Marine-continental Transitional Shale Research Articles

Influence of Sedimentary Environments on Organic Matter Enrichment in Marine-Continental Transitional Shale: A Case Study of the Upper Permian Longtan Formation in the Central Hunan Depression, China

Influence of Sedimentary Environments on Organic Matter Enrichment in Marine-Continental Transitional Shale: A Case Study of the Upper Permian Longtan Formation in the Central Hunan Depression, China

Differential characteristics of lithological assemblages and gas-bearing of the Permian Longtan Formation mudstone in well L3, southeastern Sichuan Basin

As the main reservoir of coalbed gas in southeastern Sichuan, the mudstone of the Permian Longtan Formation has been drilled to obtain industrial gas, but the level of exploration and development is low. The researches on the types of lithological assemblages, reservoir characteristics, and gas-bearing properties are poor, which limits the evaluation and selection of the sweet point area for the marine-continental transitional shale gas. In this paper, by comparing the differential of different lithological distribution in the well L3, multiple discriminant functions and logging interpretation models for different lithology are established to determine the classification criteria of lithological assemblage types of shale formations. Based on the experimental results of high-temperature and high-pressure isothermal adsorption, the reservoir space distribution and gas-bearing characteristics of mudstone in different lithological assemblages are compared and analyzed. It is indicated that the four lithological assemblage types are found in the Permian Longtan Formation, including thick mudstone with the interlayer of coalbed (Type I), rich mudstone with the interlayer of sandstone and thin coalbed (Type II), sandstone interbedded with mudstone with the interlayer of coalbed (Type III), and limestone interbedded with sandstone with the interlayer of mudstone (Type IV), which are superimposed with each other. The different pore structure characteristics of mudstone in different lithological assemblages is the main influencing factor of differential gas-bearing property. The dominant lithological assemblages are Type I and Type II. Coalbed and carbonaceous mudstone are the source rock and primary storage space of adsorbed gas. Moreover, with low porosity and permeability, high breakthrough pressure and the strong sealing capacity of regional mudstone, it is easy to form the “microtrap” to store the natural gas. The sealing capacity of mudstone provides a favorable condition for gas preserve. Under the dynamic condition of hydrocarbon generation, the pressure storage box is formed, accompanied with the fine reservoir spaces and gas-bearing contents.

The sedimentary environment and main controlling factors of organic matter enrichment of Carboniferous-Permian marine-continental transitional shale in Kaiping syncline

The shale from the Carboniferous and Permian Taiyuan–Shanxi Formation in Kaiping syncline has good prospects for shale gas exploration and development. To understand the evolutionary characteristics of the sedimentary environment in this area and their relationships with organic matter enrichment. Shale samples from the Taiyuan-Shanxi in Well KP1 were selected as research objects, followed by organic geochemical research, elemental geochemical analysis, and numerical analysis. The results show that the sedimentary period of the Shanxi Formation was marked by a medium productivity level. The sedimentary water column was composed of brackish water and seawater deposits in an anoxic reduction environment. The Taiyuan Formation shale was characterized by a medium level of paleoproductivity. The sedimentary water column was composed of brackish water deposits in an anoxic reduction environment. According to the linear correlation analysis of paleoenvironmental conditions and TOC content. This study suggests that ancient water salinity is the main factor controlling the enrichment of organic matter in the sedimentary stage of the Taiyuan-Shanxi Formation.

Weathering-induced organic matter enrichment in marine-continental transitional shale: A case study on the early Permian Taiyuan Formation in the Ordos Basin, China

A comparative analysis of the factors controlling organic matter (OM) enrichment between marine-continental transitional (transitional hereafter) and marine or lacustrine shales is lacking. The early Permian Taiyuan Formation in the Ordos Basin, deposited during a shift from marine to continental settings in northern China, provides a unique opportunity to unravel the differences in OM enrichment mechanisms among these shales. The Taiyuan Formation is characterized by high TOC content (average 4.50%) and kerogen type II2-III. Most samples are thermally mature with a few high to post-mature samples relating to the Late Jurassic–Early Cretaceous Yanshanian magmatism. Rare earth elements and yttrium (REY) are dominated by light- and medium-types enrichments, with distinctly positive Gd anomaly, likely due to seawater incursion. A warm and humid climate prevailed during deposition of the Taiyuan Formation, consistent with the tropical-subtropical location of the North China Plate in the early Permian. The climatic conditions promoted intense continental weathering as reflected by high Th/Sc ratios, chemical index of alteration values, and feldspar alteration to scaly kaolinite. The V/(V + Ni) ratio is inconsistent with the other redox proxies, presumably due to variations in the redox buffer supply in the transitional facies (e.g., OM and pyrite), varying burial rates and dissimilar redox potential of different elements. Hence, this proxy should be interpreted with caution in such settings. Most redox proxies indicate oxic bottom water during deposition of the Taiyuan Formation transitional shale, in contrast to typical OM enriched marine and lacustrine shales where redox stratification or euxinic conditions are common. Instead, the dominant factor for OM enrichment in transitional shales appears to have been a high influx of terrestrial weathering products, including abundant higher-plants OM, associated with preservation of OM due to rapid burial. This process minimizes the detrimental effects of oxic conditions on OM accumulation in the transitional shale facies. This mechanism may hold relevance for analogous basins elsewhere.

Pore Structure and Heterogeneity Characteristics of Coal-Bearing Marine–Continental Transitional Shales from the Longtan Formation in the South Sichuan Basin, China

Marine–continental transitional shale has become a new field for shale gas exploration and development in recent years. Its reservoir characteristics analysis lags significantly behind that of marine shale, which restricts the theoretical research on the accumulation of marine–continental transitional shale and the progress of exploration and development. The shale pore system is complex and has strong heterogeneity, which restricts the fine evaluation and optimization of the reservoir. Based on nitrogen adsorption–desorption experiments, the morphology and structural characteristics of coal-bearing shale pores were analyzed, and then the micro-pore structure heterogeneity was quantitatively characterized based on fractal theory and nitrogen adsorption–desorption data, and the relationship between pore structure parameters and their influence on fractal characteristics were discussed. The hysteresis loop of nitrogen desorption isotherm mainly belongs to type B, indicating ink bottle, flat plate, and slit are the main pore shapes. The pore size distribution curves are left unimodal or multimodal, with the main peak around 4 nm and 20–60 nm. Smaller pores develop a larger specific surface area, resulting in a high value of fractal dimension (2.564 to 2.677). The rougher the pore surface and the larger the specific surface area provide an adequate adsorption site for shale gas adsorption and aggregation. Thus, fractal characteristics conduced to understand the pore structure, heterogeneity, and gas-bearing property of coal-bearing shale.

Controlling Factors of Organic Matter Enrichment in Marine–Continental Transitional Shale: A Case Study of the Upper Permian Longtan Formation, Northern Guizhou, China

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.

Pore Fractal Characteristics between Marine and Marine–Continental Transitional Black Shales: A Case Study of Niutitang Formation and Longtan Formation

Marine shales from the Niutitang Formation and marine–continental transitional shales from the Longtan Formation are two sets of extremely important hydrocarbon source rocks in South China. In order to quantitatively compare the pore complexity characteristics between marine and marine–continental transitional shales, the shale and kerogen of the Niutitang Formation and the Longtan Formation are taken as our research subjects. Based on organic petrology, geochemistry, and low-temperature gas adsorption analyses, the fractal dimension of their pores is calculated by the Frenkel–Halsey–Hill (FHH) and Sierpinski models, and the influences of total organic carbon (TOC), vitrinite reflectance (Ro), and mineral composition on the pore fractals of the shale and kerogen are discussed. Our results show the following: (1) Marine shale predominantly has wedge-shaped and slit pores, while marine–continental transitional shale has inkpot-shaped and slit pores. (2) Cylindrical pores are common in organic matter of both shale types, with marine shale having a greater gas storage space (CRV) from organic matter pores, while marine–continental transitional shale relies more on inorganic pores, especially interlayer clay mineral pores, for gas storage due to their large specific surface area and high adsorption capacity (CRA). (3) The fractal characteristics of marine and marine–continental transitional shale pores are influenced differently. In marine shale, TOC positively correlates with fractal dimensions, while in marine–continental shale, Ro and clay minerals have a stronger influence. Ro is the primary factor affecting organic matter pore complexity. (4) Our two pore fractal models show that the complexity of the shale in the Longtan Formation surpasses that of the shale in the Niutitang Formation, and type I kerogen has more complex organic matter pores than type III, aiding in evaluating pore connectivity and flow effectiveness in shale reservoirs.

Comparative Study on Flow Characteristics in Deep Marine and Marine-Continental Transitional Shale in the Southeastern Sichuan Basin, China.

Permeability is a significant characteristic of porous media and a crucial parameter for shale gas development. This study focuses on deep marine and marine-continental transitional shale in the southeastern Sichuan area using the gas pulse decay testing method to systematically analyze the gas permeability, stress sensitivity, and gas transport mechanisms of shale under different pressure conditions and directions. The results show that the porosity and gas permeability of the deep marine shale are greater compared to those of the marine-continental transitional shale. The elevated fluid pressure in the deep marine shale offers superior conditions for the preservation of nanopores, while the high quartz content provides advantageous conditions for fluid transport in nanopore channels. The permeability and stress sensitivity of the deep marine shale are greater than those of the marine-continental transitional shale, and the stress sensitivity is greater in the perpendicular bedding direction than in the parallel bedding direction, possibly related to the mineral composition of shale and the compaction it has undergone. The flow mechanism of the deep marine shale is transition flow and Knudsen flow, while that of the marine-continental transitional shale is transition flow. The deep marine shale possesses smaller nanopore sizes and a higher quantity of micropores, which create advantageous conditions for gas transport within nanopores. During the process of extracting shale gas, the extraction of gas causes a decrease in pore pressure and an increase in effective stress, resulting in a reduction in permeability. However, when the pore pressure reaches a specific value, the enhanced slippage effect leads to an increase in permeability, which is advantageous for gas extraction. In the later stage of shale gas well production, intermittent production plans can be developed considering the strength of the slippage effect, leading to a significant improvement in production efficiency.

Karst topography paces the deposition of lower Permian, organic-rich, marine–continental transitional shales in the southeastern Ordos Basin, northwestern China

Karst topography paces the deposition of lower Permian, organic-rich, marine–continental transitional shales in the southeastern Ordos Basin, northwestern China

Modeling of Burial History, Source Rock Maturity, and Hydrocarbon Generation of Marine-Continental Transitional Shale of the Permian Shanxi Formation, Southeastern Ordos Basin.

The Ordos Basin is characterized by abundant natural gas resources, and the marine-continental transitional shale gas of the Permian Shanxi Formation has great exploration and development potential. However, few systematic studies have focused on the burial history, thermal maturity, and hydrocarbon generation of the shale, which limits the understanding of shale gas enrichment and resource evaluation. To reveal the shale gas resource potential, we focused on the Shanxi Formation shale in the southeastern Ordos Basin. Net erosion was estimated, and then one-dimensional (1D) and three-dimensional (3D) geological models were constructed using PetroMod to simulate the burial-thermal history and hydrocarbons generated in the Shanxi Formation shale, and finally, the gas generation intensity was evaluated. The results show that four periods of uplift and erosion events have occurred in the study area since the Mesozoic, of which the erosion in the Late Cretaceous was the most severe. The burial center gradually shifted from east to northwest in the study area, and the basin reached the maximum burial depth in the Late Cretaceous and then gradually changed to a monoclinal tilted east to west after uplift and erosion. The Shanxi Formation shale reached the hydrocarbon generation threshold at 233 Ma (Ro = 0.5%), reached the oil generation peak at 200 Ma (Ro = 1.0%), and entered the high maturity stage rapidly (Ro = 1.3%). Currently, the average maturity is approximately 2.48%, which is in the overmature stage. The center of shale maturity was in the southern part of the study area before the Late Jurassic and shifted northeast in the late Early Cretaceous. Cumulative gas generated to date is 44.0 × 1012 m3, and the center of gas generation was in the middle-eastern region of the study area before the Early-Middle Jurassic and shifted northwest in the Early Cretaceous. This study provides a theoretical basis and guidance for the exploration and development of marine-continental transitional shale in the Ordos Basin.

Study on Sedimentary Environment and Organic Matter Enrichment Model of Carboniferous–Permian Marine–Continental Transitional Shale in Northern Margin of North China Basin

The shales of the Taiyuan Formation and Shanxi Formation in the North China Basin have good prospects for shale gas exploration and development. In this study, Well KP1 at the northern margin of the North China Basin was used as the research object for rock mineral, organic geochemical, and elemental geochemical analyses. The results show that brittle minerals in the shales of the Taiyuan Formation and Shanxi Formation are relatively rare (<40%) and that the clay mineral content is high (>50%). The average TOC content is 3.68%. The organic matter is mainly mixed and sapropelic. The source rocks of the Taiyuan Formation and Shanxi Formation are mainly felsic, and the tectonic background lies in the continental island arc area. The primary variables that influenced the enrichment of organic materials during the sedimentary stage of the Taiyuan Formation were paleosalinity and paleoproductivity. Paleosalinity acted as the primary regulator of organic matter enrichment during the sedimentary stage of the Shanxi Formation.

Investigations of methane adsorption characteristics on marine-continental transitional shales and gas storage capacity models considering pore evolution

Methane adsorption is a critical assessment of the gas storage capacity (GSC) of shales with geological conditions. Although the related research of marine shales has been well-illustrated, the methane adsorption of marine-continental transitional (MCT) shales is still ambiguous. In this study, a method of combining experimental data with analytical models was used to investigate the methane adsorption characteristics and GSC of MCT shales collected from the Qinshui Basin, China. The Ono-Kondo model was used to fit the adsorption data to obtain the adsorption parameters. Subsequently, the geological model of GSC based on pore evolution was constructed using a representative shale sample with a total organic carbon (TOC) content of 1.71%, and the effects of reservoir pressure coefficient and water saturation on GSC were explored. In experimental results, compared to the composition of the MCT shale, the pore structure dominates the methane adsorption, and meanwhile, the maturity mainly governs the pore structure. Besides, maturity in the middle-eastern region of the Qinshui Basin shows a strong positive correlation with burial depth. The two parameters, micropore pore volume and non-micropore surface area, induce a good fit for the adsorption capacity data of the shale. In simulation results, the depth, pressure coefficient, and water saturation of the shale all affect the GSC. It demonstrates a promising shale gas potential of the MCT shale in a deeper block, especially with low water saturation. Specifically, the economic feasibility of shale gas could be a major consideration for the shale with a depth of <800 m and/or water saturation >60% in the Yushe-Wuxiang area. This study provides a valuable reference for the reservoir evaluation and favorable block search of MCT shale gas.

Geochemical Constraints and Astronomical Forcing on Organic Matter Accumulation of Marine–Continental Transitional Shale Deposits in the Qinshui Basin during the Carboniferous–Permian Transition

Geochemical Constraints and Astronomical Forcing on Organic Matter Accumulation of Marine–Continental Transitional Shale Deposits in the Qinshui Basin during the Carboniferous–Permian Transition

Pore system and methane adsorption capacity features of marine and marine-continental transitional shale in the Sichuan Basin, SW China

Recently, significant achievements have been made in the gas exploration of marine Longmaxi shale in China. As exploration efforts have advanced, the exploration targets have gradually expanded to other sedimentary systems (marine-continental transitional and lacustrine). Compared with marine shale, shale in other sedimentary systems shows stronger heterogeneity, rendering previous exploration experiences of marine shale ineffective in guiding exploration efforts. Therefore, there is a pressing need for comparative studies to support future exploration practices. In this paper, the marine Longmaxi Formation and the marine-continental transitional Longtan Formation shales in the Lintanchang area of the southeastern of the Sichuan Basin are selected as the research objects. The study aims to compare the mineralogical characteristics, pore systems, and methane adsorption capacities of these two sets of shales, thereby revealing the differences in controlling factors that affect their physical properties and methane adsorption capacities. Our results show that the Longtan shale exhibits a higher clay mineral content, while the Longmaxi shale demonstrates significantly higher siliceous mineral content. Compare with Longmaxi shale, the Longtan shale exhibits a wider distribution range and higher average value of TOC content. The pore system in the Longmaxi shale is primarily dominated by organic matter-related pores, whereas the Longtan shale is characterized by clay mineral-related pores as the primary pore type. Given the variance in sedimentary environments, the controlling factors of physical properties differ significantly between the two sets of shales. In the case of the Longmaxi shale, TOC content is the most influential factor governing physical properties, while clay mineral content exerts the most significant influence on physical properties in the Longtan shale. Furthermore, TOC content emerges as the primary factor affecting methane adsorption capacity in both the Longmaxi and Longtan shales, despite the presence of significant variations in their pore systems. Nevertheless, the specific mechanisms through which TOC content impacts methane adsorption capacity exhibit variations between the two distinct shale types under investigation. The difference in sedimentary environment leads to various effects of mineral composition on methane adsorption capacity. Therefore, in the future research, the influences of different factors on methane adsorption capacity should be studied in combination with the sedimentary background.

Pore Development Characteristics of Marine–Continental Transitional Shale of Benxi Formation from the Ordos Basin, China

Pore Development Characteristics of Marine–Continental Transitional Shale of Benxi Formation from the Ordos Basin, China

Sedimentology and geochemistry of the Upper Permian transitional shale, Western Guizhou, China: Implications for the environmental effect of the Emeishan large igneous province and organic matter accumulation

The marine-continental transitional shale of the Upper Permian Longtan Formation serves as a crucial target for both shale gas exploration and development, as well as a significant research object of the dramatic geological events during the Middle-Late Permian period. In this study, X-ray diffraction (XRD), polished section, scanning electron microscopy (SEM), energy spectrum analysis (EDS), total organic carbon content (TOC) and elemental geochemistry were conducted to elucidate the sedimentological and geochemical characteristics of the transitional shale in the Longtan Formation and the underlying Emeishan basalt in Western Guizhou. Additionally, the paleoenvironment conditions, the environmental effect of the Emeishan large igneous province and the organic matter enrichment mechanism of the transitional shale are investigated.The sedimentological and geochemical proxies reveal that influenced by seawater and river, the transitional shale of the Longtan Formation in western Guizhou was deposited in a marginal tidal flat-lagoon environment under a warm and humid paleoclimate, anoxic condition, brackish to saline bottom water, high terrigenous detrital input and high sedimentation rate condition. The mineral and elemental composition characteristics show that the transitional shale was predominantly derived from Emeishan basalt, supplemented by contributions from volcanic ash and hydrothermal activity. This suggests that following the eruption of the Emeishan large igneous province in Middle-Late Permian transition, the paleogeographic pattern of high in the west and low in the east was formed in the Upper Yangtze region. The weathering products of the Emeishan basalt were the main source of the Longtan Formation. Intense volcanic activities in the earliest Wuchiapingian potentially induced cooling of the climate, and the elements carried by hydrothermal input likely enhanced paleoproductivity and increased seawater salinity, and was conducive to the suboxic-anoxic water condition.The organic matter contents in transitional shale are extremely high with the highest organic matter content (>20%) of the shale deposited in mire, followed by the shale deposited in tidal flat (10%), whereas the lagoonal shale, predominant in the Longtan Formation, displays the lowest organic matter content (5%). The organic matter enrichment mechanism of transitional shale is primarily attributed to a “preservation model”. The suboxic-anoxic condition and brackish-saline water were conducive to the organic matter preservation. The primary productivity was low and organic matter mainly came from terrestrial higher plants. The warm and humid climate strengthened terrestrial weathering and erosion, consequently increasing terrigenous organic matter input and sedimentation rate. Elevated sedimentation rate reduced the decomposition time of organic matter, thereby also facilitating organic matter preservation. However, the correlations between TOC content and geochemical proxies in lagoonal shale suggest that the warmer and more humid paleoclimate may have increased terrigenous detrital influx and diluted the organic matter content.

Evolution Mechanism of Microscopic Pore System in Coal-Bearing Marine–Continental Transitional Shale with Increasing Maturation

The formation and evolution mechanisms of complex types and scales of marine–continental transitional shale pores are still indefinite, restricting the accurate evaluation of shale reservoir and the effective evaluation of coal-bearing marine–continental transitional shale gas resource quantity. Considering the Shanxi shale in Ordos basin of China as the research object, combining the FE-SEM images and petrophysical analysis, high-pressure mercury intrusion porosimetry, and CO2 and N2 adsorption–desorption experiments, the structure characteristics and differential evolution mechanisms of multiscale and multitype of coal-bearing shale pores were discussed. The results show that coal-bearing marine–continental transitional shales are rich in clay minerals and organic matters (OMs). Pores developed within organic matters, clay, and brittle minerals of coal-bearing shale have decreasing porosity values. OM pores are directly related to micro- and mesopores, with high specific surface areas, while the porosity of inorganic pores increases with the increasing pore diameter. The porosity of all pores shows a positive relationship with permeability, which changes periodically with the increase in maturity. Coal-bearing shale pores are mainly plate- and ink bottle-shaped, with multimodal pore size distributions. Controlled by both diagenesis and hydrocarbon generation, the evolution of coal-bearing shale pores could be mainly divided into four stages. Furthermore, the pore evolution model of coal-bearing marine–continental transitional shale was preliminarily constructed. This study would enhance the understanding of reservoir evolution of the coal-bearing shale and provide useful information for the assessment and evaluation of reservoir capacity.

Diagenetic Evolution Sequence and Pore Evolution Characteristics: Study on Marine-Continental Transitional Facies Shale in Southeastern Sichuan Basin

Diagenesis and pore structure are essential factors for reservoir evaluation. marine-continental transitional facies shale is a new shale gas reservoir of concern in the Sichuan Basin. The research on its diagenesis pore evolution model has important guiding significance in its later exploration and development. However, the current research on pore structure changes, diagenesis, and the evolution of marine-continental transitional facies shale is not sufficient and systematic. In order to reveal the internal relationship between pore structure changes and diagenesis, the evolution of marine-continental transitional facies shale was tested by X-ray diffraction, field emission scanning electron microscopy, low-pressure gas adsorption, nuclear magnetic resonance, and the diagenetic evolution sequence and nanopore system evolution of Longtan Formation shale was systematically studied. The results show that the Longtan Formation shale underwent short-term shallow after sedimentation, followed by long-term deep burial. The main diagenetic mechanisms of the Longtan Formation shale include compaction, dissolution, cementation, thermal maturation of organic matter, and transformation of clay minerals, which are generally in the middle-late diagenetic stage. The pore structure undergoes significant changes with increasing maturity, with the pore volumes of both micropores and mesopores reaching their minimum values at Ro = 1.43% and subsequently increasing. The change process of a specific surface area is similar to that of pore volumes. Finally, the diagenetic pore evolution model of Longtan Formation MCFS in Southeastern Sichuan was established.

Paleoenvironment of marine-continental transitional shales in the lower Permian Shanxi formation, southeastern Ordos Basin, China

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. Hf–Co, and ∑REE 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 Th–Co–Zr/10, Th–Sc–Zr/10, La–Th–Sc, 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).

Factors Controlling the Pore Development of Low-Mature Marine–Continental Transitional Shale: A Case Study of the Upper Permian Longtan Shale, Western Guizhou, South China

The Upper Permian Longtan Shale is a significant reservoir in western Guizhou. To clarify the main factors controlling the low-mature marine–continental transitional shale pore development in western Guizhou, pore types were classified with scanning electron microscopy (SEM), and the pore developmental stages and morphological structures were quantitatively characterized by nitrogen adsorption isotherm analyses. Additionally, the qualitative or semi-quantitative relationships between the pore developmental stages and the main controlling factors were established via geochemical analysis. The results showed that the Longtan Shale pores include intergranular pores, intragranular pores, organic pores, and microfractures. The intergranular pore structures were categorized into ink-bottle, slit, layered, and irregular types. The intragranular pores were found to be of the elliptical, nearly circular, ink-bottle, and irregular varieties. The organic pores were categorized into elliptical, bubble-like, and irregular polygonal variants. The microfractures were only of the elongated type. The clay-mineral-related intergranular pores were the predominant pore type. The organic pores were found to be poorly developed. The mesopores were predominant, followed by macropores. The shale pore diameters ranged between 1 nm and 100 nm, and they are characterized by multiple peaks. The specific surface area (SSA) was primarily provided by nanopores in the range of 5 nm to 10 nm, such that the smaller pores provided a greater contribution to the SSA, and they are more conducive to shale gas adsorption and accumulation. Clay mineral content was the dominant internal factor controlling pore development and the SSA, with the illite–smectite mixed layer being the most obvious controlling factor. While too low or too high clay mineral content is adverse to macropore development, brittle mineral content, carbonate mineral content, and total organic carbon (TOC) content are adverse to pore development and the SSA. Thermal maturity has no remarkable control effect on pore volume and the SSA of non-organic pores.