Dalong Formation Research Articles

Late Permian radiolarian Neoalbaillella caridroiti assemblage from the Loufanggou, Guangyuan-Shangsi area, Sichuan Province, China

A well-preserved Lopingian (Late Permian) radiolarian assemblage including Neoalbaillella has been recovered from the Loufanggou 2 section of the Dalong Formation in northern Sichuan, China. The assemblage consists mainly of spherical radiolarians with spongy tests, such as Entactinaria and Spumellaria. Albaillellaria and Latentifistularia are relatively rare in the assemblage. The occurrence of Neoalbaillella caridroiti nov. sp., Neoalbaillella antaixiangi Yao and Kuwahara and Albaillella triangularis Ishiga, Kito and Imoto indicates that the Loufanggou 2 section may be correlated with the Neoalbaillella optima assemblage zone. Until now, only Neoalbaillella caridroiti nov. sp. and Neoalbaillella antaixiangi were known to occur in the northern Sichuan area. This limited occurrence indicates the possibility that the Dalong Formation depositional basin in northern Sichuan Province was a geographically limited area, and the radiolarian assemblage shows its provincialism.

Spatial Distribution and the Key Impact Factors of Soil Selenium of Cultivated Land in Lianyuan City, China

Selenium (Se) is a micronutrient that has attracted significant attention, because the threshold for human health is low. During soil surveys in China, large areas of low-Se soil were found, and this condition may increase the probability of people suffering from Se deficiency. A multi-purpose regional geochemical survey conducted in the Lou Shao basin of Hunan Province found abundant Se-rich soils in Lianyuan City. However, as the primary grain-producing area in Hunan Province, the key factors affecting the spatial distribution of soil Se in the cultivated land of Lianyuan City remain to be elucidated. Therefore, based on the data of 5516 topsoil samples (0–20 cm) of cultivated land in Lianyuan City, we used geostatistics, correlation analysis, and a Geodetector to explore the effects of geological conditions (strata), soil types, soil properties, and topography on the distribution of Se in soil. The results showed that (1) in comparison to cultivated land in the Chinese mainland, Japan, Belgium, and Sweden, the cultivated land in Lianyuan City exhibits higher Se contents, with Se-sufficient and Se-rich areas accounting for 9.74% and 88.96% of the total area, respectively; (2) the distribution of high-Se soil was consistent with that in the Longtan Formation, Dalong Formation, and Daye Formation; (3) organic matter (OM) showed a positive correlation with Se, while both the elevation and slope were negatively correlated with Se; (4) stratum had the most significant effect on the spatial variation in soil Se, followed by OM. Lianyuan City is a typical Se-rich area, and the high level of Se in soil reduces the risk of local residents suffering with diseases caused by Se deficiency. The synergistic effect of stratum and OM is the key factor influencing Se enrichment in soils. Moreover, low-lying flat areas are more conducive to the accumulation of Se. This study will help farmers to identify suitable Se-rich cultivation areas in order to increase the Se content in crops, thereby providing a valuable basis for improvements in human health and the optimization of agricultural strategies.

Origin and Formation Mechanism of the Late Permian Black Siliceous Rocks in the Lower Yangtze Region.

The Late Permian witnessed a Permian Chert Event (PCE) and distinctive oceanic geochemical fluctuations, such as an ocean acidification event, large-scale volcanic eruption, and rapid global warming. However, the links between siliceous rock formation mechanism, ocean, and climate changes are rarely discussed. In this article, two well-preserved deeper-water sections of the Dalong Formation from the Lower Yangtze region in southeast China are selected for analysis. To document the coeval oceanic changes, we present thin section authentication, scanning electron microscope (SEM) observation, and multiple geochemical proxies, including total organic carbon contents (TOC), major element contents, trace element contents, and rare earth element contents (REEs). The results show that siliceous rocks are mainly of biological origin in this region. The low content of MgO (0.10-0.94%, mean = 0.36%) in the Fantiansi section of Tongling area indicates that it is affected by regional hydrothermal fluids. The correlation between Al2O3/TiO2, Al2O3 versus SiO2 /Al2O3, and TiO2/ΣREEs indicates that the Late Permian was greatly influenced by the continuous input of terrigenous materials. The correlations of MnO/TiO2, LaN/CeN, Ce/Ce*, and LREE/HREE imply that the Dalong Formation siliceous rocks were deposited in a continental margin setting. Redox geochemical data (EFU, EFMo, and EFV) imply the water column experienced widespread anoxic/euxinic during the Late Permian, which aided in the preservation of organic matter following biological decay. The accumulation of siliceous rocks is related to South China experiencing a hot tropical climate, coastal upwelling and continental weathering-enriched marine nutrients, fostering high primary productivity, and benefiting abundant siliceous zooplankton. Volcanic and regional hydrothermal activity further enhanced nutrient flux, and the simultaneous ocean acidification event provided favorable chemical conditions for the preservation of silica, leading to the formation of siliceous rocks in the Dalong Formation.

Origin of Authigenic Quartz in Organic-Rich Shales of the Dalong Formation (NW Middle Yangtze Basin), South China: Implications for Pore Network Development

Origin of Authigenic Quartz in Organic-Rich Shales of the Dalong Formation (NW Middle Yangtze Basin), South China: Implications for Pore Network Development

Organic matter accumulation in response to tectonism: New data from the Upper Permian Dalong Formation black shales in the Western Hubei Basin, South China and its implications for the end-Permian mass extinction

Redox conditions and primary productivity are proposed to be the major controls of organic matter accumulation in sediments. Studies of the impacts of tectonic processes on organic matter accumulation are still limited. We took the Late Permian-Early Triassic Western Hubei Basin, located in the north of South China, as an example. In black shales in the Dalong Formation, bottom-water redox conditions and surface-water productivity are constrained as shown by the analysis of total organic carbon (TOC) as well as geochemical (enrichment of Mo, U, Ni, Cd and Zn) and mineralogical characteristics. The highest productivity is present in the lower Dalong Formation, and the most intensively anoxic and restricted conditions are found in the middle Dalong Formation. The correlation of the sedimentary successions on the two sides of the basin's boundary fault indicates a subsidence of the basement during the deposition of the lower-middle Dalong Formation, followed by an uplift during the deposition of the upper Dalong Formation. When looked at in conjunction with the tectonic background of the basin, the new data in the present study support that: (1) the collision of the South and North China plates controlled the paleoceanographic and hydrographic evolution of the Western Hubei Basin, as well as organic matter accumulation in sediments; (2) upwelling-induced high productivity and good preservation in an anoxic environment jointly contributed to organic matter enrichment in the lower Dalong Formation, while good preservation in an anoxic environment was the major cause of organic matter enrichment in the middle-upper Dalong Formation. This study indicates that tectonism can act as the ultimate drive mechanism for organic matter accumulation in black shales. Our data also suggest that plate tectonism in related to the assembly of East Asian blocks with the main body of Pangea can also be treated as one of the ultimate drive mechanisms of the end-Permian mass extinction as it created the restricted, anoxic Paleo-Tethys Ocean, which greatly disturbed- global seawater circulation.

Pore Structure and Fractal Dimension in Marine Mature Silicon-Rich Shale of the Dalong Formation in Western Hubei.

How shale reservoirs and gas contents are affected by the pore structure of shale is very important. Low-temperature nitrogen isothermal adsorption experiments were conducted by us to investigate the pore structure of the Dalong Formation shale. We measured the specific surface area and fractal dimension of the pores and also considered the mineral fraction and organic matter content of the rock. The results show that the Dalong Formation shale contains a lot of organic carbon, with a total organic carbon (TOC) value between 1.20 and 10.82% (mean: 5.02%). Quartz and clay minerals are the main components of the shale, with quartz making up 40.30 to 85.60% (mean: 67.21%) and clay minerals making up 9.20 to 34.10% (mean: 20.26%) of the shale. Most of the pore space in the shale of the Dalong Formation is formed by intragranular and intergranular pores, organic matter pores, and some microfissures. The pore structure is complex, with parallel-plate and ink-bottle pores being the most common types. Most of the pores are 0-2 or 2-5 nm in size. D1 and D2 are the fractal dimensions, with averages of 2.66 and 2.81, respectively. D1 can range from 2.55 to 2.78, while D2 can range from 2.66 to 2.94. The TOC content, mineral composition, and pore structure characteristics determine the fractal dimension. Higher levels of the TOC content, quartz mineral content, and specific pore surface area result in a higher fractal dimension, while higher levels of feldspar content result in a lower one. There is no apparent correlation to clay minerals or other mineral compositions.

Organic Matter Accumulation in the Upper Permian Dalong Formation from the Lower Yangtze Region, South China

AbstractThe Late Permian was marked by a series of important geological events and widespread organic‐rich black shale depositions, acting as important unconventional hydrocarbon source rocks. However, the mechanism of organic matter (OM) enrichment throughout this period is still controversial. Based on geochemical data, the marine redox conditions, paleogeographic and hydrographic environment, primary productivity, volcanism, and terrigenous input during the Late Permian in the Lower Yangtze region have been studied from the Putaoling section, Chaohu, to provide new insights into OM accumulation. Five Phases are distinguished based on the TOC and environmental variations. In Phase I, anoxic conditions driven by water restriction enhanced OM preservation. In Phase II, euxinic and cycling hydrological environments were the two most substantial controlling factors for the massive OM deposition. During Phase III, intensified terrestrial input potentially diluted the OM in sediment and the presence of oxygen in bottom water weakened the preservation condition. Phase IV was characterized by a relatively higher abundance of mercury (Hg) and TOC (peak at 16.98 wt%), indicating that enhanced volcanism potentially stimulated higher productivity and a euxinic environment. In Phase V, extremely lean OM was preserved as a result of terrestrial dilutions and decreasing primary productivity. Phases I, II and IV are characterized as the most prominent OM‐rich zones due to the effective interactions of the controlling factors, namely paleogeographic, hydrographic environment, volcanism, and redox conditions.

Pore Structure and Fractal Characteristics of the Middle and Upper Permian Dalong and Gufeng Shale Reservoirs, Western Hubei Province, South China

The Middle and Upper Permian Dalong and Gufeng Formations in South China have recently been considered as potential gas-producing shales. However, their pore structure characteristics remain poorly understood. To investigate the pore structure and fractal characteristics of the pores in these two formations, a suite of shale samples from the Dalong and Gufeng Formations in the western Hubei Province, South China were analyzed by multiple techniques, namely, TOC content, X-ray diffraction (XRD) mineralogy analysis, optical microscopy observations, major elemental analysis, field emission-scanning electron microscopy (FE-SEM), and low-pressure gas adsorption measurements (N2 and CO2). The identified major shale lithofacies include siliceous mudstone, carbonaceous mudstone, argillaceous-siliceous mixed mudstone, and calcareous-siliceous mixed mudstone. SEM images show that the dominant pore types include the pores between brittle minerals, slit-shaped pores between clay sheets, and secondary organic matter (OM) pores within solid bitumen. The pore size distribution is dominated by micropores and mesopores (<30 nm), which are the major contributors to total pore volume and surface area for the Dalong and Gufeng Formations. Based on the Frenkel–Halsey–Hill (FHH) method, fractal dimensions (D1, D2) calculated from the nitrogen adsorption data have a range of from 2.489 to 2.772 (D1) and from 2.658 to 2.963 (D2), and are higher in the Gufeng Formation (average TOC = 8.3 wt.%) due to a higher TOC content comparing to the Dalong Formation (average TOC = 6.2 wt.%). The pore development and fractal characteristics are primarily controlled by organic matter (OM), carbonate minerals, and clay minerals for both the Dalong and Gufeng Formations. Shale samples with high TOC content, low carbonate content, and high clay content tend to develop more heterogeneous micropores and mesopores, which is ascribed to the generation of clay-related and OM-hosted pores, along with the destruction of primary pores by pore-filling carbonate cements.

Evolution of organic pores in Permian low maturity shales from the Dalong Formation in the Sichuan Basin: Insights from a thermal simulation experiment

Shale oil and gas exploration is greatly impacted by the development and evolution of organic pore structures. In order to study the evolution of shale pore structures over maturities, we conducted thermal simulation experiments on low-mature organic-rich shale (Ro = 0.73%, TOC = 9.57%) obtained from the Dalong Formation. Various maturity stages were covered in these experiments, from low to over-maturity. By analyzing mineral composition, organic geochemistry, and characteristics of pore structures at various temperatures used for thermal simulation, we examined how organic matter pores generate and evolve during the formation of shale hydrocarbons. As a result, an integrated model of the evolution of pore structures in organic-rich shale was developed. Our study revealed the following insights: With increasing thermal simulation temperature, shale pore development exhibited three distinct stages: initial development, rapid development, and slow development. These stages correspond respectively to the three stages of organic matter thermal evolution. The distribution of pores in the shale transitioned from a unimodal structure dominated by larger pores to a bimodal structure with both micropores and macropores. Decomposition of organic matter and pressure compaction play a controlling role on organic porosity. Pore structure parameters and fractal dimensions of the pores were primarily influenced by organic matter decomposition, residual liquid hydrocarbons, and diagenetic processes. Additionally, kerogen type, morphology of organic matter debris, symbiosis of minerals-organic matter also impacted pore development.

Geological Significance of Rare Earth Elements in Marine Shale of the Upper Permian Dalong Formation in the Lower Yangtze Region, South China

The rare earth elements (REEs) provide significant geological information and serve as a reliable indicator for predicting the paleoclimate, paleoenvironment, and paleotectonic evolution of sedimentary rocks. The REEs, major elements, and trace elements of 75 marine shale samples collected from the Late Permian Dalong Formation at the Fantiansi (FTS) and Putaoling (PTL) sections in the Lower Yangtze region were analyzed. The results revealed that the major elements Al2O3, K2O, and TiO2 were primarily influenced by clay minerals associated with terrigenous detrital, whereas SiO2 and Na2O were not affected by clay minerals. The ΣREE values obtained from the Late Permian Dalong Formation at the Fantiansi (FTS) and Putaoling (PTL) sections in the Lower Yangtze region were found to be lower than those of the Post-Archean Average Shale (PAAS) (184.8 μg/g). The study of REE indicators related to the source, redox conditions, and tectonic settings led to the following conclusions: (a) ΣREE showed strong positive correlations with TiO2, Al2O3, K2O, and Na2O, but exhibited relatively weak correlations with Fe2O3 and P2O5, suggesting that REEs were mainly associated with clay minerals, but were also influenced by pyrite, phosphates, and other minerals; (b) The high values of Ce/Ce*, MoEF, UEF, and Corg/P were mainly concentrated in Stages I, II, III and V of the Dalong Formation at the FTS and PTL sections, and the corresponding strong anoxic environment was more conducive to the preservation of organic matter; (c) The diagram between La/Yb and ΣREE, Al2O3-TiO2, TiO2-Zr, and La/Th-Hf bivariate diagrams indicated that the provenance of the rocks from the Dalong formations was primarily felsic igneous rocks; (d) Discriminant-function diagrams and La-Th-Sc, Th-Sc-Zr/10 and Th-Co-Zr/10 triangular diagrams show that the clastic sediments of the Dalong formations were derived most likely from continental island arcs. This study corresponds to the background of the transition from continental margin to continental collision structure in South China during the Late Permian.

Fracability evaluation of the middle–upper Permian marine shale reservoir in well HD1, western Hubei area

Taking the HD1 well as the research target, through intensive core sampling, experimental test analysis, comprehensive logging interpretation and other methods, the fracability of shale reservoirs was discussed in this paper. The results show that the Gufeng–Dalong Formation in the HD1 well has a high organic carbon content, and the organic-rich shale developed in the lower part of the Gufeng and Dalong Formations with thicknesses of 30 m and 15 m, respectively. For the low-porosity and ultralow-permeability shale reservoir type, the natural fractures are undeveloped in the lower part of the Dalong Formation, with a lower linear density, while they are well developed in the Xiayao and Longtan Formations and the lower part of the Gufeng Formation, and the interlayer bedding fractures are relatively developed. The Gufeng–Dalong Formation shale also has a high mineral brittleness index (average of 44.5%), high static Young’s modulus (20–70 GPa), low static Poisson’s ratio (0.10–0.31), and high horizontal pressure difference coefficient of two phases (0.17–0.56). It is concluded that the shale reservoir is favorable for fracture development in the lower part of the Dalong Formation, with depths of 1249–1289.5 m, and the lower part of the Xiayao, Longtan and Gufeng Formations, with depths of 1300–1335.3 m. Overall, the fracable shale section with high brittleness and rock strength is beneficial to fracturing. However, the existence of a large number of shale bedding fractures increases the complexity of the fractures, and at the same time, it has a certain negative impact on fracture generation. Double wing fractures are easily formed because of the high two-phase horizontal pressure difference. Therefore, the leakage caused by shale bedding fractures and the influence on the fracture height and extension length should be considered comprehensively in fracturing design.

Quartz types and origins of the Upper Permian Dalong Formation shale of the Sichuan Basin: Implications for pore preservation in deep shale reservoirs

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

Reservoir Characteristics and Influencing Factors of Organic-Rich Siliceous Shale of the Upper Permian Dalong Formation in Western Hubei

To elucidate the reservoir characteristics of organic-rich siliceous shale of the Upper Permian Dalong Formation in western Hubei, this study focused on the drilling cores of Well ED-2. Various techniques, including a mineral composition analysis, an organic carbon content analysis, a vitrinite reflectance measurement, a total porosity determination, field emission scanning electron microscopy (FE-SEM), and low-pressure CO2 and N2 physical adsorption tests, were employed to analyze the mineralogy, organic geochemistry, total porosity, and pore structure characteristics. Additionally, the factors influencing the reservoir performance of the Dalong Formation shale were investigated. The results indicated that the Dalong Formation’s shale was characterized as an organic-rich siliceous shale. Organic matter was mainly of sapropelic type, with a relatively high thermal evolution degree and Ro ranging from 2.59% to 2.76%. The total porosity of the Dalong Formation’s siliceous shale was low, indicating poor reservoir properties. Organic matter pores were highly developed, mainly the ones formed after the hydrocarbon generation of solid asphalt. Micropores and mesopores were the dominant pore types in the shale, with macropores being significantly less abundant. The study further revealed that the pore volume and specific surface area exhibited a significantly positive correlation with total organic carbon (TOC) content and clay minerals, while demonstrating a weak negative correlation with quartz content. The comprehensive analysis revealed that there were two factors contributing to the poor physical properties of organic-rich siliceous shale in the Dalong Formation. Firstly, in siliceous shale with a high quartz content, the siliceous component was partly derived from the siliceous cementation of hydrothermal fluids. This process led to the formation of secondary quartz that filled intergranular pores, resulting in a decrease in macropore volume, total porosity, and a weak negative correlation with quartz content. Secondly, in siliceous shale with a relatively high clay mineral content, the organic matter was subjected to stronger compaction due to the relatively low content of brittle minerals. This compaction caused the destruction of most macropores, leaving behind primarily micropores and mesopores. Consequently, the average pore size decreased, leading to poor physical properties.

Nanopore Evolution of the Upper Permian Organic-Rich Shales from Dalong Formation, Sichuan Basin, during Artificial Hydrous Pyrolysis

Nanopore Evolution of the Upper Permian Organic-Rich Shales from Dalong Formation, Sichuan Basin, during Artificial Hydrous Pyrolysis

Application of biomarkers in characterization of organic matter source, maturity and palaeoredox condition of black shales from the upper Permian Dalong Formation in Southern Anhui Province, South China

The organic‐rich shale in the upper Permian Dalong Formation in Southern Anhui Province is not only regarded as a high‐quality hydrocarbon source rock, but one of the potential targets for shale oil and gas exploration in Southern China. Organic matter types in such a setting remain a subject of debate. The major point of controversy is whether they are dominated by oil‐prone types or gas‐prone types. A total of 20 rock samples collected from Gangdi‐1 well were conducted for analysis of organic geochemistry. Results show that Dalong Formation shale extracts exhibit a wide range of C12‐C35 n‐alkanes and acyclic isoprenoids. The chromatograms display an unimodal distribution with carbon peak in the range of nC18‐nC22 and low quantities of long‐chain (nC25+) n‐alkanes. Relatively high ratios of nC21‐/nC22+ and C27/C29 regular steranes demonstrate that organic matter is mainly originated from lower aquatic organisms and followed by higher plants in a local area. This is also supported by the variations of the δ13C values of C13‐C26 n‐alkanes for the target samples being in the range of 3.0‰–5.6‰ offset, which is slightly higher than the corresponding value (1.6‰) of n‐alkanes from a single source. All these pieces of information decipher that the organic matter in Dalong Formation shale may be dominated by type I and followed by type II1. Moreover, the maturity proxies according to both organic petrography (Ro and Tmax) and biomarkers such as ratios of C29 20 S/(20 S + 20R) and C29 ββ/(αα + ββ) together indicate that organic matter in Dalong Formation shale is in a mature stage of hydrocarbon generation. Furthermore, a cross‐plot of total organic carbon versus total sulphur as well as Pr/Ph ratio and Pr/nC17‐Ph/nC18 diagram elucidates that Dalong Formation shales were mainly formed in a marine reducing environment. Finally, two depositional models of organic matter enrichment are proposed. The abundant lower aquatic organisms with few terrestrial detrital inputs and the reducing water environment together promote organic matter accumulation.

Reservoir Characteristics and Their Controlling Factors in Siliceous Shales of the Upper Permian Dalong Formation, Western Hubei Province, South China

To evaluate the reservoir characteristics of siliceous shale in the Dalong Formation within the late Permian intra-platform rift trough in Western Hubei (China), we studied a drill core from well ED-2 in Western Hubei. To analyze the physical characteristics, pore structure, methane adsorption performance, and their influences on the siliceous shale reservoir, we performed X-ray diffraction, total organic carbon (TOC) content, vitrinite reflectance (Ro, indicating thermal evolution), total porosity and permeability, field emission scanning electron microscopy, CO2 and N2 physical adsorption, and methane isothermal adsorption analyses, among others. Our results show that the Dalong Formation in Western Hubei is an organic-rich (2.6–14.3 wt.%), highly thermally evolved (Ro = 2.59–2.76%), siliceous shale containing mainly type-I and type-II1 organic matter. The Dalong siliceous shale has low porosity and permeability and belongs to a larger reservoir with low horizontal permeability (0.002–335.209 mD) and porosity (1.2–7.8%). Pores in the shale are mainly organic, inorganic, and microfractures; the organic pores are very developed. The pore volume and specific surface area of the shale are mainly due to micropores and mesopores and are positively correlated with TOC and clay mineral contents and weakly negatively correlated with quartz and carbonate contents. The micropores and mesopores are well developed, improving the methane adsorption capacity, which, in turn, is strongly positively correlated with TOC content. Comprehensive analysis shows that the high organic matter content of the Dalong siliceous shale has the greatest influence on its pore structure; the many organic pores generated after hydrocarbon generation have controlled the development of micropores and mesopores, which is conducive to the adsorption and storage of shale gas. The development of brittle minerals resistant to compaction, such as siliceous minerals, helps preserve organic pores. This study is informative for basin-scale petroleum system investigations, which are essential for understanding oil and gas exploration possibilities and regional petroleum systems.

Characteristics and controlling factors of the Upper Permian Dalong Formation in northwestern Sichuan Basin, China

It has been generally accepted that the Permian shale is an important target for shale oil and gas exploration. In order to precisely predict the distribution of potential excellent source rocks of shale oil and gas, it is necessary to reveal the forming process and mechanism of high-quality source rocks, such as the Dalong Formation in the Scihuan Basin. In this study, 17 samples were collected from the Dalong Formation in Longfeng Quarry, Northwest Guangyuan City, Sichuan Province. The lithofacies, organic geochemistry, and pyrolysis of these samples were analyzed to evaluate the main factors controllingthe source rock formation. In particular, the influences of hydrothermal and paleoenvironmental conditions on the accumulation of organic matter were analyzed. The total organic carbon (TOC) content of the samples, which ranges from 0.05% to 12.21%, is closely related to the lithology. High TOC has been generally observed in rock intervals developed in a deep-water sedimentary environment, including siliceous rock, dark shale, and siliceous shale. The peak pyrolysis temperature of the samples ranges from 444°C to 462°C, indicating a mature stage. Thin-section observation, combined with the results of previous studies, has confirmed that the source of the organic matter is mainly marine phytoplankton, such as algae and radiolarians, ostracods, and other marine organisms, which consist of these high-quality source rocks. The ratio of trace elements, such as Ni/Co, V/Cr, U/Th, and V/(V + Ni) indicate that high-quality source rocks are mainly formed in anoxic restricted environments. The hydrothermal activity of submarine volcanoes provided many nutrients and compounds, resulting biological prosperity and improving primary productivity. This is simultaneously conducive to the formation of anoxic environments and the preservation of organic matter.

Climate-driven paleoceanography change controls on petrology and organic matter accumulation in the upper Permian Dalong Formation, western Hubei Province, southern China

The influence of paleoclimate on shale sedimentary processes and organic matter accumulation is of great significance for reconstructing the “deep time” paleoclimate as well as for oil and gas exploration. Here, a detailed study on the Lopingian Dalong Formation organic-rich shales in the western Hubei area of the Middle Yangtze region is performed to understand how paleoclimate affects the depositional process and organic matter accumulation of shales. By combining a detailed core description and microscopic observations, along with mineralogical and geochemical analysis, two major lithological facies were identified from the H1 drilling core, including siliceous shale and clay/carbonate bearing mixed shale. The changes in depositional observations and geochemical indicators (including detritus index, Ln (Al2O3/Na2O), CuEF, ZnEF, UEF, MoEF, Cd/Mo and Co × Mn) indicate that the siliceous shale was deposited in an anoxic basin with well-developed upwelling under warm and semihumid conditions. However, the clay/carbonate bearing mixed shale was deposited in a hypoxic basin with weakly developed upwelling, and the climate was arid and hot. These deposits suggest that paleoclimates developed into hot and arid climates over time and that such climate change trends may be a response to global warming at the end of the late Permian. High temperatures inhibit the development of upwelling but increase the input of terrigenous clastic materials by enhancing weathering, and controlling the shale deposition. The average total organic carbon (TOC) content of the basin varies significantly in different lithological facies. We infer that the organic matter accumulation of clay/carbonate bearing mixed shale is comprehensively controlled by the carbonate production rate and primary productivity, while the organic matter accumulation of siliceous shale is primarily controlled by the authigenic quartz production rate. When the detrital index in the study area reaches 80 %, TOC does not increase or even decrease. Therefore, in the presence of authigenic quartz silicon, the increased silicate content is not always conducive to the accumulation of organic matter.

Effect of Volcanic Activity on Hydrocarbon-Forming Organisms in Organic-Rich Shale: A Case Study of Dalong Formation in Northwestern Sichuan Basin, China

Hydrocarbon-generating material determines the elemental composition and hydrocarbon-generating potential of kerogens in a source rock, and it is the key material basis to control the hydrocarbon-generating capacity of the source rock. Previous studies have shown that many intervals of high-quality source rocks generally contain a varying number of volcanic ash layers. The impact of these volcanic ash layers on the development of high-quality source rocks has attracted extensive attention. However, these studies mainly focused on the development of hydrocarbon-forming organisms and the preservation of organic matters but rarely dealt with the differential development of hydrocarbon-forming organisms between multiple volcanic ash layers. The Permian Dalong Formation in the northwestern Sichuan Basin, China, is a set of high-quality source rocks with multiple volcanic sedimentary layers. To understand the differential development of hydrocarbon-forming organisms between volcanic ash layers, with the Dalong Formation in the Longfeng quarry section, Guangyuan, the Sichuan Basin as an example, this study analyzes the types of hydrocarbon-generating materials of high-quality source rocks of the Dalong Formation and the reasons why volcanic activities affected the growth of hydrocarbon-forming organisms and explores the impact of volcanic activities on the development of different types of hydrocarbon-forming organisms. The results show that the TOC of the Longfeng quarry section is 0.05%–15.46%, with an average of 4.76%, and the average TOC of the Dalong Formation is as high as 5.16%, which belongs to high-quality source rocks. The hydrocarbon-forming organism association in the Dalong Formation source rocks is mainly composed of floating algae and benthic algae and vertically divided into three parts: a few radiolarians and trace fossils occasionally in the lower part; a large number of radiolarians, calcium spheres, and algal debris in the middle part; and mainly foraminifera and gastropods in the upper part. There are 36 volcanic ash layers in the Dalong Formation of the study area. In the process of volcanic activity, the “fertilization” effect of volcanic ash made the nutrient elements in the volcanic ash enter water, increasing the number of hydrocarbon-forming organisms, causing the differential development of various hydrocarbon-forming organisms, and promoting the formation of reducing water to support the preservation of organic matters. The development of hydrocarbon-forming organisms is controlled by the changes of element types and abundances caused by magmatic properties. The time interval of volcanic activities also has an important impact on the development of hydrocarbon-forming organisms.

Changhsingian (Lopingian, Permian) radiolarian paleobiogeography on and around the Yangtze Platform

We report a Changhsingian radiolarian assemblage from the Rencunping section of the Dalong Formation, in the northern margin of the Yangtze Platform, South China. The assemblage is composed of 37 species belonging to 20 genera, including two new species (Copicyntroides rencunpingensis n. sp. and Copicyntroides subplanus n. sp.). Together with the well documented radiolarian faunas of the Dalong Formation around the Yangtze Platform, we applied Sørensen Coefficient, an incidence-based measures of relative similarity, to examine the similarity of species associations among the sections of the Dalong Formation, including those at Rencunping, Shangsi, Hushan, Pingdingshan, Xinmin and Dongpan. We find that 165 species have been reported in the entire Dalong Formation of all sections, with variable paleobiogeographic setting from section to section within the same formation. The Sørensen Coefficient showed a high similarity between the Rencunping and Pingdingshan sections and low similarity in the Xinmin or Dongpan sections. This is interpreted as due to water mass separation between the northern and southern sides of the Yangtze Platform, probably by the physical barrier of the shallow carbonate platform in the region. We reviewed taxonomic groups and depositional conditions in each section, and determined “low latitudinal common species” or “cosmopolitan” species including Entactinia itsukaichiensis, Albaillella angusta, A. excelsa, A. protolevis, A. triangularis, Ishigaum obesum, I. trifustis, Trilonche crassispinosa, Triplanospongos musashiensis and “Hegleria-shaped” radiolarian. The radiolarian fauna of Rencunping was concluded as a typical fauna in the Dalong Formation, and its higher similarity to faunas from Japanese Islands and Southeast Asia indicates close paleogeographic positions and similar paleogeographic environments in these areas during the Changhsingian.