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Three-end-member diagrams of the shale lithofacies of the Permian Shanxi Formation in the eastern margin of the Ordos Basin.
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Based on core description, thin section identification, X-ray diffraction analysis, scanning electron microscopy, low-temperature gas adsorption and high-pressure mercury intrusion porosimetry, the shale lithofacies of Shan2³ sub-member of Permian Shanxi Formation in the east margin of Ordos Basin was systematically analyzed in this study. The Shan...
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... and evaluation [3] . Both parameters can be accurately obtained through experiments, and they provide accurate lithofacies division. For this reason, in the study, we selected mineral composition and TOC as criteria for shale lithofacies division and established a comprehensive shale lithofacies division scheme for the Shanxi Formation (Fig. ...
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... minerals, clay minerals and carbonate minerals. Mineral composition, as the primary indicator of lithofacies division, can be accurately and quantitatively obtained by bulk rock XRD analysis. The shale lithofacies is divided based on the three-end-member diagram covering siliceous minerals (quartz+feld-spar), carbonate minerals, and clay minerals (Fig. 2). The shale lithofacies is siliceous shale (S) when siliceous minerals account for more than 50%, clay shale (C) when clay minerals account for more than 50%, carbonaceous shale (CA) when carbonate minerals account for more than 50%, and mixed shale (M) when all three kinds of minerals account for less than 50% but more than 25%. TOC is ...
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... account for more than 50%, and mixed shale (M) when all three kinds of minerals account for less than 50% but more than 25%. TOC is the second important indicator for lithofacies division. Using TOC of 5.5% and 8.5% as boundary values, there are high TOC (>8.5%) shale (H), medium-TOC (5.5%TOC8.5%) shale (M), and low-TOC (<5.5%) shale (L) (Fig. 2a). Gas-bearing capacity is the key measure for evaluating whether a shale gas reservoir is worth developing. For the Lower Silurian Longmaxi Formation shale gas exploration in China, 2.0 m 3 /t is the lowest gas content of the reservoir commercial development [8] . According to the statistics used in this study, for 17 wells in the ...
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Shale gas exploration requires studies on the enrichment mechanism of sedimentary organic matter. The Lower Cambrian shale is taken as a study object to analyze the effect of organic matter on gas content using TOC content and porosity analyses, isothermal adsorption experiments, and FIB-HIM scanning electron microscopy observations. Then, we selec...
Citations
... Energies 2024, 17, 2470 2 of 15 resultant potential hydrocarbon reserves are greatly considerable [11][12][13][14][15]. The exploration summary of China National Petroleum Corporation states that the geological resources of shale reservoir in China are abundant and have great development potential [16][17][18][19]. The large-scale cost-effective development of shale reservoir is a decisive contributor to the sustainable development of the petroleum industry. ...
... Energies 2024,17, 2470 ...
... Energies 2024, 17, 2470 ...
CO2 pre-injection fracturing is a promising technology for shale reservoirs development, with multiple advantages for improving the complexity of fractures, the production of crude oil, and the sequestration of CO2. Previous research mostly focused on the CO2 effect on macroscopic mechanical properties of shale. However, there are many phenomena closely related to shale micro mechanical behavior. Therefore, this study presents a systematic investigation into the effects of CO2 on both macro and micro mechanical properties, as well as pore-fracture structures during CO2 pre-injection fracturing in shale reservoirs. The results show that CO2 can significantly decrease the tensile strength, uniaxial compressive strength, and elastic modulus of shale. With the increasing CO2 treatment time, the macro mechanical properties of shale decrease gradually. The microscopic experiments show that this significant decrease may be due to two mechanisms. The first is the significant decrease in the micro-mechanical properties of shale. The results of indentation analysis show that the microscopic elastic modulus and hardness of shale decrease by 51.3% and 63.3% after CO2 treatment. The second is the changes of the original shale framework. Pore-fractures structure analysis showed that after CO2 treatment, a large number of dissolution pores are generated in the shale matrix. Meanwhile, there are propagation of original fractures and opening of structural weak planes, which lead to the form of new fractures. Under the action of these two mechanisms, the macro mechanical strength of shale is reduced significantly. Therefore, in the field application, proper soaking following CO2 injection could lead to a significant overall reduction in mechanical strength, potentially lowering formation breakdown pressure, easing the requirements for treatment equipment, and enhancing fracturing effects.
... Conventional vertical well extraction methods are not effective in recovering natural gas resources, necessitating the use of large-scale hydraulic fracturing technology to create a network of fractures within the reservoir and enabling economically efficient development [5][6][7]. Marine and continental transitional shale gas reservoirs provide a stable depositional environment in which tight sandstone gas and coalbed gas are often vertically stacked [8][9][10]. These reservoirs have become a focal point for the exploration and development of unconventional natural gas resources. ...
... When U is enriched but Mo is not, a possible anoxic sedimentary environment may be indicated; when they are significantly enriched at the same time, a sulfide sedimentary environment is indicated (that is, a sedimentary environment containing a certain amount of H 2 S in the water body) [32]. During the calculation process, trace elements were first standardized with relatively stable Al elements during diagenesis [32][33][34][35] and compared with the average shale value (according to Wedepohl). The calculation formula was as follows: ...
... The distribution of sedimentary facies, organic matter content, mineral composition, and tectonic evolution together influence the micropore type, shale gas occurrence state, and enrichment degree of the reservoir [33,34]. Among them, the lithology and mineral composition under the control of the sedimentary environment are the main factors controlling the pore distribution [35]. As the main components of shale, quartz and clay minerals have different degrees of influence on the pore structure and shale reservoir quality. ...
Marine–continental transitional shale is one of the most promising targets for shale gas exploration in the Lower Yangtze region. To investigate the sedimentary environments and the regularity of the enrichment of the Longtan shale, multiple techniques including core and thin-section observations, geochemical and elemental analyses, X-ray diffraction, scanning electron microscopy (SEM), and low-pressure nitrogen adsorption (LPNA) were used to analyze the sedimentology, mineralogy, and pore structure of the Longtan shale. The core descriptions and thin-section observations showed that the Longtan shale was deposited in marine–delta transitional environments including delta-front, shore swamp, mixed tidal flat and shallow shelf environments. The Sr/Cu, V/Cr, CIA, EF (Mo), EF (U), and other major and trace element results indicated warm and moist climates and water-reducing conditions in the Longtan period. Both the climate and water conditions were favorable for organic matter production and preservation. The geochemical results showed that the Longtan shale was in the overmature stage (Ro values ranging from 2.4% to 3.57%) and that the average total organic carbon (TOC) content was 5.76%. The pore system of the Longtan shale consisted of inorganic pores with a small number of organic pores and microfractures. The porosity and specific surface area were mainly affected by the TOC and clay mineral contents. An effective combination of brittle mineral particles, organic matter, and clay minerals provided the necessary conditions for pore preservation. The organic pores, intergranular pores in clay minerals, and brittle mineral pores formed the main network system for the Longtan shale. In summary, the lithological combinations, organic geochemistry, and pore structure system were all affected by the sedimentary environments.
... Gas content is a key indicator of whether the shale gas reservoirs have development potential; the lower limit of the gas content for the commercial exploitation of shale gas in the Longmaxi Formation of the Lower Silurian in China is 2.0m 3 /t. 39,40 The gas content of shale in the Wufeng Formation−Long1 1 sub-member of the Zhaodi 1 well and Bucan 1 well in the Xichang Basin was 0.58−2.56 m 3 /t. ...
The shale of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Xichang Basin is the main replacement horizon for the shale gas exploration being conducted in the Sichuan Province, except for the Sichuan Basin. The fine identification and classification of the types of shale facies are important for shale gas exploration and development evaluation. However, the lack of systematic experimental studies on rock physical characteristics and micro-pore structures leads to a lack of physical evidence for the comprehensive prediction of shale sweet spots. Therefore, the present study used different means, such as core observation, total organic carbon content (TOC), helium porosity measurement, X-ray diffraction analysis, and mechanical properties analysis, in combination with the analysis of the whole rock mineral composition and characteristics of shale, for the identification and classification of the lithofacies of the shale layer, the systematic petrology and hardness measurement of the shale samples with different lithofacies, and discussion of the dynamic and static elastic properties of the shale samples and the control factors. It was revealed that nine types of lithofacies existed in the Wufeng Formation- the Long11 sub-member in the Xichang Basin, among which moderate organic carbon content-siliceous shale facies, moderate organic carbon content-mixed shale facies, and high-organic carbon content-siliceous shale facies were the best lithofacies with the optimum reservoir conditions, providing sufficient space for shale gas accumulation. The siliceous shale facies mainly developed organic pores and fractures, and the pore texture was excellent overall. The mixed shale facies mainly developed intergranular pores and mold pores, with a preference toward pore texture. The argillaceous shale facies mainly developed dissolution pores and interlayer fractures, and the pore texture was relatively poor. The geochemical characteristics of the organic-rich shale samples with TOC > 3.5% revealed that the sample was composed of microcrystalline quartz grains as the rock support framework, while the intergranular pores were located between the rigid quartz grains, which exhibited hard pores in the analysis of their mechanical properties. In the relatively organic-poor shale samples with TOC < 3.5%, the quartz source was mainly terrigenous clastic quartz, and the sample was composed of plastic clay minerals as the rock support skeleton, while the intergranular pores were located between argillaceous particles, which exhibited soft pores in the analysis of their mechanical properties. The difference in the rock fabric of the shale samples resulted in an ″initial increase followed by a decrease″ trend of velocity with quartz content, with the organic-rich shale samples exhibiting low velocity-porosity and velocity-organic matter content change rate, and the two kinds of rocks were easier to distinguish in the correlation diagram of the combined elastic parameters such as the P-wave impedance-Poisson ratio and the elastic modulus-Poisson ratio. The samples dominated by biogenic quartz exhibited greater hardness and brittleness, while the samples dominated by terrigenous clastic quartz exhibited lower hardness and brittleness. These results could serve as a basis for logging interpretation and seismic ″sweet spot″ prediction of high-quality shale gas reservoirs in Wufeng Formation-Member 1 of the Longmaxi Formation.
... Processes 2023, 11, 1424 2 of 24 recoverable resources of shale gas in China, and it is an important replacement field for future shale gas exploration [4]. Shale is a heterogeneous porous material with a complex pore structure that includes a range of pore types, a broad range in pore size, and a continuous distribution of microscopic pores that are between nanometres and microns in size and are linked to macroscopic pores [5,6]. ...
To study the microscopic pore characteristics of marine–continental transitional shale, we studied the Daning–Jixian block of the Shanxi Formation using low-pressure CO2 adsorption (LP-CO2A) and low-temperature N2 adsorption (LT-N2A) methods in conjunction with field emission scanning electron microscopy (FE-SEM), geochemistry, and mineral composition analysis in order to obtain pore structure characteristic parameters. The fractal dimension of the pores was calculated using the Frankel–Halsey–Hill (FHH) model, and the study also discusses the factors that influence the pore structure. The study found that the marine–continental transitional phase shale of the Shanxi Formation has clay mineral contents ranging from 36.24% to 65.21%. The total organic carbon (TOC) contents range from 0.64% to 9.70%. Additionally, the organic matter maturity is high. The FE-SEM and gas adsorption experiments revealed that the transitional shale of the Shanxi Formation possesses a diverse range of pore types with relatively large pore sizes. The dominant pore types are organic and intragranular pores, with pore morphologies predominantly appearing as slit and parallel plate structures. According to the experimental data on gas adsorption, the total SSA values range from 11.126 to 47.220 m2/g. The total PV values range from 0.014 to 0.056 cm3/g. Micropores make up a greater proportion of the total SSA, whereas mesoporous pores make up a greater proportion of the total PV. The distribution of shale pore fractal dimensions D1 and D2 (D1 is 2.470 to 2.557; D2 is 2.531 to 2.755), obtained through LT-N2A data, is relatively concentrated. D1 and D2 have a positive correlation with the TOC content, clay mineral content, and BET-SSA, and D1 and D2 have a negative correlation with the quartz content. D2 is positively correlated with the Langmuir volume, showing that D2 can be used to evaluate the methane adsorption capacity.
... In addition, in recent years, shale gas flow has been discovered from exploration of the Permian Taiyuan Formation in the Ordos Basin. Many wells have obtained high production, which shows the resource potential of shale gas, and exhibits the coexistence of multiple gas types including shale gas, tight gas, and coal bed methane [51] . ...
Based on the analysis of Upper Paleozoic source rocks, source-reservoir-caprock assemblage, and gas accumulation characteristics in the Ordos Basin, the gas accumulation geological model of total petroleum system is determined. Then, taking the Carboniferous Benxi Formation and the Permian Taiyuan Formation and Shanxi Formation as examples, the main controlling factors of gas accumulation and enrichment are discussed, and the gas enrichment models of total petroleum system are established. The results show that the source rocks, faults and tight reservoirs and their mutual coupling relations control the distribution and enrichment of gas. Specifically, the distribution and hydrocarbon generation capacity of source rocks control the enrichment degree and distribution range of retained shale gas and tight gas in the source. The coupling between the hydrocarbon generation capacity of source rocks and the physical properties of tight reservoirs controls the distribution and sweet spot development of near-source tight gas in the basin center. The far-source tight gas in the basin margin is mainly controlled by the distribution of faults, and the distribution of inner-source, near-source and far-source gas is adjusted and reformed by faults. Generally, the Upper Paleozoic gas in the Ordos Basin is recognized in four enrichment models: inner-source coalbed gas and shale gas, inner-source tight sandstone gas, near-source tight gas, and far-source fault-transported gas. In the Ordos Basin, inner-source tight gas and near-source tight gas are the current focuses of exploration, and inner-source coalbed gas and shale gas and far-source gas will be important potential targets in the future.
... The study area, with an area of 4.5 × 10 4 km 2 , is located in the Southeastern Ordos Basin, and the studied well is located in Central part of the study area ( Figure 1B). From Late Carboniferous to Early Permian Shanxi Formation period, the Ordos Basin enters the subsidence period, and a large area of transgression occurs, which is dominated by marinecontinental transitional facies deposits, forming thick organic-rich marine-continental transitional shale (Chen et al., 2011;Wu et al., 2021). Until the Middle Permian Shihezi period, the Ordos Basin entered continental sedimentary evolution stage ( Figure 1C). ...
The Lower Permian Shanxi Formation in the Eastern Ordos Basin is a set of transitional shale, and it is also a key target for shale gas exploration in China. Three sets of organic-rich transitional shale intervals (Lower shale, Middle shale and Upper shale) developed in Shan 2³ Submember of Shanxi Formation. Based on TOC test, X-diffraction, porosity, in-situ gas content experiment and NMR experiments with gradient centrifugation and drying temperature, the reservoir characteristics and pore fluid distribution of the three sets of organic-rich transitional shale are studied. The results show that: 1) The Middle and Lower shales have higher TOC content, brittleness index and gas content, reflecting better reservoir quality, while the Upper shales have lower gas content and fracturing ability. The total gas content of shale in the Middle and Lower shales is high, and the lost gas and desorbed gas account for 80% of the total gas content. 2) The Middle shale has the highest movable water content (32.58%), while the Lower shale has the highest capillary bound water content (57.52%). In general, the capillary bound water content of marine-continental transitional shale in the Shan 2³ Submember of the study area is high, ranging from 39.96% to 57.52%. 3) Based on pore fluid flow capacity, shale pores are divided into movable pores, bound pores and immovable pores. The Middle shale and the Lower shale have high movable pores, with the porosity ratio up to 27%, and the lower limit of exploitable pore size is 10 nm. The movable pore content of upper shale is 25%, and the lower limit of pore size is 12.6 nm. It is suggested that the Lower and Middle shales have more development potential under the associated development technology.
... The requirements for shale reservoirs to be the potential industrial exploitation target are TOC ≥ 2.0%; gas content > 2.0 m 3 /t. 31 Combining the current situation of mountain shale research in the study area and referring to previous research results, 12 shale lithofacies are subdivided into four types in this paper, using 4.0, 3.0, and 2.0% as TOC division thresholds. These include the high-carbon shale (TOC > 4.0%), medium-carbon shale (TOC between 3.0 and 4.0%), low-carbon shale (TOC between 2.0 and 3.0%), and extra-low-carbon shale (TOC < 2.0%). ...
... At TOC greater than 4.0%, the lithofacies in the study area are mainly siliceous shale; at TOC ranges from 3.0 to 4.0%, the lithofacies are mainly clay siliceous shale, calcareous siliceous shale, and siliceous shale; while at TOC ranges from 2.0 to 3.0%, the shale lithofacies are mainly clay siliceous shale. A condition for a shale reservoir to be a potential target for industrial exploitation is that the TOC is greater than 2.0%, 31 so the lower limit of the TOC content of the advantageous lithofacies shale reservoir in the mountain shale in the study area is 2.0%. ...
The mountain shale in Wufeng Formation-Member 1 of Longmaxi Formation in the complex structural area of northern Yunnan-Guizhou has great potential for exploration and development. In order to clarify the differences of reservoir quality and the longitudinal distribution law of different lithofacies, the lithofacies in Wufeng Formation-Member 1 of Longmaxi Formation was divided combined with core, logging, and analytical test data. Based on the data of total organic carbon, laminate structure, reservoir porosity types and physical properties, and gas content, the reservoir characteristics and advantageous lithofacies shale reservoir distribution were determined. The results show that the mountain shale lithofacies in the study area are divided into 7 major types and 20 subtypes. The high-carbon siliceous shale has the highest degree of organic pore development, specific surface area (average 28.55 cm2/g), and pore volume (average 0.0397 cm3/g). Two types of advantageous lithofacies shale reservoir in the study area were identified. The high-carbon siliceous shale reservoir could be considered as an excellent shale reservoir for shale gas and is mainly concentrated in the first section. It is mainly developed in layer-1 and the bottom of layer-2 of the Longmaxi Formation, providing favorable source-reservoir conditions for shale gas enrichment. The medium-carbon siliceous shale, medium-carbon clay siliceous shale, and medium-carbon calcareous siliceous shale, developed in the top of layer-2 and the bottom of layer-3 of the Longmaxi Formation, are assumed to be moderately promising for shale gas. Therefore, the research results deepen the understanding of the longitudinal distribution of the dominant shale reservoirs in the study area and are of great significance in promoting strategic transfer of the main shale gas exploration system in the south from the Sichuan basin to the outer basin.
... A success of shale gas in North America started a "shale revolution" all over the world. In China, a strategic breakthrough on shale gas has also been made along with the sustained progress in exploration and development technologies [1][2][3][4][5][6]. In fact, China has the largest technically recoverable reserves in the world, and Sichuan Basin possesses the most abundant shale gas resources in China [7][8][9][10][11][12][13][14]. ...
... The numerical simulation of the resistivity in fractured reservoirs with various fracture geometries [6,[55][56][57] is more complicated than that in a single porous medium. However, most of the simulations merely cared about regular and complete fractures without considering the spatial relations between fractures and the position of the fractures in the rock. ...
In the oil and gas industry, traditional logging mostly deems that oil and gas reservoirs are characterized by high resistivity, whereas the water layer is often by low resistivity. However, a lot of exploration and development practices on shale gas reservoirs in Sichuan Basin, China, prove that it is hard to characterize a functional relation between resistivity and water saturation using the Archie equation. Therefore, to make clear the mechanism to form low resistivity in shale gas reservoirs, the matrix resistivity was calculated through the percolation network simulation based on pore structure characteristics and mineral compositional parameters. Moreover, the resistivity in low-resistivity laminations of shale was measured through the finite element simulation. In addition, the reasons for such low resistivity in shale were analyzed according to the resistivity-forming mechanism, and the effects of penetration degree, width, quantity, and spatial distribution of the laminations on the resistivity were worked out. Those may provide theoretical support for explaining the phenomenon of low-resistivity gas reservoirs.
... Kuang et al. [4] qualitatively described the MTC shale pore classification. Wu et al. [25] and Gu et al. [26] mainly analyzed the mesoporous and microporous characteristics of MTC shale in the different lithofacies. However, previous studies lack the quantitative pore development of MTC shale with different OM spatial distribution and do not quantitatively clarify the main controlling factors of MTC shale pore development. ...
The pore characteristics are studied in the overmatured marine-continental transitional (MCT) shale and simulated shale under different thermal maturity conditions, based on transitional and simulated shale samples in the eastern margin of Ordos Basin. The work uses high-pressure mercury intrusion (MICP), field emission scanning electron microscopy (FESEM), helium-mercury method, X-ray diffraction of whole-rock minerals, and hydrocarbon-generating thermal simulation to quantitatively analyze pore characteristics and main controlling factors of pore development. The results show that the shallow bay and lake facies (SBLF) shale has great exploration potential, while the delta facies (DF) shale has poor exploration potential. The SBLF shale is mainly characterized by pie shale, high quartz and carbonate, low clay, high porosity, and pore volume. The DF shale mainly develops dot shale with low quartz and carbonate content, high clay content, low porosity, and pore volume. Kaolinite has the strongest inhibition on MTC shale pore development. The pore volume of MTC shale decreases first and then increases with maturity. The pie shale is more conducive to the increase of pore volume than the dot shale. The effect of doubled TOC on porosity is greater than that of maturity in the dot shale. The effect of doubled TOC on porosity is less than that of maturity in the pie shale. Organic matter (OM) has the greatest impact on pore development, controlled by the OM content, sedimentary facies, and maturity. OM content, sedimentary facies, and maturity can be used to jointly characterize the MTC shale pore development, providing guidance for multiparameter quantitative characterization of pore development and determining the enrichment area of shale gas.
