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The transitional shale of the Permian Shanxi Formation is an important source rock for shale gas, tight sandstone gas, and coalbed methane in the Ordos Basin. This study takes third Sub-Member of Shanxi Formation as an example, and uses continuously collected shale core samples and matching geochemical data to restore the paleo-environmental conditions of its depositional period, clarify the formation mechanism of organic rich shale in different sedimentary environments under transitional facies background. According to different types of cross-plot patterns, paleo-weathering, provenance sources, and tectonic backgrounds were determined in this study. The results indicate that Shanxi organic-rich shale can be divided into two categories: estuarine shale in Unit 1 and lagoon shale in Units 2–4. The average TOC value of estuarine shale is significantly higher than that of lagoon shale, reaching 5.55%. Estuarine shale is mainly deposited in an suboxic-anoxic bottom water environments, with high surface seawater productivity, which is conducive to the formation and preservation of a large amount of organic matter. Lagoon shale is mainly composed of Type II2 and III kerogen, with a weakly oxic bottom water environment and low water surface paleo-productivity. The organic matter mainly comes from terrigenous plant debris, and its high sedimentation rate can effectively accumulate and preserve it. The chemical weathering indicators of Shanxi transitional shale are controlled by sedimentary recycling, hydrodynamic sorting, and diagenetic alteration, and the influence of changes in the source area is relatively small. The Shanxi transitional shale is mainly deposited on the active continental margin under collision background, and its source is mainly granodiorite from the Qinling orogenic belt. [ABSTRACT FROM AUTHOR]

The Ordos Basin, characterized by its abundant transitional shale gas resources, plays a significant role in Chinese oil and gas exploration industry. However, the complex sedimentary environment and lithofacies combination of transitional shale make it highly challenging for reservoir quality evaluation. Acknowledging the rapid development of artificial intelligence, particularly the extensive use of machine learning in geology, this study proposes a new approach to assess the quality of transitional shale reservoirs through the utilization of the Random Forest algorithm (RF). Firstly, the lithology identification chart and reservoir quality evaluation standard were established using data and logging curves, and the relevant datasets were constructed. Four logging curves (Acoustic curve (AC), Compensated Neutron Log (CNL), Density curve (DEN), Gamma Ray (GR)), which serve as input variables to reflect reservoir characteristics, were carefully selected, while reservoir quality classification was used as the output results. Subsequently, the RF model was constructed and trained using this dataset. By analyzing the confusion matrix, it was observed that the RF model achieved an impressive accuracy level of approximately 90%. The study confirmed RF's superiority through comparisons with five methods: Factor analysis, Bayesian discriminant analysis, Gaussian Mixture Model (GMM), K-Nearest Neighbors (KNN), Gradient Boosting Decision Tree (GBDT). The comparison results revealed that the RF model exhibited high reliability and practical efficiency. Additionally, the RF model is utilized to predict the thickness of Type I reservoirs in the study area. The results demonstrated remarkable success in confirming production data, further emphasizing the proficiency of the RF within the field of machine learning for evaluating transitional shale reservoirs. This method presents a valuable tool for assessing transitional shale reservoirs. [ABSTRACT FROM AUTHOR]

Upper Permian Longtan Formation transitional shale has become an important exploration layer, but the Longtan Formation shale (LFS) has a complex mineralogical composition, which affects the subsequent diagenesis and diagenetic evolution, and restricts the subsequent geologic exploration of shale gas. In this article, the observation of drilling cores, argon ion polishing-scanning electron microscope, Vitrinite reflectance, and X-ray diffractometer were used to analyze the type and characteristics of diagenesis of the LFS and clarify the stage of diagenetic evolution. The results show that the main diagenesis in the LFS is compaction, cementation, thermal maturation of organic matter (OM), dissolution, and transform of clay minerals. Among them, OM hydrocarbon generation, clay mineral transformation, and dissolution are pore-enhancing diagenetic events. Compaction and cementation are pore-reducing diagenetic events. The transitional and marine shales have similar characteristics of diagenesis, but there are big variations in the diagenesis of OM hydrocarbon, authigenic quartz, and siderite. The complex depositional environments of the marine-continental transition environment have resulted in a variety of rock types, which in turn influenced the diagenesis types and diagenetic evolution process. Compared with the transitional shales of the Shanxi and Taiyuan formations in the South China North Basin, the LFS are characterized by high clay content, low quartz content, complex mineral compositions, and a higher degree of thermal evolution.

The transitional shale of the Permian Shanxi Formation is an important source rock for shale gas, tight sandstone gas, and coalbed methane in the Ordos Basin. This study takes third Sub-Member of Shanxi Formation as an example, and uses continuously collected shale core samples and matching geochemical data to restore the paleo-environmental conditions of its depositional period, clarify the formation mechanism of organic rich shale in different sedimentary environments under transitional facies background. According to different types of cross-plot patterns, paleo-weathering, provenance sources, and tectonic backgrounds were determined in this study. The results indicate that Shanxi organic-rich shale can be divided into two categories: estuarine shale in Unit 1 and lagoon shale in Units 2–4. The average TOC value of estuarine shale is significantly higher than that of lagoon shale, reaching 5.55%. Estuarine shale is mainly deposited in an suboxic-anoxic bottom water environments, with high surface seawater productivity, which is conducive to the formation and preservation of a large amount of organic matter. Lagoon shale is mainly composed of Type II2 and III kerogen, with a weakly oxic bottom water environment and low water surface paleo-productivity. The organic matter mainly comes from terrigenous plant debris, and its high sedimentation rate can effectively accumulate and preserve it. The chemical weathering indicators of Shanxi transitional shale are controlled by sedimentary recycling, hydrodynamic sorting, and diagenetic alteration, and the influence of changes in the source area is relatively small. The Shanxi transitional shale is mainly deposited on the active continental margin under collision background, and its source is mainly granodiorite from the Qinling orogenic belt.

Horizontal principal stress is a fundamental parameter for reservoir reconstruction. For improving single well productivity, accurate evaluation of reservoir stress characteristics is of great importance. One of the main challenges in predicting the magnitude of the in situ stress is how to obtain the rock mechanical parameters accurately. An intelligent fusion model was proposed to predict rock mechanical parameters to address the issue that traditional approaches are not very reliable at predicting the rock mechanical parameters of complex lithology reservoirs, using transitional shale reservoir rocks as the research object. Machine learning algorithms such as nearest neighbor regression, support vector machine, and random forest were selected to construct intelligent fusion models of different rock mechanics parameters based on the laboratory test data. Finally, the logging profile of transitional shale reservoir horizontal principal stress in the study area was obtained under the constraints of the empirical physical model and measured in situ stress data. The results showed that the fusion models outperformed the single model on rock mechanics parameters and had higher accuracy in both training and test sets, meeting the engineering requirements for predicting the horizontal principal stress in the study area. [ABSTRACT FROM AUTHOR]

The paleoenvironment of shales can be reconstructed to some extent using the combinations or concentrations of elements that correlate strongly with environmental conditions. In this study, we analyzed rare earth elements (REEs), major elements, and trace elements in the marine-continental transitional shales (transitional shales for short) of the Shan 2³ submember of the Shanxi Formation in the southeastern Ordos Basin. The purpose is to deduce the paleoenvironmental conditions of the shales, encompassing paleoredox, paleoclimate, paleoproductivity, and paleo-provenance. The Shan 2³ submember comprises four sections, namely Shan 2³-1, Shan 2³-2, Shan 2³-3, and Shan 2³-4. The Ba/Al, P/Al, and Cu/Al ratios, along with biogenic barium (Babio), indicate that the paleoproductivity of the submember peaked during the Shan 2³-1 deposition and exhibited a downtrend upward in other sections. Trends in the Uau and the Ni/Co, V/Cr, U/Th, and V/Sc ratios suggest that suboxic conditions prevailed during the Shan 2³-1 deposition, with the oxidation level gradually increasing from Shan 2³-1 to Shan 2³- 4. C-value and the Sr/Cu vs. Ga/Rb cross-plot indicate a warm and arid paleoclimate during the Shan 2³-1 deposition, which transitioned to cooler, drier conditions during the deposition of other sections. Indicators sensitive to paleoclimate, such as the K/Rb and Th/U ratios, along with the ICV, PIA, and Chemical Index of Alteration (CIA), highlight elevated weathering from Shan 2³-2 to Shan 2³-4, with Shan 2³-1 exhibiting the weakest weathering during its deposition. As suggested by the REE data, the Zr/Sc vs. Th/Sr cross-plot, provenance discriminant functions, and the cross-plots of Hf vs. La/Th, Th vs. HfeCo, and PREE vs. La/Yb, the sedimentary provenance for the transitional shales of the Shan 2³ submember is of multiple origins, with significant contributions from the Upper Continental Crust (UCC). Discriminant diagrams, including those of TheCoeZr/10, TheSceZr/10, LaeTheSc, and K2O/Na2O vs. SiO2, suggest that the transitional shales of the Shan 2³ submember were primarily deposited under tectonic settings such as continental island arcs (CIAs) and passive continental margins (PCMs). [ABSTRACT FROM AUTHOR]

The marine–continental transitional shale of the Upper Permian Longtan Formation in northern Guizhou is an important source rock in the upper Yangtze region of China, and it holds significant potential for the exploration of shale gas. To investigate the correlation between sedimentary conditions and the accumulation of organic matters in marine–continental transitional shale, this paper performed an extensive analysis using organic geochemical testing, organic petrology examination, a cross-section polisher–scanning electron microscope (CP-SEM), and geochemical analysis. The Jinsha and Dafang drilling cores were selected as the research subjects. The results showed that the TOC of the Longtan Formation in the study area was relatively high, and the TOC content of the tidal flat–lagoon environment (average of 8.37%) was significantly higher than that of the delta samples (average of 2.77%). The high content of Al2O3 (average of 17.41% in DC-1, average of 16.53% in JC-1) indicated strong terrigenous detrital input. The proxies indicated that the Longtan Formation shale in northern Guizhou was deposited in a climate that was both warm and humid, with oxic–dysoxic sedimentary water characterized by high biological productivity and a rapid sedimentation rate. The organic-rich shales during the marine and continental transitional phases were affected by various factors, including the paleo-climate, water redox properties, paleo-productivity, sedimentation rate, and other variables, which directly or indirectly impacted the availability, burial, and preservation of organic matter. [ABSTRACT FROM AUTHOR]

Reservoir in-situ stress is crucial in oil and gas exploration and development. With the transitional shale as the research object, the ultrasonic transmission test and uniaxial compression test of rock samples from different sampling angles were performed. Further, the transversely isotropic in-situ stress logging calculation method for transitional shale reservoirs was established based on the transversely isotropic constitutive relationship. Finally, the in-situ stress profile of the transitional shale reservoir in the study area was analyzed, and the reliability of the results was verified. The results indicated that the mechanical characteristics of rock and its acoustic velocity varied significantly depending on the sample angle, with some regularity in the changes between vertical and horizontal mechanical properties. The logging profile revealed that the transverse Young modulus was greater than the vertical, and the vertical Poisson's ratio was typically greater than the transverse. The suggested transversely isotropic model performed better than the conventional in-situ stress model at predicting the in-situ stress of transitional shale reservoirs, with an error of less than 2%. This demonstrated the applicability of the model's in-situ stress calculation of the transitional shale reservoir. [ABSTRACT FROM AUTHOR]

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Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica 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 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 intensive study of the pore structure and its controlling factors of shale reservoir has important guiding significance for further exploration and exploitation of shale gas. This work investigated the effects of organic and inorganic compositions on the development of pore structures of the Upper Permian Shanxi shale in the southern Yan’an area, Ordos Basin. Based on the results of high-pressure mercury intrusion, low-pressure N2 and CO2 adsorption and organic geochemical experiments, X-ray diffraction and scanning electron microscope observations, the mineral composition, pore structure and its influencing factors of the transitional shale were studied systematically. The results indicate that the total organic carbon (TOC) content of the shale is between 0.12% and 5.43%, with an average of 1.40%. The type of the organic matter (OM) belongs to Type III and has over maturity degree with an average Ro of 2.54%. An important character of this kind of shale is the large proportion of clay mineral content, which ranges from 40.70% to 87.00%, and with an average of 60.05%. Among them, illite and kaolinite are the main components, and they account for 36.6% and 36.7% of the total clay minerals respectively, followed by chlorite and illite/smectite (I/S) mixed layer. The quartz content is between 10.6% and 54.5%, with an average of 35.49%. OM (organic matter) pores are mostly circular bubble-shaped pores, and most of them are micropores, while inorganic pores are well developed and mainly contributed by clay mineral pores and have slit-type, plate-like and irregular polygon forms. Mesopores are the major contributor to pore volume (PV), while micropores contribute the least to PV. The contribution of micropores to the specific surface area (SSA) is greater than 61%, followed by mesopores. Macropores have almost no contribution to the development of SSA. OM pores are the main contributor to the total specific surface area of the shale, with an average contribution rate of 61.05%, but clay mineral pores contribute more to the total pore volumes. In addition, both the content of chlorite and illite/smectite (I/S) mixed layer is positively correlated with the volume ratio of mesopores. It was found that high TOC, I/S mixed layer and chlorite content are all favorable conditions for the target shale.

Fracability is a property widely used to evaluate whether reservoirs can effectively fracture to increase production capacity. The brittleness index, which is used to evaluate reservoir fracability, is calculated via various methods, and the evaluation process of the brittleness index is complicated and not sufficiently intuitive. Thus, we used triangle method to evaluate reservoir fracability. The shale composition is classified into strong (quartz + pyrite), moderate (carbonate + plagioclase + siderite), and poor (clay + TOC + porosity) fracability based on the mechanical parameters of each shale component, which were integrated as the endpoints of the triangle method. Meanwhile, the triangle method is divided into four fracability evaluation grades: strong (I), moderate (II), weak (III) and poor (IV) fracability; each major evaluation grade has four sub-categories: best for fracturing (1), better for fracturing (2), poor for fracturing (3) and worst for fracturing (4). The triangle method is simpler and more convenient than the conventional method, dividing the fracability evaluation grades more specifically. The difference in fracability between samples can be shown intuitively, which enhances the accuracy and reliability of the evaluation of transitional shales and provides theoretical support for reservoir reconstruction. [ABSTRACT FROM AUTHOR]

Quantitative characterization of pore structure and analysis of influencing factors of methane adsorption are important segments in shale gas reservoir and resources evaluation and have not been systematically carried out in marine–continental shale series. A series of integrated methods, including total organic carbon (TOC) contents, Rock-Eval pyrolysis, mineral composition analysis, pore structure measurement, high-pressure CH4 adsorption analysis and FE-SEM observation, were conducted on 12 transitional shale samples of well WBC-1 in the southern North China Basin (SNCB). The results indicate that TOC contents of the transitional shales range from 1.03 to 8.06% with an average of 2.39%. The transitional shale consists chiefly of quartz, white mica and clay minerals. Interparticle pore, intraparticle pore, dissolution pore and microfracture were observed in the FE-SEM images. The specific surface area (SSA) of BET for the samples ranges from 3.3612 to 12.1217 m2/g (average: 6.9320 m2/g), whereas the DR SSA for the samples ranges from 12.9844 to 35.4267 m2/g (average: 19.67 m2/g). The Langmuir volume (VL) ranges from 2.05 to 4.75 cm3/g (average = 2.43 cm3/g). There is unobvious correction between BET and DR SSA with TOC contents, which means inorganic pores are the main component of pore space in the transitional shale from the SNCB. The relationship of SSA and pore volume shows that micropore has a greater impact on the CH4 adsorption capacity than mesopore–macropore in the transitional shale. Different from shales in other petroliferous basin, clay minerals are the primary factor affecting adsorption capacity of CH4 for transitional shale in this study. The pore structure of the transitional shale for this study is characterized by higher fractal dimension and more heterogeneous pore structure compared to shale in other petroliferous basin. This study provides an example and new revelation for the influencing factors of pore structure and methane adsorption capacity of marine–continental transitional shale. [ABSTRACT FROM AUTHOR]

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Marine–continental transitional (hereinafter referred to as transitional) Permian shales are important targets for shale gas in China because of the considerable volumes of shale gas resources present in them. In this study, transitional shale samples from the Permian Shanxi Formation in the Daning–Jixian block along the eastern margin of the Ordos Basin were collected to investigate the effects of organic and inorganic compositions on the development of their pore structures through organic petrographic analysis, X–ray diffraction, scanning electron microscope (SEM) observation, gas (N2 and CO2) adsorption, high-pressure mercury injection (HPMI), and methane adsorption experiments. The organic petrographic analysis reveals that the Permian Shanxi shale comprises Type-II2-III kerogens, and the average vitrinite reflectance (Ro) is 2.3% at the overmature stage or in the dry gas window. The shale interval at the bottom of the lagoon facies is considered the most favorable interval throughout the entire section because of its high total organic carbon (TOC) content (4.19–43.9%; an average of 16.9%) and high brittle mineral content (38.3–73.2%; an average of 55.8%). N2 and CO2 gas adsorption and HPMI tests reveal the pore size distribution characteristics of the shale. The full pore size distribution by the gas adsorption and HPMI test reveals that micropores (

Shale gas occurrence is mainly affected by pore heterogeneity caused by material composition. Therefore, transitional shale samples collected from the Taiyuan Formation in the Linxing area were carried out with the low-temperature nitrogen adsorption (LT-N2GA), geochemical composition, and scanning electron microscopy (SEM) observation tests. Surface and structural fractal dimensions (D1 and D2) were obtained by using the Frenkel-Halsey-Hill (FHH) model. Based on this, the influence of pore development characteristics and material composition on pore homogeneity was studied. The maturity of organic matter (OM) is beneficial to the development of pore volume (PV) and specific surface area (SSA), and it increases pore surface roughness and pore structure complexity. The total organic carbon content is negatively associated with PV, SSA, and fractal dimensions. The influence of clay minerals on the fractal dimension is complex. D1 increases with increasing chlorite and illite contents but decreases with increasing kaolinite and illite mixed-layer contents. D2 increases with increasing chlorite, illite, and kaolinite contents and decreases with increasing illite mixed-layer content. Brittle minerals are mainly composed of quartz; an increased quartz content is not conducive to the development of pores, but it increases the homogeneity of pores. [ABSTRACT FROM AUTHOR]

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The evolution of shale reservoirs is mainly related to two functions: mechanical compaction controlled by ground stress and chemical compaction controlled by thermal effect. Thermal simulation experiments were conducted to simulate the chemical compaction of marine-continental transitional shale, and X-ray diffraction (XRD), CO2 adsorption, N2 adsorption and high-pressure mercury injection (MIP) were then used to characterize shale diagenesis and porosity. Moreover, simulations of mechanical compaction adhering to mathematical models were performed, and a shale compaction model was proposed considering clay content and kaolinite proportions. The advantage of this model is that the change in shale compressibility, which is caused by the transformation of clay minerals during thermal evolution, may be considered. The combination of the thermal simulation and compaction model may depict the interactions between chemical and mechanical compaction. Such interactions may then express the pore evolution of shale in actual conditions of formation. Accordingly, the obtained results demonstrated that shales having low kaolinite possess higher porosity at the same burial depth and clay mineral content, proving that other clay minerals such as illite–smectite mixed layers (I/S) and illite are conducive to the development of pores. Shales possessing a high clay mineral content have a higher porosity in shallow layers (< 3500 m) and a lower porosity in deep layers (> 3500 m). Both the amount and location of the increase in porosity differ at different geothermal gradients. High geothermal gradients favor the preservation of high porosity in shale at an appropriate Ro. The pore evolution of the marine-continental transitional shale is divided into five stages. Stage 2 possesses an Ro of 1.0%–1.6% and has high porosity along with a high specific surface area. Stage 3 has an Ro of 1.6%–2.0% and contains a higher porosity with a low specific surface area. Finally, Stage 4 has an Ro of 2.0%–2.9% with a low porosity and high specific surface area. [ABSTRACT FROM AUTHOR]

In this study, matrix-related pores from differing depositional shales were explored comparatively. Among of them, Lower Cambrian shale (3.83%Ro) and Lower Silurian shale (2.61%Ro) were marine sediments with abundant oil-prone kerogen and rich siliceous minerals, while Upper Permian shale (2.44%Ro) were transitional sediments with redundant gas-prone kerogen and rich clay. The morphology and geometry of pores were investigated via fractal analyses based on N 2 adsorption and direct imaging. The effects of organic matter (OM) within different shales were also highlighted through N 2 adsorption before and after OM isolation. Lower Cambrian shale possessed the lowest pore volumes (PV) (averaging 0.0109 ml/g) and the lowest pore surface areas (PSA) (averaging 9.09 m2/g) as well as the smallest average pore diameters (APD) (averaging 5.47 nm). Dissolved pore with dead-end openings was the main type. The PV and PSA of isolated OM were only approximately 1 and 2 times higher than that of corresponding samples, respectively. In contrast, Lower Silurian shale possessed the highest PV (averaging 0.0109 ml/g) and the highest PSA (averaging 9.09 m2/g) as well as relatively large APD (averaging 13.43 nm). Organic-hosted pores (OMP) with cellular structure is the main type. The PV and PSA of isolated OM were approximately 8.5 and 3 times higher than that of corresponding samples, respectively. Upper Permian shale with the largest averaging pore diameters (averaging 18.82 nm) presented high PV (averaging 0.0209 ml/g) similar to that of Lower Silurian shale, and a low PSA (averaging 10.85 m2/g) like that of Lower Cambrian shale. Pore associated with clay flakes was the main type. The PV and PSA of isolated OM were only approximately 0.6 times and 1 times higher than that of corresponding samples, respectively. For marine shale, matrix-related pore features are synergy effects of the matrix basis where pre-existing space controls the occurrence of porous OM and functions as the shelter for OMP with an appropriate thermal maturity. However, extensive diagenesis can overprint the effects of matrixes on pore properties, because oil-prone kerogen is sensitive to thermal maturity. The specific material composition of transitional shale limit pore properties, because dominant structured OM is thermally stable with limited migration ability and pore contribution. Hence, diagenetic differences and material diversities may be attributed to the discrepancies of pore properties between marine shale and transitional shale. • Pores from differing depositional shales were explored comparatively. • Pore features were analyzed via N 2 adsorption and direct imaging. • Effects of OM were highlighted via N 2 adsorption before and after OM isolation. • Diagenetic differences and material diversities cause discrepancies of porosity. [ABSTRACT FROM AUTHOR]

Transitional shale is very different from marine shale with respect to mineral composition and pore structure. Insights into the effects of rock composition on pore structure can aid understanding of the storage and migration mechanisms of shale gas in a reservoir. To investigate the compositional effects on pore structure, a combination of geochemical and petrophysical experiments were conducted on four bulk shale samples and their corresponding extracted organic matter (OM) and clay mineral samples. The experimental results show that the removal of OM using H 2 O 2 during clay mineral extraction oxidizes and removes pyrite and siderite from the samples while other inorganic phases remain unaffected. OM hosted more abundant micropores compared with clay minerals, while mesopores and fine-macropores are dominant in clay minerals. Effects of the OM and clay minerals on pore volume and surface area are quantitatively evaluated on the basis of weight-normalized pore-size distribution (PSD) curves of extracted OM and clay mineral samples. In terms of the CO 2 adsorption, contribution from OM and clay minerals to micropore volume (MIV) are 19.43–32.92% and 18.75–39.05% respectively, and to micropore surface area (MIS) are 19.30–32.28% and 16.42–35.72%. Other minerals (other than organic matter and clay minerals) account for 34.29–48.93% of the MIV and 39.75–52.98% of the MIS. N 2 adsorption results reveal that the pore volume and surface area are mainly attributable to the clay minerals, providing 54.37–80.75% of the BJH pore volume (BJHV) (average 65.83%) and 70.68–87.04% of the BET surface area (BETS) (average 78.71%). The primary medium for pore development in marine shale is OM, whereas pores in the transitional shale are mostly found in the clay and other inorganic minerals. Therefore, the hydraulic fracturing design and drilling-related operations involved in the exploitation of transitional shale gas should be appropriately adjusted and improved in order to accommodate intrinsic characteristics of the transitional shale gas reservoir. • Organic matter(OM) and clay minerals were isolated from shale samples. • Pore size distribution of bulk, and corresponding OM and clay samples were tested. • Composition effects on pore structure were quantitatively calculated. [ABSTRACT FROM AUTHOR]

Pore connectivity of shale controls shale gas migration and production behavior. The pore connectivity and water accessibility in clay-rich Upper Permian transitional shales remain unclear. Contrast matching small-angle neutron scattering (CM-SANS) tests were used to determine the accessibility of water to pores in the transitional shales. Complementary analyses with SANS, gas (CO 2 and N 2) physisorption isotherm, mercury intrusion capillary pressure (MICP), helium ion microscopy (HIM), as well as field emission-scanning electron microscopy (FE-SEM) were conducted to study the pore connectivity and distribution characteristics of closed pores. The results show that closed pores (inaccessible to nitrogen molecules and mercury) are mainly distributed in pore diameters ＜10 nm and associated with organic pores and interlayer spaces of illite-smectite mixed-layer mineral. The pore volume values obtained from MICP and N 2 adsorption underestimate the large pores (pore diameters ＞100 nm) in shales. Based on deuterated water CM-SANS tests, 87–98% of the pores (2–200 nm diameters) are water-connected in transitional samples. The low accessibility to water is at pore-sizes of 5–10 nm and 20–30 nm. Results from in-situ gas contents show that closed pores have a certain gas bearing capacity, but micropores (pore diameters ＜2 nm) control the gas occurrence in transitional samples. The connectivity of the organic pore network and between organic pores and surrounding interparticle pores is directly supported by FE-SEM and HIM imaging. Overall, improving pore-fracture connectivity through effective fracturing techniques is a means of mitigating the rapid decline in shale gas production. • H 2 O/D 2 O contrast matching SANS tests were conducted on pore connectivity and water accessibility of transitional shale. • Closed-pore porosity and PSD were compared from SANS, MICP, and gas sorption approaches. • Inaccessible pores to N 2 and mercury have a certain gas-bearing capacity. • Pore connectivity of gas shales will affect hydrocarbon production. [ABSTRACT FROM AUTHOR]

Organic-rich shale samples from marine-continental transitional facies of the Late Permian Longtan Formation in southwestern Guizhou were investigated using organic petrography analysis, X-ray diffraction analysis, mercury intrusion capillary pressure experiments, gas adsorption experiments, nuclear magnetic resonance, scanning electron microscopy image analysis, and multi-fractal theory to quantitatively and qualitatively investigate the full-size pore structure and fractal characteristics of the shale. The correlations among the total organic carbon (TOC) content, mineral composition, and pore structure were discussed. The fractal dimension was used to evaluate the production potential of a shale reservoir. Results show that samples have a high TOC content (4.38 wt % on average), low permeability (less than 0.001 mD), and complex pore structure. The pore volume (PV) varies from 0.001997 cm3/g to 0.03653 cm3/g, with an average of 0.02864 cm3/g, and the specific surface area (SSA) varies from 14.144 m2/g to 29.740 m2/g, with an average of 21.533 m2/g. Mesopores (diameter between 2 and 50 nm according to IUPAC) are the predominant contributors to the pore volume, while mesopores and micropores (diameter less than 2 nm) are the main contributors to the specific surface area. The shale samples with higher TOC and clay mineral content have larger PVs and SSAs and higher fractal dimensions than those that do not. A high fractal dimension indicates a rough pore surface and complicated pore structure. Shale with higher D 1 and D A (representing the pore surface roughness and the adsorption space, respectively) has high adsorption capacity and favors gas accumulation, while shale with higher D 2 and D S (representing the pore structure complexity and the seepage space, respectively) has greater flow capability and favors gas diffusion. • Quantitative methods were combined with qualitative methods in this study to investigate the full-size pore structure characteristics of marine-continental transitional shale. • The fractal characteristics of transitional Longtan shale were investigated using multi-fractal concept and different experiments. • The fractal dimension was used to evaluate the production potential of shale reservoir. [ABSTRACT FROM AUTHOR]

We analyzed the tectonic evolution characteristics, sedimentary environment, geochemical characteristics, petrological characteristics, and gas-bearing properties of three mudstone sections of the Lower Paleozoic in Ningwu Basin, NE China, and determined the geologic characteristics and resource potential of the transitional facies shale gas. Geochemical analysis of the organic carbon content, kerogen macerals, and vitrinite reflectance of the shale samples showed that the total organic content was generally over 2.0%, the main organic type was type III, and the vitrinite reflectance values (Ro) were between 1.20 and 1.90%. Thus, the mudstones are good shale gas source rocks. The thickness of the three mudstone sections was approximately 30–70 m, and the average porosity was 3.10%. The pore types were diverse with good reservoir capacity. The shale gas resources of the Carboniferous-Permian transitional facies estimated by the volumetric method were approximately 2798.97 × 108–4643.09 × 108 m3. Through a comparison with shales in SW China, where shale gas has been successfully exploited, we determined the preferred criteria for favorable shale gas areas, as well as favorable areas for shale gas enrichment. [ABSTRACT FROM AUTHOR]

The Carboniferous Ceshui formation and Permian Longtan and Dalong formations were deposited in transitional settings preserved in what is now central Hunan Province, South China, as they are potential natural gas plays. In this study, we analysed the total organic carbon (TOC), vitrinite reflectance (Ro), kerogen type, mineralogy, porosity, permeability, and methane adsorption of representative shale samples from these rock units. Our results indicate that TOC content can be as high as 9.2%, with a mean (x̄) of 3.5%. The Permian shale formations were deposited in more strongly reducing environments than the Carboniferous Ceshui shale. The kerogen composition of the Carboniferous Ceshui shale is dominated by Type III, while both of the Permian shales contain primarily Type II kerogens; Ro values range from 1.1% to 2.4% (x̄ = 1.6%). The organic matter in all the studied shales is in the wet gas window of thermal maturity and is relatively less mature than Lower Palaeozoic marine shales in south China. Mineral compositions are dominated by quartz (x̄ = 53.8%) and clay (x̄ = 35.6%), suggesting a high brittleness index. Porosity ranges from 0.5% to 14% (x̄ = 6.4%), while permeability varies from 0.0026 micro Darcy (mD) to 0.0640 mD (x̄ = 0.0130 mD). The gas adsorption capacity varies from 1.24 to 4.53 cm 3 /g (x̄ = 2.40 cm 3 /g). Relatively less mature shale samples (Ro<1.5%) have low methane adsorption capacities, regardless of their TOC values. However, the methane adsorption capacity of more mature (Ro>1.5%) shales samples exhibit a positive correlation with TOC content. [ABSTRACT FROM AUTHOR]

Shale samples collected from seven wells in the southeastern Ordos Basin were tested to investigate quantitatively the pore structure and fractal characteristics of the Lower Permian Shanxi Shale, which was deposited in a marine-continental transitional (hereinafter referred to as the transitional) environment. Low-pressure nitrogen adsorption data show that the Shanxi Shale exhibits considerably much lower surface area (SA) and pore volume (PV) in the range of 0.6–1.3 m 2 /g and 0.25–0.9 ml/100 g, respectively. Type III kerogen abundant in the transitional Shanxi Shale were observed to be poorly developed in the organic pores in spite of being highly mature, which resulted in a small contribution of organic matter (OM) to the SA and PV. Instead, I/S (illite-smectite mixed clay) together with illite jointly contributed mostly to the SA and PV as a result of the large amount of inter-layer pores associated with them, which were evident in broad-ion-beam (BIB) imaging and statistical analysis. Additionally, the Shanxi Shale has fractal geometries of both pore surface and pore structure, with the pore surface fractal dimension (D1) ranging from 2.16 to 2.42 and the pore structure fractal dimension (D2) ranging from 2.49 to 2.68, respectively. The D1 values denote a pore surface irregularity increase with an increase in I/S and illite content attributed to their more irregular pore surface compared with other mineralogical compositions and OM. The fractal dimension D2 characterizing the pore structure complexity is closely related to the pore arrangement and connectivity, and I/S and illite decrease the D2 when their contents increase due to the incremental ordering degree and connectivity of I/S- or illite-hosted pores. Meanwhile, other shale constituents (including kaolinite, chlorite, and OM) that possess few pores can significantly increase the pore structure complexity by way of pore-blocking. [ABSTRACT FROM AUTHOR]

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A series of methane sorption experiments were conducted at reservoir temperature, 60 ° C and pressures up to 18 MPa for seven fresh, over mature Lower Permian-Upper Carboniferous shale core samples collected from the Ordos Basin with depths over 3000 m and TOCs ranging 0.49–3.82 wt%. The measured maxima of excess sorption capacity of methane range from 1.36 to 2.38 m 3 /t. Maximum sorption capacity did not correlate only with total organic carbon (TOC) but showed a more complex dependence on petrophysical and mineralogical properties. Three different models were investigated to match the measured excess sorption isotherms and to correct those to absolute sorption isotherms: the 1) Ono-Kondo (OK), 2) supercritical Dubinin-Radushkevich (SDR) and 3) Langmuir-based excess sorption models. All three models are found to match the experimental data with comparable accuracy and to agree on the correction to absolute sorption isotherms. The relationship between sorbed gas and depth is also investigated based on a combination of sorbed gas data from previous research and from this investigation. A theoretical model is proposed to predict the evolution of methane sorption capacity with burial depth (or time) as a function of TOC, vitrinite reflectance (Ro), type of kerogen, pressure and temperature. The results show that methane sorption capacity will first increase with depth, and then decrease with depth after reaching a critical depth. [ABSTRACT FROM AUTHOR]

The purpose of this article is to make a comparative study on the pore structure of different sedimentary facies shales developed in typical areas, China. Results show that the surface area (SA) and pore volume (PV) of the over-mature marine shales are in the range of 17.83–29.49 m 2 /g and 3.65–18.26 ml/kg, respectively, which are much higher than that of the highly mature transitional and less mature terrestrial shales in the range of 0.9–1.9 m 2 /g and 2.5–10.9 ml/kg, respectively. In the view of the pore-size distribution (PSD), marine shales concentrate at the fine mesopores < 10 nm, while the terrestrial and transitional shales focus on the large-mesopores to fine-macropores (30–70 nm). Additionally, the qualitative analysis of hysteresis loops from the nitrogen isotherms reveals that the marine shales are dominated by the inkbottle-shaped pores, which are further confirmed to be the organic pores. While the terrestrial and transitional shales investigated give priority to the slit-shaped pores or wedge-shaped pores, which are further identified as the interlayer pores within clay minerals. Furthermore, different shale components have great differences in the contribution to the SA and PV in different sedimentary facies shales due to the different development of nanopores within (or associated with) different shale compositions. In post-mature marine shales, organic matters (OMs) bearing a large amount of smaller organic nanopores contribute mostly to the SA and PV. For the highly mature transitional shales, it is the illite-smectite mixed clay (I/S) that contributes mostly to the SA and PV because the I/S-hosted nanopores are in the dominated position of the pore system and OMs are almost free of organic pores. As to the less mature terrestrial shales, the I/S minerals together with the OMs jointly contribute to the SA and PV due to the large amount of I/S-hosted pores and some existence of organic pores. Finally, the differences among the three sedimentary facies shales in terms of the pore space, suggest that the marine shales in the South China likely possess greater potential for shale gas than do the terrestrial and transitional shales in North China. [ABSTRACT FROM AUTHOR]

Shale reservoirs have different properties and production performance depending on their geologic nature. Study of Precambrian to Jurassic shales in the Sichuan Basin in China indicates that the marine, transitional environment and lacustrine shales have tremendous shale gas and oil potential. Regional shale property mapping, recent drilling and production data, lab tests of 187 samples for geochemistry, mineralogy and petrophysics, and comparison of properties between shales reveal the distinctive reservoir characteristics between different type of shales: the shale of marine origin exampled by widespread Precambrian to Silurian shales has high to over maturity, high TOC, highest brittle mineral content, lowest clay content (<40%), and high gas content; The shale of coal-associated transitional origin exampled by Upper Permian shale has the highest clay content (usually >50%) and high to over maturity; The shale of lacustrine origin exampled by Triassic to Jurassic shales is relatively clay-rich compared to the marine shale but is less clay-rich compared to transitional shale. The maturity of Triassic lacustrine shale is high, while the Jurassic shale is mature in the oil to wet gas window. All of the shales in the Sichuan Basin have porosity less than 9% and the marine shale has more micron-scale to nano-scale organic pores than the transitional and lacustrine shales. The marine shale has the lowest permeability, dominated by nanodarcy range permeability, while the lacustrine shale has the highest permeability, ranging from microdarcy to millidarcy. The marine shale in the Sichuan Basin has the best reservoir and completion quality. [ABSTRACT FROM AUTHOR]

Organic-rich shales, deposited in marine, terrestrial, and marine-terrestrial transitional (hereinafter referred to as transitional) sedimentary environments, were selected to evaluate the nature of organic-associated pores (OAP) and their developmental regularity using broad ion beam milling and scanning electronic microscopy (BIB-SEM). At least seven organic matter (OM) components (alginite, exinite, vitrinite, inertinite, amorphinite otherwise solid bitumen, bitumen, and bioclast) were identified under BIB-SEM based on their in-situ morphology in shale matrix. Besides, OAP were classified into four subtypes based on their differences in genetic mechanism, i.e., hydrocarbon-bubble pores (HBP), hydrocarbon-dissolution pores (HDP), hydrocarbon-shrinkage cracks (HSC), and bioclast-skeleton pores (BSP). HBP formed within OM are generated by the gas-exsolution from the OM surface under the gas-expansion force at the gas window stage, and thereby are widely developed in highly matured marine shales that are rich in structure-alterable liptinite (alginite, amorphinite, exinite, solid bitumen). HDP are essentially the secondary pores in minerals formed by the corrosion of organic-acids before or at the oil window stage and therefore are mostly developed in immature terrestrial shales containing certain amount of organic acids. HSC formed at the OM periphery are caused by the volumetric contraction of OM in case that they cannot generate the HBP within them, and thereby are commonly found in transitional shales which are rich in structure-stable huminite. BSP are the inherited internal structure pores of some bioclasts (planktonic algae or plant tissues), and they occupy only a small part of the total OAP in all of the three kinds of sedimentary facies shales. [ABSTRACT FROM AUTHOR]

The organic-rich shales in the Sichuan Basin in China include Pre-Cambrian Sinian to Middle Permian marine shales deposited in passive margin to foreland settings, Upper Permian transitional shales deposited in a coastal swamp setting, and Triassic and Jurassic lacustrine shales deposited in a foreland setting. Regional shale property mapping, analysis of geochemistry, mineralogy and petrophysics based on sample tests, reservoir characterization of potential shale intervals, and recent exploration and production (E&P) results reveal that: Pre-Cambrian to Middle Permian marine shales, especially the Lower Silurian Longmaxi shale, deposited during transgressive systems tract to early highstand systems tract period have significant reservoir storage, high TOC, high maturity, high brittle mineral content and high gas content, and are similar to the Barnett shale in USA to hold a huge amount of shale gas. The coal-associated Upper Permian transitional shales and Triassic to Jurassic lacustrine shales are relatively clay-rich compared to marine shales, but possess some shale gas potential in organic-rich shales. The shale gas resource potentials and emerging production have been confirmed by the reservoir characteristics and test results of recent drilled wells targeting marine, transitional and lacustrine shale gas in the Sichuan Basin. [ABSTRACT FROM AUTHOR]

Organic-rich shales spanning in age from Pre-Cambrian to Quaternary were widely deposited in China. This paper elaborates the geology and unique characteristics of emerging and potential hydrocarbon-bearing shales in China. The Pre-Cambrian Sinian Doushantuo to Silurian black marine shales in the intra-shelf low to slope environments were accumulated in South China and Tarim Platform in Northwest China. These marine shales with maturity (Ro) of 1.3–5% are in dry gas window. During Carboniferous to Permian, the shales associated with coal and sandstones were mainly deposited in coastal swamp transitional setting in north China, NE China, NW China and Yangtze platform in South China. These transitional shales are generally clay rich and are potential gas-bearing reservoirs. Since Middle Permian, the lacustrine shales with total carbon content (TOC) up to 30% and Ro mainly in oil window are widely distributed in all the producing basins in China. The lacustrine shales usually have more clay mineral content than marine shales and are characterized by rapid facies change and are interbedded with carbonates and sandstone. The high quality shale reservoir with high TOC, hydrocarbon content and brittle minerals content is usually located at transgressive systems tract (TST) to early highstand systems tract (EHST) interval deposited in anoxic depositional setting. Recent commercial shale gas production from the Silurian Longmaxi marine shale in the southeastern Sichuan Basin, preliminary tight oil production associated with lacustrine hydrocarbon-bearing shale intervals and hydrocarbon shows from many other shales have proven the hydrocarbon-bearing shales in China are emerging and potential shale gas and tight (shale) oil plays. Tectonic movements could have breached the early hydrocarbon accumulation in shales and tectonically stable areas are suggested to be favorable prospects for China shale plays exploration and production. [ABSTRACT FROM AUTHOR]

With the purpose of characterizing the organic geochemistry and investigating the shale gas potential, we applied multiple methods, including quantifying organic matter richness, polarizing microscope observations, maceral composition analysis, kerogen stable carbon isotope analysis, vitrinite reflectance analysis, and Rock-Eval pyrolysis were performed on black shale samples from the Lower Permian Shanxi and Taiyuan formations in the Mouye-1 well of the Southern North China Basin. The results indicate that the average total organic carbon (TOC pd ) content at the present time is 1.73% and 2.41% for Shanxi and Taiyuan shale samples, respectively. The microscopic distribution of organic matter includes scattered types and local enrichment types, as well as layered enrichment types. These types were revealed by polarizing microscope observations and exhibit an excellent correlation with the TOC content of shale samples. Based on maceral compositions and kerogen stable carbon isotopes, organic matter in Shanxi and Taiyuan shales is characterized by gas-prone, inertinite-dominated type III kerogen. The thermal maturity, as indicated by measuring vitrinite reflectance (3.2–3.6%R o ), suggests that shale samples from Shanxi and Taiyuan formation in the Mouye-1 well have evolved far into the metagenesis stage, and no significant amounts of hydrocarbons can be generated. The remaining hydrocarbon generative potential, S2 (0.02–0.77 mg HC/g Rock), which was determined by Rock-Eval pyrolysis, also supports this conclusion. Additionally, the original total organic carbon (TOC o ) content and hydrocarbon generative potential (S2 o ) were reconstructed based on Jarvie's equations for these thermally over-mature shales; this reconstruction indicated poor to fair original source rock potentials based on the correlations of TOC o and S2 o . Furthermore, the total volume of gas generated during thermal maturation was calculated using a conversion formula based on molar mass and resulted in a yield of 5.69 cm 3 /g Rock and 3.54 cm 3 /g Rock for Shanxi and Taiyuan shale samples, respectively. Overall, inertinite-dominated maceral compositions of kerogen with extremely high thermal maturity can have a negative effect on the gas potential of shale in the Southern North China Basin. [ABSTRACT FROM AUTHOR]

Whether exploration practices of marine shale can be applied to transitional shale remain unclear. To address this issue, it is first necessary to clarify similarities and differences in dominant pore types between the two shales. In this paper, samples were selected from Lower Silurian marine shale and Upper Permian transitional shale, in China. Methods utilized include organic geochemistry, X-ray diffraction, physical property tests and scanning electron microscopy. Results showed that marine shale and transitional shale were similar in OM abundances and thermal maturities, but were different in OM types, mineral components and pore properties. Further, facilitated by image statistics, OM-hosted pores (OMPs) and mineral-hosted pores (MPs) were separated and quantified between the two shales. In particular, more than 90% of pores were OMPs in OM-rich/medium lithofacies of marine shale, whereas above 90% of pores were supplied by MPs in transitional shale. Initially, differences in OM origins and OMP genetic characteristics may trigger the porosity differences. Thermal-genetic OMP, mainly developing in marine shale, features wide distributions, complex surfaces, various sizes, and extensive contributions to pore volume (PV)/pore surface area (PSA). Inheritance OMP, mainly developing in transitional shales, features isolated distributions, smooth surfaces, large diameters, and limited contributions to PV/PSA. Likewise, differences in mineral compositions and MP preservation mechanisms exert influence on the porosity differences. In marine shale, MP is rare and contributes little to PV/PSA due to fierce compaction, cementation and OM occupation. In transitional shale, MP (almost associated with clay flakes) can be protected from cementation and OM occupation, and contributes considerable PV but minimal PSA. It implies that different pore types may impact the reservoir characteristics. Consequently, the exploration targets in marine shales should be OM-rich siliceous lithofacies, whereas targets in transitional shale should be mainly silt-clay lithofacies. • Differences of dominant pore types and their impacts on reservoirs were carried out between marine and transitional shales. • OM-hosted pores and mineral-hosted pores were separated and quantified by image statistics. • Different exploration strategies were conducted to counter the particular controlling issues of each type of shale. [ABSTRACT FROM AUTHOR]

The marine-continental transitional shale in the upper Permian Longtan Formation is not only regarded as an excellent source rock, but also one of the crucial layers of shale gas exploration in the Lower Yangtze region, South China. However, controlling factors and mechanisms of transitional organic matter (OM) accumulation in such a setting are not well understood. In this study, the characteristics of total organic carbon (TOC) and elemental geochemistry of 22 rock samples collected from one core and two different types of outcrops are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that shales developed in a tidal flat-lagoon environment display high TOC contents ranging from 6.16% to 10.10% (average 8.50%), whereas these formed in a deltaic environment exhibit medium-high TOC contents varying from 0.93% to 6.70% (average 3.26%). These data also elucidate that the target shales in the Longtan Formation were deposited in a complex paleoenvironment with strong water-mass restriction that was mainly characterized by warm and humid paleoclimate, high primary productivity, oxic-to-dysoxic conditions, and a high sedimentary rate. Besides, the transitional OM accumulation is not determined by a single factor, but is the result of the mutual configuration and coupling of multiple factors such as paleoclimate, paleoproductivity, paleoredox, and sedimentary rate. All these factors will directly or indirectly affect OM supply or preservation. Finally, the "integrated model" for transitional OM accumulation is proposed. The model stresses two aspects: on one hand, the warm and humid paleoclimate not only facilitates the growth of higher plants, but also accelerates the chemical weathering rate of the parent rocks and increases the input of nutrients to the water column, which is conducive to the blooms of lower aquatic organisms. Both higher plant debris and lower aquatic organisms together provide abundant OM sources. On the other hand, although the oxidized water environment is usually unfavorable for OM preservation, a higher sedimentary rate can greatly shorten OM exposure time in the decomposition region of aerobic bacteria, and a number of organic matters cannot be oxidized or degraded through the rapid burial. These findings also add to our knowledge that despite the oxygenated water environment during transitional shale deposition, TOC contents are not necessarily lower. • The transitional shales are geochemically characterized. • The paleoenvironmental conditions are comprehensively analyzed. • The "integrated model" for transitional OM accumulation is proposed. [ABSTRACT FROM AUTHOR]

Marine-continental transitional shale gas is an important resourceinaddition to shale gas in marine facies. Transitional facies strata are rich in type III organic matter and shales with obvious lamellation, but there has been no "breakthrough" for shale gas. The main reason for this may be that the sedimentary environment and characteristics of favorable shale reservoirs differ from those of marine shale. It is imperativeto understand the environment and reservoir characteristics to advance the exploration of transitional shale gas.Typical lower Permian transitional shale has been collected continuously in an exploratory well in the Southern North China Basin. A complete set of experimental procedures, including elemental geochemical methods (XRF and ICP-MS) and petrophysical techniques (N 2 /CO 2 adsorption, MIP, NMR and FE-SEM), were used. The results indicate that, in addition to the contribution of mesopores (2–50 nm), transitional shale in the SNCB has a higher macropore (>50 nm) volume (average = 0.00616 cm3/g) and proportion (average45.85%) than in marine shale. The precipitation and dissolution of dispersed pyrite in the Shanxi Formation formed more macropores (average = 0.00649 cm3/g) than pyrite and calcite aggregates (filled with bitumen and clays) in the Taiyuan Formation. Total organic carbon (TOC) has no obvious relationship with micropores and mesopores, but has a weak positive correlation with macropores, unlike marine shale. Sulfur enrichment in the presence of organic matter mixed with terrigenous iron causes dispersed pyrite (FeS) precipitation under anoxic conditions, which improves the reservoir (intergranular and dissolution pores). With decreasing sea levels, the salinity of water decreases gradually, while weathering increases gradually; however, the shale reservoir conditions do not simply improve or deteriorate. Favorable shale reservoirs are mainly lagoonal facies characterized by a low SiO 2 content (<58 wt%), high TOC (>2.0%), intermediateV/Cr ratios (1.5–2), and a high chemical index of alternation (CIA; >88) and formed in an environment with little clastic input, hypoxia and strong weathering.However, due to the strong connectivity, high proportions of macropores and thin single-layers, it is necessary to study the preservation capacity in future to determine whether the natural gas formed in shale is lost. • Macropore volume and movable fluid ratio were substantial in transitional shales. • Dispersed pyrite facilitated macropores and microcracks. • The favorable shale reservoirs mainly occurred in the lagoonal environment. • The vertical evolution of environment and reservoir was discussed. [ABSTRACT FROM AUTHOR]

Sea level changes more frequently during the deposition of transitional shale than marine shale, resulting in the strong heterogeneity and instability of the sedimentary environment and making transitional shale gas evaluation difficult. The appropriate identification and classification of transitional shale lithofacies types can improve evaluation of shale reservoirs and can provide the geological basis for the evaluation and potential of shale gas reservoirs during exploration and development. The lower Permian shale in the northeastern Ordos Basin is selected as the focus of this research. Based on X-ray diffraction (XRD), total organic content (TOC), vitrinite reflectance (R o), nitrogen and carbon dioxide adsorption, and argon ion polishing scanning electron microscopy (AIP−SEM) data, the lower Permian transitional shale in the study area is divided into silty mudstone and muddy siltstone lithofacies. On this basis, the differences in the organic geochemical characteristics, pore development characteristics and influencing factors of the shale reservoir in the two lithofacies are compared. The results show that the mean values of w (TOC) and w (R o) in the muddy siltstone lithofacies are 1.9% and 1.0%, respectively. Siliceous minerals dominate the composition of the rock, and the main types of reservoir spaces are mineral matrix pores, including interparticle (interP) pores and intraparticle (intraP) pores related to inorganic minerals. Macropores and mesopores are the main components of the pore volume, and account for 40.8% and 49.8% of the total volume, respectively. The pore structure is mainly affected by the TOC and siliceous mineral contents. In the silty mudstone lithofacies, the average values of w (TOC) and w (R o) are 2.4% and 1.2%, respectively. The clay mineral content is relatively high. The main types of reservoir spaces are organic pores and interP pores. Additionally, the pore volumes of macropores, mesopores and micropores are almost the same, accounting for 24.9%, 44.6% and 30.4% of the total pore volume, respectively. The pore structure is mainly affected by the organic matter content and clay mineral content. • Lithofacies classification is an effective method to evaluate the reservoir development characteristics of the transitional shale. • The effect of organic carbon content and inorganic mineral composition on pore volume of different lithofacies shale is various. • Although the reservoir capabilities of transitional shale are considerable, the recoverable reserves are still lower than marine shale. [ABSTRACT FROM AUTHOR]

Nano-scale pore structure and fractal characteristics are significant information to understand shale gas reservoirs. Therefore, seventeen marine-continental transitional shale samples of Leping Formation in Qingjiang Basin, southeast China were investigated to study the potential relationship between pore structure and fractal characteristics. In this work, a dual-pore-shape method was firstly applied to construct pore structure in the shapes of cylinder and slit, which is closer to reality. Meanwhile, non-local density function theory method and Barrett-Joyner-Halenda method were employed for comparison as well. Fractal dimensions obtained from nitrogen isotherms via Frenkel-Halsey-Hill method were divided into two parts, one characterizes larger pores and the other characterizes smaller pores. Geochemical index, pore structure information, and fractal characteristics showed different correlated behaviors with each other. Total organic carbon content strongly affects pore structure of transitional shale, but clay and quartz content have no clear influence. Correlations between fractal dimensions and geochemical parameters were affected by the degree of isotropy. Pore parameters have conspicuous relationships with the fractal dimension of larger pores but not with that of smaller pores. It is found that slit pores play a significant role in pore space in shale matrix. Moreover, the connections between different shapes of pores and fractal dimensions are displayed. Pore volume of cylindrical pores positively correlates with fractal dimension of larger pores, whereas pore volume of slit pores negatively correlates with fractal dimension of smaller pores. The comparison of all pore parameters obtained by different methods shows that dual-pore-shape method could better construct real pore structure and obtain the pore volume distributions more accurately. • Dual-pore-shape method was firstly applied to characterize pore structure of shale. • Besides cylindrical pores, slit pores play an important role in nano-pore space. • Smaller pores have stronger correlation between volumes and fractal dimensions. • Fractal dimensions of larger and smaller pores have connections with pore shape. • Pore volumes and specific surface areas were compared by using three methods. [ABSTRACT FROM AUTHOR]