the Wufeng Formation-Longmaxi Formation shales in the southeastern region of the Sichuan Basin as an example, the thin section, X-ray, geochemical testing and TIMA techniques are used to systematically study the source analysis and characterization of quartz fractions, and in turn, the mechanism of the development of highly brittle shale reservoirs is revealed. The results of the study show that the high quartz mineral content segments are well stacked with the organic matter enriched segments. Excess silica occurs in the lower part of the Wufeng -Longmaxi Formation, and some samples in the middle and upper part of the Wufeng and Longmaxi Formation have excess silica deficit; the K2O/Na2O ratio is generally >1, reflecting that the reservoir quality is affected by the biogenic silica. Land- sourced detrital quartz grains, on the other hand, are significantly larger and are generally distributed over a range of tens to more than a hundred micrometers, often with clay minerals and organic matter around the periphery. Biogenic siliceous fossils are mainly developed in the lower part of the Wufeng-Longmaxi Formation, dominated by biogenic siliceous remains such as radiolarians. TIMA test results show that quartz belongs to the mineral type with absolutely dominant content ratio in the bottom samples of the Wufeng-Longmaxi Formation and reflects the characteristics of fine-grained-muddy/ carbonaceous horizontal laminar deposition. There are three different types of quartz grain distribution patterns in shale reservoirs, including excess silica type, multi-source silica type, and land-source silica type. The layers of highly brittle shale reservoirs are all at the bottom of Wufeng-Longmaxi Formation, which is rich in biogenic silica, and at the same time, they are also in the active layers of montmorillonite and other abundant volcanic activities, and they are developed in the closed basins as well as the deep-water land scaffolds, so that the reservoirs have a good self-sealing; at the same time, the combination of microcrystalline quartz+organic matter makes the reservoirs have an excellent mechanical brittleness. [ABSTRACT FROM AUTHOR]

Lacustrine shale has garnered significant attention due to its significant resource potential. The Jurassic shale in the Sichuan Basin is an important target for lacustrine shale exploration in China. However, previous studies have predominantly focused on the Da'anzhai member of the Ziliujing Formation, and little attention has been paid to the shale of other strata. This paper aims to address this gap by investigating the Jurassic Dongyuemiao member in the Sichuan Basin. The study systematically characterizes the geological properties of the Dongyuemiao shale system, reconstructs the paleosedimentary environment, identifies the key factors influencing organic matter (OM) enrichment and physical properties, and assesses its resource potential through comparative analysis. Our results show that the Dongyuemiao shale was deposited in an oxic and semi-humid sedimentary environment characterized by intense weathering conditions. The enrichment of OM in the shale is primarily controlled by redox conditions and salinity, with redox conditions playing the most crucial role in OM accumulation. In terms of pore system characterization, clay mineral-associated pores dominate the pore types in the Dongyuemiao shale, while two types of organic matter-associated pores are also widely observed. The development of framework grain-associated pores is limited and only present in certain siliceous and carbonate minerals. The shales of the Dongyuemiao member and the Da'anzhai member exhibit slight differences in TOC content. However, the kerogen in the Dongyuemiao member displays higher hydrocarbon generation potential, and the Dongyuemiao shale exhibits more favorable pore structure parameters. Overall, the Dongyuemiao shale does not exhibit any significant disadvantages compared to the Da'anzhai shale. Therefore, it deserves greater attention in future exploration endeavors. The research findings presented in this paper provide a solid theoretical foundation for expanding the exploration scope of lacustrine shale in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Organic matter serves as the hydrocarbon-generating parent material for shale reservoirs, in which organic pores are also important reservoir spaces. Different types of organic matter have wide differences in hydrocarbon generation and pore-forming ability. Based on the occurrence state of organic matter, in the over-mature Marine shale organic matter mainly includes in situ and migrated organic matter. It has been extensively studied on in situ organic matter and organic matter migrating into inorganic pores, while there are few reports of organic matter migrating into microfractures. In this study, the over-mature Marine shale reservoir in the first sub-member of the Silurian Longmaxi Formation in the Luzhou area of the Sichuan Basin is taken as an example. Core observation, optical microscope, high-precision large-view scanning (MAPS, modular automated processing system) and mineral analysis scanning (QEMSCAN, quantitative evaluation of minerals by scanning electron microscopy) were conducted to observe the morphological characteristics of organic matter veins, and then analyze the genesis and pore-forming characteristics of such organic matter. The results show that: ① Organic matter veins (OM veins) are soluble organic matter with fractures as an effective channel, and OM veins in the study section is easy to form under the condition of micro-fractures in the shale sweet segment after organic matter generating oil and before gas generation ② Organic matter in the OM veins are less efficient in pore-forming, with sparse pores and smaller pore sizes. The occurrence of fractures varies greatly, including horizontal fractures, oblique fractures and high-angle fractures, which are mostly developed in the Long111 and Long112 layers. ③ The development of OM veins can indicate better reservoir conditions, that is, the layers have strong hydrocarbon generation intensity (strong pore-forming ability of organic matter) and high brittle mineral content (strong reservoir compressibility). The new findings in this paper reveal that OM veins are instructive for the determination of geological–engineering sweet spots in the Longmaxi Formation in the Sichuan Basin, and also provide guidance for future research on occurrence form and geological significance of different types of organic matter. [ABSTRACT FROM AUTHOR]

The Permian shale in Northeast Sichuan is an important shale oil and gas resource potential area, and the study of its organic matter characteristics and paleoenvironmental analysis is of great significance for revealing the shale oil and gas formation mechanism and resource evaluation. In this study, the organic matter of Permian shales in northeast Sichuan was carefully studied based on various analytical tools, such as petrology, laser Raman, microscopy, and principal trace elements, and the paleoenvironmental parameters of the shales were comprehensively analysed. A detailed study of the organic matter characteristics of Permian shales in northeast Sichuan reveals important features such as organic matter fractions, structural characteristics, maturity and sources. The results show that the organic matter of the shale consists mainly of solid bitumen, putrescine group, specular group and multicellular planktonic algae. Petrological observations and laser Raman analyses indicate a high maturity of the organic matter and a high content of organic carbon (TOC), showing good hydrocarbon potential. In this study, we reconstructed the palaeoenvironmental parameters and inferred the palaeoenvironmental evolution through palaeoenvironmental analyses of Permian shales in northeast Sichuan. The results of the comprehensive multi-indicator study show that the type of palaeoenvironment at the time of shale deposition was mainly an anoxic-reducing environment, and the depositional conditions were favourable to the enrichment and preservation of organic matter. In summary, the organic matter characteristics and paleoenvironmental analyses of the Permian shales in Northeast Sichuan provide important geological background information for an in-depth understanding of the formation mechanism, exploration potential and development prospect of shale hydrocarbons in this area. The results of this study can provide a scientific basis for the evaluation and development of shale oil and gas resources in this area, which is of great significance to geologists and the energy industry. [ABSTRACT FROM AUTHOR]

The sealing capacities of shale, roof, and floor strata are essential for the preservation of shale gas. Sealing capacity development and its influence on shale gas composition and organic matter pore development are important but have not been addressed by existing studies. In this paper, the Upper Ordovician Wufeng-the first member of the Lower Silurian Longmaxi Fm. in the Yangtze region is selected as the study object. We dissected the J1 well in the Jiaoshiba block inside the Sichuan Basin and the Y6 well in the Youyang block outside the Sichuan Basin to analyze the gas components and carbon isotopes in recovered shale gas. In this study, two microscope techniques (i.e., FIB-SEM and FIB-HIM), porosity measurement, TOC content analysis, permeability tests, isothermal adsorption measurements, are utilized. The analytical results show that both the roof and floor strata have vertical sealing effects on the organic-rich shale due to their different physical properties compared to the shale. The shale overburden pressure increases with burial depth, and the adsorbed gas increases with increasing TOC content and thickness: all of these can lead to a decrease in permeability and an increase in self-sealing capacity. This paper determined the formation mechanism of the sealing capacity for shale, roof and floor strata. Furthermore, the control patterns of "sealing capacity strength" on the shale gas source and organic matter pore development are clarified. The main findings are as follows: when the shale, roof, and floor strata have a strong sealing capacity, only a small amount of liquid hydrocarbons can be discharged from the shale, but the natural gas formed due to liquid hydrocarbon cracking is abundant. Under such circumstances, shale gas has a mixed origin of liquid hydrocarbon cracked gas and kerogen cracked gas; the carbon isotopes are inverted; abundant pyrobitumen pores with high porosity and good connectivity are developed. When the sealing capacities are weak, the amount of liquid hydrocarbons discharged from the shale are high, and the amount of natural gas is low. The shale gas is mainly derived from kerogen cracking with weak or no carbon isotope inversion, and kerogen pores are small with poor connectivity. These findings will advance the study of shale gas formation mechanisms and provide a scientific basis for selecting shale gas exploration area. • Due to different physical properties, the roof and floor strata provide a seal for the organic-rich shale. • As the burial depth, TOC content, and the shale thickness increase, the shale has a stronger sealing capacity. • Sealing capacity plays an important role in the characteristics of shale gas and organic matter pore development. [ABSTRACT FROM AUTHOR]

Marine organic-rich shale is widely distributed in the Upper Ordovician Wufeng Formation (WF-F) and Silurian Longmaxi Formation (LMX-F), making it an important target for shale gas exploration and development. In order to clarify the paleoenvironment evolution characteristics and the effect of depositional environment on organic matter (OM) accumulation of the marine shale in the Wufeng and Longmaxi Formations, a series of geochemical and petrological experiments were carried out, including TOC, Rock-Eval pyrolysis, XRD, and major and trace element analyses. Research results show that based on the variation characteristics of TOC, mineral composition, and paleoenvironment evolution characteristics, four units can be identified from bottom to top: Wufeng Formation (WF-F), Lower Longmaxi Formation (L-LMX-F), Middle Longmaxi Formation (M-LMX-F) and Upper Longmaxi Formation (U-LMX-F). The high-quality marine shale developed in WF-F and LMX-F in the Zigong area (TOC: 0.65–4.56%, avg. 2.15%) contains type I kerogen (kerogen type index: 86.0–98.3, avg. 92.7) and OM in mature stage (average of Rb and Tmax are 2.94%, 646 °C, respectively). Clay minerals (avg. 42.5%) and quartz (avg. 37.7%) dominate the mineral compositions, with subordinated dolomite (avg. 6.3%), feldspar (avg. 6.0%), calcite (avg. 4.0%), and pyrite (avg. 3.5%). Paleoenvironment indicators suggest that during the sedimentary period of WF-F and L-LMX-F, the paleoclimate condition was humid; the weathering condition, paleosalinity, and redox conditions were the strongest; and there was a relatively high level of paleoproductivity and a relatively low level of terrigenous detritus influx. However, during the period of M-LMX-F and U-LMX-F, the climate gradually changed from warm and humid to hot and dry; the intensity of weathering conditions, paleosalinity, and redox conditions was relatively reduced; terrigenous detritus influx increased; and the paleoproductivity decreased. Relationships between TOC and paleoclimate condition, paleosalinity, redox condition, paleoproductivity, and terrigenous detritus influx suggest that redox condition is most important controlling factor for OM enrichment. A combination of anoxic bottom water conditions and high primary productivity and a relatively low terrigenous input resulted in the enrichment of OM in the WF-F and L-LMX-F, making it a potential exploration and development target. The research can provide scientific guidance for the selection of potential shale gas development targets in the Zigong area. [ABSTRACT FROM AUTHOR]

Both complete and uncompleted radiolarian siliceous shells were developed at Wufeng-Longmaxi radiolarian siliceous shale laminae in Sichuan Basin. Micro- and ultra-micropetrological observation suggests that they were successively filled by calcite, pyrite and organic–silicon complex, where pyrite and organic–silicon complex filled dissolved pores associated with calcite during sedimentation. Calcite was derived from calcium carbonate produced by microbial activities at the seawater surface. The environment of radiolarian siliceous shell cavities, which was suitable for sulfate reducing bacterial growth or dissolved hydrogen sulfide reducing Fe3+, contributed positively to pyrite development. Organic–silicon complex development was related to microorganism metabolism that was an important silica source. Honeycomb-like organic pores were developed in cavities with complete shells, but were not developed in cavities with uncompleted shells. This is because the latter could not withstand overburden pressure compared with the former. The only approach to figure out organic pore carriers and understand sequences and development processes of minerals and organic matter is to select weakly compacted radiolarian siliceous shale laminae to carry out micro- and ultra-micropetrological observation and geochemical testing via various technologies. [ABSTRACT FROM AUTHOR]

The upper Permian Longtan Formation (LTF) shale is a new shale gas exploration and development focus in China, which was formed in a transitional depositional environment. Because of the difference in the sedimentary environment between marine-continental transitional shale and marine shale, the enrichment characteristics and mechanism of organic matter (OM) are very different. Because of a weak research foundation, the enrichment mechanism of OM in the LTF shale in the Sichuan Basin is still unclear. In this paper, shale samples of the upper Permian LTF from the Well F5 were collected, and the abundance, source and sedimentary environment of organic matter were studied by using paleoclimate, detrital input, redox, paleoproductivity and sedimentation rate parameters. The results show that the TOC content of the LTF shale is high (average = 3.04%) and reaches the high to overmature stage (average = 2.46%), indicating a favorable source rock. The LTF shale was deposited in a humid and hot climate, which was conducive to the growth and prosperity of a large number of plants. The source of organic matter was mainly terrestrial plant fragments. The sedimentary environment of the LTF shale was oxic, it had a high sedimentation rate, which reduced the oxidation rate of OM, promoted the rapid deposition of OM, and ensured the preservation of organic matter. In summary, the enrichment of OM of LTF shale in the Sichuan Basin was affected by the sedimentary environment, which was complicated. The main controlling factors were detrital input, redox conditions and sedimentation rate. • The geochemical characteristics of Permian Longtan Formation (PLTF) shale in eastern Sichuan are analyzed. • The sedimentary paleoenvironment of PLTF shale in eastern Sichuan is reconstructed. • The main controlling factors of organic matter enrichment of PLTF in eastern Sichuan are studied for the first time. [ABSTRACT FROM AUTHOR]