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21 pages, 5618 KB

Open AccessArticle

Brittleness Evaluation Method and Brittle–Plastic Transition Law of Deep Shale Based on Energy Evolution

by Wen-Ping Liu, Cheng-Lin Liu, Bo Peng, Yi Song, Yong-Zhi Huang and Xue-Lian You

Minerals 2026, 16(3), 291; https://doi.org/10.3390/min16030291 - 10 Mar 2026

Viewed by 352

Abstract

In order to accurately evaluate the brittleness and plasticity during hydraulic fracturing of deep shale reservoirs, this study constructs a brittleness evaluation index for deep shales based on the energy evolution features from the complete stress–strain curve. Both the pre- and post-peak elastic [...] Read more.

In order to accurately evaluate the brittleness and plasticity during hydraulic fracturing of deep shale reservoirs, this study constructs a brittleness evaluation index for deep shales based on the energy evolution features from the complete stress–strain curve. Both the pre- and post-peak elastic energy ratios and the stress drop effect were considered in this index. The brittle–plastic deformation characteristics was fundamentally reflected during rock failure. Due to further comparison between the brittleness index and sample fracture patterns with corresponding stress–strain curves, a quantitative evaluation model (0–1 scale) for deep shale brittleness–plasticity deformation was built. Using this model, the brittle–plastic transition patterns of different shale facies are investigated, creating a three-parameter diagram of the brittleness index, clay mineral content, and depth. The results show that siliceous and carbonate shales undergo a brittle–plastic transition at approximately 4500–5000 m depth, while mixed shales transition at around 3500 m. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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15 pages, 4140 KB

Open AccessArticle

Study on Mineral Characteristics and Lithofacies Identification of Shallow Shale in the Taiyang Gas Field, Zhaotong Demonstration Zone

by Gaocheng Wang, Honglin Shu, Zhenxue Jiang, Hongyan Wang, Liwei Jiang, Xinsheng Zhao, Xiaomin Gu, Chao Zhang, Chen Zou and Jue Mei

Minerals 2026, 16(3), 290; https://doi.org/10.3390/min16030290 - 10 Mar 2026

Viewed by 319

Abstract

Shale lithofacies is a key geological factor controlling the hydrocarbon generation capacity and fracturing effectiveness of shale reservoirs. Current research on lithofacies remains insufficient for the exploration and evaluation of shallow shale gas in complex structural areas of southern China. This study focuses [...] Read more.

Shale lithofacies is a key geological factor controlling the hydrocarbon generation capacity and fracturing effectiveness of shale reservoirs. Current research on lithofacies remains insufficient for the exploration and evaluation of shallow shale gas in complex structural areas of southern China. This study focuses on the shale of the Wufeng-Longmaxi Formation in the Taiyang anticlinal structure within the Zhaotong National Shale Gas Demonstration Zone. By utilizing logging data to calculate mineral content characteristics in the study area and identifying lithofacies types based on the DEI (Dual-Energy Index) derived from dual-energy computed tomography scanning analysis of full-diameter core samples, this paper comprehensively analyzes the control of different lithofacies on key reservoir attributes. The results indicate that the target shale in the Taiyang area exhibits high brittle mineral content (averaging 66.07%), while the clay mineral content averages 30.89%, and the content of other minerals is only 3.04%, showing distinct regularity in both vertical and planar distribution. Based on the DEI method, lithofacies are classified into biogenic siliceous shale, argillaceous shale, calcareous shale, and limestone. Among these, biogenic siliceous shale, characterized by abundant brittle minerals and bioclast laminations, has the highest brittleness index and the richest organic matter content, making it the optimal lithofacies for reservoirs. Calcareous shale exhibits moderate brittleness and low organic matter content, while argillaceous shale, dominated by clay matrix, has the lowest brittleness index and moderate organic matter content. The findings highlight the critical influence of lithofacies on reservoir quality and provide important guidance for the preferential selection and exploration of favorable lithofacies in shallow shale gas reservoirs in similar complex structural regions in southern China. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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30 pages, 47854 KB

Open AccessArticle

Genesis and Reservoir Implications of Multi-Stage Siliceous Rocks in the Middle–Lower Ordovician, Northwestern Tarim Basin

by Jinyu Luo, Tingshan Zhang, Pingzhou Shi, Zhou Xie, Jianli Zeng, Lubiao Gao, Zhiheng Ma and Xi Zhang

Minerals 2026, 16(1), 107; https://doi.org/10.3390/min16010107 - 21 Jan 2026

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Abstract

Siliceous rocks of various colors and types are extensively developed within the Middle–Lower Ordovician carbonate along the Northwest Tarim Basin. Their genesis provides important insights into the evolution of basinal fluids and the associated diagenetic alterations of the carbonates. Based on petrographic, geochemical, [...] Read more.

Siliceous rocks of various colors and types are extensively developed within the Middle–Lower Ordovician carbonate along the Northwest Tarim Basin. Their genesis provides important insights into the evolution of basinal fluids and the associated diagenetic alterations of the carbonates. Based on petrographic, geochemical, fluid inclusion, and petrophysical analyses, this study investigates the origin of siliceous rocks within the Middle–Lower Ordovician carbonate formations (Penglaiba, Yingshan, and Dawangou formations) in the Kalpin area, Tarim Basin, and investigates the impact on hydrothermal reservoirs. The results reveal two distinct episodes of siliceous diagenetic fluids: The first during the Late Ordovician involved mixed hydrothermal fluids derived from deep magmatic–metamorphic sources, formation brines, and seawater. Characterized by high temperature and moderate salinity, it generated black chert dominated by cryptocrystalline to microcrystalline quartz through replacement processes. The second episode developed in the Middle–Late Devonian as a mixture of silicon-rich fluids from deep heat sources and basinal brines. In conditions of low temperature and high salinity, it generated gray-white siliceous rocks composed of micro- to fine crystalline quartz, spherulitic-fibrous chalcedony, and quartz cements via a combination of hydrothermal replacement and precipitation. A reservoir analysis reveals that the multi-layered black siliceous rocks possess significant reservoir potential amplified by the syndiagenetic tectonic fracturing. In contrast, the white siliceous rocks, despite superior petrophysical properties, are limited in scale as they predominantly infill late-stage fractures and vugs, mainly enhancing local flow conduits. Hydrothermal alteration in black siliceous rocks is more intense in dolostone host rocks than in limestone. Thus, thick (10–20 m), continuous black siliceous layers in dolostone and the surrounding medium-crystalline dolostone alteration zones, are promising exploration targets. This study elucidates the origins of Ordovician siliceous rocks and their implications for carbonate reservoir properties. The findings may offer valuable clues for deciphering the evolution and predicting the distribution of hydrothermal reservoirs, both within the basin and in other analogous regions worldwide. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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26 pages, 5245 KB

Open AccessArticle

Sedimentary Environment and Organic Matter Enrichment of the First Member in the Upper Triassic Xujiahe Formation, Southeastern Sichuan Basin

by Hao Huang, Zhongyun Chen, Tingshan Zhang, Xi Zhang and Jingxuan Zhang

Minerals 2025, 15(10), 1071; https://doi.org/10.3390/min15101071 - 13 Oct 2025

Cited by 1 | Viewed by 1032

Abstract

The Xujiahe Formation (FM) is a significant source rock layer in the Sichuan Basin. In recent years, a growing number of scholars believe that the shale gas potential of the Xujiahe Formation is equally substantial, with the first member of the formation being [...] Read more.

The Xujiahe Formation (FM) is a significant source rock layer in the Sichuan Basin. In recent years, a growing number of scholars believe that the shale gas potential of the Xujiahe Formation is equally substantial, with the first member of the formation being the richest resource. The deposition of Member (Mbr) 1 of Xujiahe FM represents the first and most extensive transgression event within the entire Xujiahe Formation. This study investigates the sedimentary environment and organic matter (OM) enrichment mechanisms of the dark mud shales in the Mbr1 of Xujiahe FM on the southeastern margin of the Sichuan Basin, utilizing methods such as elemental geochemistry and organic geochemistry analyses. The results indicate that these dark mud shales possess a relatively high OM abundance, averaging 2.20% and reaching a maximum of 6.22%. The OM is primarily Type II₂ to Type III. Furthermore, the paleoclimate during the Mbr1 period in the study area was warm and humid with lush aquatic vegetation. Intense weathering and ample precipitation transported large amounts of nutrients into the lacustrine/marine basin, promoting the growth and reproduction of algae and terrestrial plants. Correlation analysis between the Total Organic Carbon (TOC) content and various geochemical proxies in the Mbr1 mud shales suggests that OM enrichment in the study area was primarily controlled by the climate and sedimentation rate; substantial OM accumulation occurred only with abundant terrigenous OM input and a relatively high sedimentation rate. Redox conditions, primarily productivity, and terrigenous detrital input acted as secondary factors, collectively modulating OM enrichment. Event-driven transgressions also played an important role in creating conditions favorable for OM preservation. Synthesizing the influence of these multiple factors on OM enrichment, this study proposes two distinct composite models for OM enrichment, dominated by climate and sedimentation rate. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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17 pages, 13918 KB

Open AccessArticle

Occurrence State and Controlling Factors of Methane in Deep Marine Shale: A Case Study from Silurian Longmaxi Formation in Sichuan Basin, SW China

by Junwei Pu, Tongtong Luo, Yalan Li, Hongwei Jiang and Lin Qi

Minerals 2025, 15(8), 820; https://doi.org/10.3390/min15080820 - 1 Aug 2025

Cited by 1 | Viewed by 744

Abstract

Deep marine shale is the primary carrier of shale gas resources in Southwestern China. Because the occurrence and gas content of methane vary with burial conditions, understanding the microscopic mechanism of methane occurrence in deep marine shale is critical for effective shale gas [...] Read more.

Deep marine shale is the primary carrier of shale gas resources in Southwestern China. Because the occurrence and gas content of methane vary with burial conditions, understanding the microscopic mechanism of methane occurrence in deep marine shale is critical for effective shale gas exploitation. The temperature and pressure conditions in deep shale exceed the operating limits of experimental equipment; thus, few studies have discussed the microscopic occurrence mechanism of shale gas in deep marine shale. This study applies molecular simulation technology to reveal the methane’s microscopic occurrence mechanism, particularly the main controlling factor of adsorbed methane in deep marine shale. Two types of simulation models are also proposed. The Grand Canonical Monte Carlo (GCMC) method is used to simulate the adsorption behavior of methane molecules in these two models. The results indicate that the isosteric adsorption heat of methane in both models is below 42 kJ/mol, suggesting that methane adsorption in deep shale is physical adsorption. Adsorbed methane concentrates on the pore wall surface and forms a double-layer adsorption. Furthermore, adsorbed methane can transition to single-layer adsorption if the pore size is less than 1.6 nm. The total adsorption capacity increases with rising pressure, although the growth rate decreases. Excess adsorption capacity is highly sensitive to pressure and can become negative at high pressures. Methane adsorption capacity is determined by pore size and adsorption potential, while accommodation space and adsorption potential are influenced by pore size and mineral type. Under deep marine shale reservoir burial conditions, with burial depth deepening, the effect of temperature on shale gas occurrence is weaker than pressure. Higher temperatures inhibit shale gas occurrence, and high pressure enhances shale gas preservation. Smaller pores facilitate the occurrence of adsorbed methane, and larger pores have larger total methane adsorption capacity. Deep marine shale with high formation pressure and high clay mineral content is conducive to the microscopic accumulation of shale gas in deep marine shale reservoirs. This study discusses the microscopic occurrence state of deep marine shale gas and provides a reference for the exploration and development of deep shale gas. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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29 pages, 10402 KB

Open AccessArticle

Depositional and Paleoenvironmental Controls on Shale Reservoir Heterogeneity in the Wufeng–Longmaxi Formations: A Case Study from the Changning Area, Sichuan Basin, China

by Chongjie Liao, Lei Chen, Chang Lu, Kelin Chen, Jian Zheng, Xin Chen, Gaoxiang Wang and Jian Cao

Minerals 2025, 15(7), 677; https://doi.org/10.3390/min15070677 - 24 Jun 2025

Cited by 1 | Viewed by 999

Abstract

Numerous uncertainties persist regarding the differential enrichment mechanisms of shale gas reservoirs in southern China. This investigation systematically examines the sedimentary environments and reservoir characteristics of the Wufeng–Longmaxi formations in the Changning area of the Sichuan Basin, through the integration of comprehensive drilling [...] Read more.

Numerous uncertainties persist regarding the differential enrichment mechanisms of shale gas reservoirs in southern China. This investigation systematically examines the sedimentary environments and reservoir characteristics of the Wufeng–Longmaxi formations in the Changning area of the Sichuan Basin, through the integration of comprehensive drilling data, core samples, and analytical measurements. Multivariate sedimentary proxies (including redox conditions, terrigenous detrital influx, basinal water restriction, paleoclimatic parameters, paleowater depth variations, and paleo-marine productivity) were employed to elucidate environmental controls on reservoir development. The research findings demonstrate that during the depositional period of the Wufeng Formation in the Changning area, the bottom water was characterized by suboxic to anoxic conditions under a warm-humid paleoclimate, with limited terrigenous detrital input and strong water column restriction throughout the interval. Within the Longmaxi Formation, the depositional environment evolved from intensely anoxic conditions in the LM1 through suboxic states in the LM3 interval, approaching toxic conditions by the LM2 depositional phase. Concurrently, the paleoclimate transitioned towards warmer and more humid conditions, accompanied by progressively intensified terrigenous input from the LM1-LM6, while maintaining semi-restricted water circulation. Both paleowater depth and paleoproductivity peaked from the Wufeng Formation to the LM1 interval, followed by gradual shallowing of water depth and declining productivity during the LM3–LM6 depositional phases. Comparative analysis of depositional environments and reservoir characteristics reveals that sedimentary conditions exert a controlling influence on multiple reservoir parameters, including shale mineral composition, organic matter enrichment, pore architecture, petrophysical properties (e.g., porosity, permeability), and gas-bearing potential. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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25 pages, 5622 KB

Open AccessArticle

Differential Geochemical Features of Lacustrine Shale and Mudstone from Triassic Yanchang Formation, Ordos Basin, China: Insights into Their Sedimentary Environments and Organic Matter Enrichment

by Ziming Wang, Hongfei Cheng and Yang Wang

Minerals 2025, 15(6), 656; https://doi.org/10.3390/min15060656 - 18 Jun 2025

Cited by 1 | Viewed by 1363

Abstract

The lacustrine mudstones and shales of the Triassic Yanchang Formation in the Ordos Basin serve as critical hydrocarbon source rocks. However, previous studies predominantly focus on individual lithologies, with comparative investigations into the sedimentary environments of dark mudstones and black shales remaining relatively [...] Read more.

The lacustrine mudstones and shales of the Triassic Yanchang Formation in the Ordos Basin serve as critical hydrocarbon source rocks. However, previous studies predominantly focus on individual lithologies, with comparative investigations into the sedimentary environments of dark mudstones and black shales remaining relatively limited. The study systematically compares sedimentary environment parameters (e.g., paleoclimate, paleosalinity, paleoredox conditions, paleowater depth, and paleoproductivity characteristics) between mudstones and shales, and how these distinct environmental factors governed the differential enrichment mechanisms of organic matter within the depositional aquatic system has been elucidated. Geochemical proxies (e.g., CIA, Sr/Cu, Rb/Sr, Sr/Ba, V/Ni, U/Th, V/Cr, Rb/Zr, P/Ti, Cu/Ti) reveal marked contrasts: In comparison with the Chang 7 and Chang 8 dark mudstones, the Chang 7 black shales exhibit (1) warmer–humid paleoclimatic regimes, (2) higher paleosalinity, (3) intensely anoxic conditions, (4) deeper paleowater depth, and (5) elevated paleoproductivity. These environmental divergences directly govern the significant total organic carbon content disparity between black shales and dark mudstones. Organic enrichment in the Chang 7 dark mudstones and black shales is primarily controlled by paleoproductivity and paleoredox conditions, with secondary influences from paleoclimate and paleowater depth. Based on the above studies, this research established a differential organic matter enrichment model. This research is of significant importance for guiding oil and gas exploration and development in the Ordos Basin. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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16 pages, 9568 KB

Open AccessArticle

Enrichment Mechanism and Development Technology of Deep Marine Shale Gas near Denudation Area, SW CHINA: Insights from Petrology, Mineralogy and Seismic Interpretation

by Haijie Zhang, Ziyi Shi, Lin Jiang, Weiming Chen, Tongtong Luo and Lin Qi

Minerals 2025, 15(6), 619; https://doi.org/10.3390/min15060619 - 9 Jun 2025

Cited by 2 | Viewed by 647

Abstract

As an important target for deep marine shale gas exploration, shale reservoirs near denudation areas have enormous resource potential. Based on the impression method, the sedimentary paleogeomorphology near the denudation area is identified as three units: the first terrace, the second terrace, and [...] Read more.

As an important target for deep marine shale gas exploration, shale reservoirs near denudation areas have enormous resource potential. Based on the impression method, the sedimentary paleogeomorphology near the denudation area is identified as three units: the first terrace, the second terrace, and the third terrace. At the second terrace where Well Z212 is located, the thickness of the Longmaxi Formation first section is only 0.8 m, and the continuous thickness of the target interval is only 4.3 m, making it a typical thin shale reservoir. By integrating petrology, mineralogy and the seismic method, the thin shale reservoir is characterized. Compared to shale reservoirs far away from the denudation area, the Well Z212 (near denudation area) production interval (Wufeng Formation first section) has high porosity (6%–10%), moderate TOC (3%–4%), a high carbonate mineral content (10%–35%), and a high gas content (>7 m³/t). The correlation between the total porosity of shale and the density of high-frequency laminations is the strongest, indicating that the silt laminations have a positive effect on pore preservation. There is a significant positive correlation between carbonate content and the volume of mesopores and macropores, as well as the porosity of inorganic pores. It is suggested that carbonate minerals are the main carrier of inorganic pores in Well Z212, and the pores are mainly composed of mesopores and macropores. Under the condition of being far away from the fault zone, even near the denudation area, it has good shale gas preservation characteristics. The key development technologies consist of integrated geo-steering technology, acidification, and volume fracking technology. Based on geological characteristics, the fracturing process optimization of Well Z212 has achieved shale reservoir stimulation. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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26 pages, 14460 KB

Open AccessArticle

Identifying the Key Control Factors of Deep Marine Shale Gas Reservoirs: A Case Study on Lower Cambrian Fine-Grained Sedimentary Rocks in Cen Gong, Guizhou, China

by Dahai Wang, Lichi Ma, Qian Yu, Tao Zhang, Jian Bai, Chuan An, Chuntang Li and Jun Peng

Minerals 2025, 15(5), 505; https://doi.org/10.3390/min15050505 - 9 May 2025

Viewed by 1079

Abstract

This study identifies the following three key factors controlling shale gas reservoirs in the lower Cambrian Niutitang Formation, northern Guizhou, China: sedimentary features, diagenetic modification, and stable tectonic conditions. This research addresses gaps in previous studies by investigating how tectonic and diagenetic conditions [...] Read more.

This study identifies the following three key factors controlling shale gas reservoirs in the lower Cambrian Niutitang Formation, northern Guizhou, China: sedimentary features, diagenetic modification, and stable tectonic conditions. This research addresses gaps in previous studies by investigating how tectonic and diagenetic conditions contribute to the unique characteristics of shale gas enrichment in tectonically complex areas with high thermal maturity (R_o > 2.5%). Sedimentary conditions revealed a positive correlation between total organic carbon (TOC) content and gas adsorption capacity, with higher TOC enhancing adsorption. Experimental data indicate that the TOC content (2.33%–9.07%) significantly correlates with methane adsorption capacity (Langmuir volume VL = 1.87–8.78 cm³/g at 30 °C and 10 MPa), as evidenced by the linear relationship between TOC and VL in shale samples. Clay mineral content exhibited a dual role as moderate levels (15%–25%) improved adsorption, while excessive amounts (>30%) reduced efficiency due to pore occlusion. Diagenesis, including compaction, cementation, and thermal evolution of organic matter, significantly reshaped reservoir porosity. Quantitative analysis of core samples demonstrates that compaction caused a porosity reduction of 18%–25% in samples with burial depths exceeding 1500 m, thereby influencing gas retention capacity. The reservoir has entered the anchizone (average vitrinite reflectance R_o = 2.54%), characterized by advanced organic matter maturation and widespread organic porosity development. Tectonic activity was critical for gas retention; intense tectonic activity led to shallower burial depths and gas loss, whereas structurally stable areas favored preservation. This study emphasizes the significance of tectonic conditions and their role in maintaining gas reservoirs in the anchizone, reconciling discrepancies in gas storage mechanisms observed in basins with similar TOC and thermal maturity. In summary, deep marine shale gas enrichment relies on the synergistic effects of high-quality sedimentary foundations (TOC > 4%, quartz > 30%), diagenetic evolution optimizing pore structures, and stable tectonic conditions ensuring gas retention. These findings provide new insights into the exploration of shale gas in complex tectonic regions and offer a framework for improving prediction models in shale gas enrichment by integrating micro-scale organic–inorganic interactions with macro-scale tectonic controls. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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20 pages, 16179 KB

Open AccessArticle

Source-Reservoir Characteristics and Pore Evolution Model of the Lower Paleozoic Shales in the Neijiang–Rongchang Area, Sichuan Basin

by Shizhen Chen, Zhidian Xi, Fei Huo and Bingcheng Jiang

Minerals 2025, 15(5), 499; https://doi.org/10.3390/min15050499 - 8 May 2025

Cited by 1 | Viewed by 1014

Abstract

The Wufeng–Longmaxi formations in the Sichuan Basin have emerged as China’s principal shale gas exploration target, with drilling results confirming substantial resource potential. Although the Neijiang–Rongchang Block demonstrates promising production, significant performance variations among lithofacies and reservoir types highlight the need for enhanced [...] Read more.

The Wufeng–Longmaxi formations in the Sichuan Basin have emerged as China’s principal shale gas exploration target, with drilling results confirming substantial resource potential. Although the Neijiang–Rongchang Block demonstrates promising production, significant performance variations among lithofacies and reservoir types highlight the need for enhanced understanding of reservoir evolution. This study integrates petrological analyses, SEM imaging, XRD characterization, seismic interpretation, and production data from multiple wells targeting the Wufeng–Long 1-1 Sub-member. Key insights reveal the following: (1) reservoir lithology consists predominantly of siliceous shale (68% occurrence), characterized by high quartz content (48% avg), low carbonates (<15%), and low clay (<30%); (2) organic-rich intervals contain Type I kerogen derived from planktonic algae, with thermal maturity indicating post-mature evolution; (3) premium reservoirs develop multi-scale pore networks combining organic-hosted pores, interparticle pores, and micro-fractures. Despite high brittle mineral content, mechanical competence decreases stratigraphically from the Wufeng Formation (78%) to Long 1-1⁷ (63%); (4) depositional redox conditions facilitated exceptional organic preservation. Core analyses reveal low porosity (5.5% avg) and ultra-low permeability (0.27 × 10⁻³ μm² avg), classifying reservoirs as multiple tight unconventional systems in the study area. The proposed lithofacies-controlled pore evolution model elucidates reservoir heterogeneity mechanisms, providing critical geological criteria for optimized shale gas development. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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22 pages, 112804 KB

Open AccessArticle

Lacustrine Gravity-Flow Deposits and Their Impact on Shale Pore Structure in Freshwater Lake Basins: A Case Study of Jurassic Dongyuemiao Member, Sichuan Basin, SW China

by Qingwu Yuan, Yuqiang Jiang, Zhujiang Liu, Xiangfeng Wei and Yifan Gu

Minerals 2025, 15(5), 473; https://doi.org/10.3390/min15050473 - 30 Apr 2025

Cited by 1 | Viewed by 977

Abstract

In recent years, the successful application of gravity-flow deposit theory in major petroliferous basins in China had attracted extensive attention in the field of sedimentology and had become a key research frontier. This study utilized core, drilling, logging, and microphotograph data, along with [...] Read more.

In recent years, the successful application of gravity-flow deposit theory in major petroliferous basins in China had attracted extensive attention in the field of sedimentology and had become a key research frontier. This study utilized core, drilling, logging, and microphotograph data, along with low-temperature nitrogen adsorption and high-pressure mercury injection experiments. It discussed the characteristics of gravity-flow deposits, sedimentary microfacies, sedimentary models, and the significance of gravity-flow deposits to pore heterogeneity in shale reservoirs, focusing on the first submember of the Dongyuemiao Member (referred to as the Dong 1 Member) in the Fuling area of the Sichuan Basin. The results indicated the development of four types of mudrock in the Dong 1 Member: massive to planar laminated shell mudrock (F1), planar laminated bioclastic mudrock (F2), planar laminated silty mudrock (F3), and massive mudrock (F4). These corresponded to debris flow deposits (F1, F2), turbidite deposits (F3), and suspension deposits (F4). According to the characteristics of lithofacies combinations and sedimentary features, four sedimentary microfacies were identified: gravity-flow channel, tongue-shaped, lobate, and semi-deep lake mud. The Shell Banks were disturbed by earthquakes, tides, storms, and other activities. Silt, clay, fossil fragments, plant debris, and other materials were deposited under the influence of gravity, mixing with surrounding water to form an unbalanced and unstable fluid. When pore pressure exceeded viscous resistance, the mixed fluid became unbalanced, and gravity flow began to migrate from the slope to the center of the lake basin. A sedimentary unit of gravity-flow channel-tongue-shaped-lobate was developed in the Fuling area. The Fuling area’s gravity-flow depositional system resulted in distinct microfacies within the Dongyuemiao Member, each exhibiting characteristic lithofacies associations. Notably, lobate deposits preferentially developed lithofacies F3, which is distinguished by significantly higher clay mineral content (60.8–69.1 wt%) and elevated TOC levels (1.53–2.45 wt%). These reservoir properties demonstrate statistically significant positive correlations, with clay mineral content strongly influencing total pore volume and TOC content specifically enhancing mesopore development (2–50 nm pores). Consequently, the F3 lithofacies within lobe deposits emerges as the most prospective shale gas reservoir unit in the study area, combining optimal geochemical characteristics with favorable pore-structure attributes. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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23 pages, 6741 KB

Open AccessArticle

The Formation of Organic Matter Pores in Shales: Implications from Combined Thermal Heating and Scanning Electron Microscopy Imaging

by Hui Han, Chunchen He, Suqi Xiao, Jintao Chen, Anton Kalmykov, Wenjie Wu, Yixing Wang and Jiang Wang

Minerals 2025, 15(4), 336; https://doi.org/10.3390/min15040336 - 23 Mar 2025

Cited by 2 | Viewed by 1278

Abstract

To reveal the forming process of organic matter pores in shales, an experiment combining thermal heating and scanning electron microscopy (SEM) was conducted on an oil shale sample with a vitrinite reflectance value of 0.46% from the Huadian Formation in the Huadian Basin, [...] Read more.

To reveal the forming process of organic matter pores in shales, an experiment combining thermal heating and scanning electron microscopy (SEM) was conducted on an oil shale sample with a vitrinite reflectance value of 0.46% from the Huadian Formation in the Huadian Basin, northeastern China. The heating temperatures were from 417.8 °C to 700.8 °C, and the corresponding Easy%Ro values were between 1.00% and 3.70%. Four pieces of macerals in the SEM images, including vitrinite (one piece), funginite (one piece), and solid bitumen (two pieces), were observed during the whole heating process. The results showed that organic pores started to appear and increased in all the studied macerals. Each piece of maceral had two rapid growth points of organic matter pores. During heating, organic pores were initially isolated and then became connected. Among the three types of macerals, solid bitumen was more porous, which may be related to the fact that solid bitumen was more easily thermally degraded. Funginite had more pores than vitrinite at all the heating temperatures. Cracks were observed in vitrinite and funginite during heating, and the vitrinite had more cracks, which may be attributed to its stiffness and brittleness. Almost all the organic matter pores were irregular in this study, but bubble-like or sponge-like organic pores have been reported in natural shales. The difference in shapes of organic matter pores may be derived from our experimental system as it cannot consider pressure. These results provide some implications for the mechanism of formation of organic matter pores. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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26 pages, 28443 KB

Open AccessEditor’s ChoiceArticle

Diagenetic Evolution and Formation Mechanism of Middle to High-Porosity and Ultralow-Permeability Tuff Reservoirs in the Huoshiling Formation of the Dehui Fault Depression, Songliao Basin

by Siya Lin, Xiaobo Guo, Lili Li, Jin Gao, Song Xue, Yizhuo Yang and Chenjia Tang

Minerals 2025, 15(3), 319; https://doi.org/10.3390/min15030319 - 19 Mar 2025

Cited by 2 | Viewed by 1532

Abstract

The fluid action mechanism and diagenetic evolution of tuff reservoirs in the Cretaceous Huoshiling Formation of the Dehui fault depression are discussed herein. The fluid properties of the diagenetic flow are defined, and the pore formation mechanism of the reservoir space is explained [...] Read more.

The fluid action mechanism and diagenetic evolution of tuff reservoirs in the Cretaceous Huoshiling Formation of the Dehui fault depression are discussed herein. The fluid properties of the diagenetic flow are defined, and the pore formation mechanism of the reservoir space is explained by means of thin sections, X-ray diffraction, electron probes, scanning electron microscopy (SEM), cathodoluminescence, and stable carbon and oxygen isotopic composition and fluid inclusion tests. The results reveal that the tuff reservoir of the Huoshiling Formation is moderately acidic, and the physical properties of the reservoir are characterized by middle to high porosity and ultralow permeability. The pore types are complex, comprising both primary porosity and secondary porosity, with dissolution pores and devitrification pores being the most dominant. Mechanical compaction and cementation are identified as key factors reducing reservoir porosity and permeability, while dissolution and devitrification processes improve pore structure and enhance pore connectivity. Diagenetic fluids encompass alkaline fluids, acidic fluids, deep-seated CO₊-rich hydrothermal fluids, and hydrocarbon-associated fluids. These fluids exhibit dual roles in reservoir evolution: acidic fluids enhance the dissolution of feldspar, tuffaceous materials, and carbonate minerals to generate secondary pores and improve reservoir quality, whereas alkaline fluids induce carbonate cementation, and clay mineral growth (e.g., illite) coupled with late-stage mineral precipitation obstructs pore throats, reducing permeability. The interplay among multiple fluid types and their varying dominance at different burial depths collectively governs reservoir evolution. This study underscores the critical role of fluid–rock interactions in controlling porosity–permeability evolution within tuff reservoirs. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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20 pages, 6150 KB

Open AccessArticle

Evolution of Permian Sedimentary Environment in South China: Constraints on Heterogeneous Accumulation of Organic Matter in Black Shales

by Weibing Shen, Weibin Shen, Xiao Xiao and Shihao Shen

Minerals 2025, 15(3), 296; https://doi.org/10.3390/min15030296 - 14 Mar 2025

Cited by 3 | Viewed by 1147

Abstract

Permian black shale, as a potential target for marine shale gas exploration in South China, is characterized by its great thickness and organic matter (OM) content. To understand the constraints on the heterogeneous accumulation of OM in Permian black shale, high-resolution geochemical data [...] Read more.

Permian black shale, as a potential target for marine shale gas exploration in South China, is characterized by its great thickness and organic matter (OM) content. To understand the constraints on the heterogeneous accumulation of OM in Permian black shale, high-resolution geochemical data related to paleoenvironment variations are collected on the Gufeng and Dalong Formations of the Putaoling area, the Anhui province, and the Lower Yangtze area. The OM was heterogeneously enriched in the Permian shales, as shown by the highly organic-matter-rich Gufeng Formation and the moderately organic-matter-rich Dalong Formation. The distribution patterns of rare earth elements (REEs) indicate a stably high sedimentary rate throughout the shale deposition. Redox indexes, including Mo_EF, U_EF, V/Sc, and U/Th, indicate anoxic conditions for the deposition of the Gufeng and Dalong Formations, and that seawater oxygenation has occurred. The stratigraphic decreases in the (Fe+Mn)/Ti ratios, the index of chemical alteration (CIA), and the content of nutrient elements demonstrate the upward weakening patterns of hydrothermal activity and chemical weathering, which result in a reduction in the primary production. The redox state combined with the primary production jointly control the heterogeneous accumulation of OM in the Permian shales. Our paleoenvironmental evolution model for OM accumulation in the black shales indicates that the Gufeng Formation might be the priority object for the exploration of shale gases in the Permian strata within the Lower Yangtze area. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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Open AccessArticle

Sedimentary Environment and Organic Matter Enrichment Mechanism of the Silurian Gaojiabian Black Shales in the Lower Yangtze Region, South China

by Wenjuan Wang, Jianwen Chen, Xiuli Feng, Xiangying Ge, Yinguo Zhang, Jie Liang and Lei Xu

Minerals 2025, 15(3), 204; https://doi.org/10.3390/min15030204 - 21 Feb 2025

Cited by 2 | Viewed by 1401

Abstract

A set of organic-rich black shales has developed in the lower Gaojiabian Formation (Lower Silurian) in the Lower Yangtze region, South China. However, limited research on its paleoenvironment and the mechanisms of organic matter enrichment has hindered further analysis of shale gas exploration [...] Read more.

A set of organic-rich black shales has developed in the lower Gaojiabian Formation (Lower Silurian) in the Lower Yangtze region, South China. However, limited research on its paleoenvironment and the mechanisms of organic matter enrichment has hindered further analysis of shale gas exploration prospects in this area. Utilizing samples from the Akidograptus ascensus to Cystograptus vesiculosus graptolite biozones in the Gaojiabian Formation obtained from well SY-1, we analyzed geochemical elements and pyrite framboids to reconstruct paleoenvironmental characteristics and paleoproductivity, revealing factors influencing organic matter enrichment. The results indicate that the total organic carbon (TOC) content and paleoproductivity levels of the shale are both high, with a significant enrichment of redox-sensitive elements. Additionally, the pyrite framboids are well developed, characterized by small particle sizes and a narrow range of variation. This study reveals that during this period, the region generally developed a perennial oxygen minimum zone (P-OMZ) environment, accompanied by transient euxinic conditions during the Akidograptus ascensus and the early stages of Parakidograptus acuminatus and Cystograptus vesiculosus. This situation represents the extension of the Rhuddanian Oceanic Anoxic Event (R-OAE) into the Lower Yangtze region, where the water body exhibited moderate restrictions. Compared to paleoproductivity indicators, there is a stronger positive correlation between TOC and the redox-sensitive elements vanadium (V), molybdenum (Mo), and uranium (U) in the samples. This finding indicates that the P-OMZ and euxinic environments of the Lower Yangtze Sea were the key factors influencing organic matter enrichment in the Akidograptus ascensus to Cystograptus vesiculosus biozones. Consequently, the overall pattern of organic matter enrichment was predominantly determined by preservation conditions. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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18 pages, 11606 KB

Open AccessArticle

Influence of Tectonic Setting and Sedimentary Environment on Organic-Rich Shale in the Wufeng Formation, Fenggang Area

by Qian Zhang, Haiquan Zhang, Yupeng Men, Qian Yu, Junfeng Cao, Yexin Zhou, Xintao Feng, Ankun Zhao and Daorong Zhou

Minerals 2025, 15(2), 191; https://doi.org/10.3390/min15020191 - 19 Feb 2025

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Abstract

Drawing on extensive geological surveys, as well as systematic mineralogical, petrological, and geochemical analyses, the study evaluates the provenance, tectonic setting, paleo-redox conditions, and paleoclimate characteristics of the Wufeng Formation black shale in the Fenggang area. The analysis of mineral components reveals that [...] Read more.

Drawing on extensive geological surveys, as well as systematic mineralogical, petrological, and geochemical analyses, the study evaluates the provenance, tectonic setting, paleo-redox conditions, and paleoclimate characteristics of the Wufeng Formation black shale in the Fenggang area. The analysis of mineral components reveals that quartz content is notably the highest, suggesting that the shale is primarily siliceous. The TOC content was highest in the YI Outcrop, ranging from 2.19 to 5.56, with an average of 3.23, followed by the SX Outcrop. Redox-sensitive indices including Mo_EF, U_EF, V/Cr, and U/Th exhibit considerable variability, indicating significant heterogeneity of the redox conditions, which is primarily characterized as a restricted marine shelf setting. The bottom water of YI Outcrop has the strongest reducing property, and mainly deposited in an anoxic environment. Organic-rich siliceous mudstone is widely distributed across the region. The provenance analysis indicates that the study area is predominantly sourced from felsic igneous rocks. Additionally, paleoclimate and paleo-weathering analyses suggest that the region underwent intense chemical weathering under warm climatic conditions. We found that the sedimentary environment exhibits pronounced spatial variability. In the northern part of the study area, water conditions were deeper, anoxic, and reducing. Toward the east, water depth gradually decreased, transitioning to weakly oxidizing and suboxic conditions. Furthermore, significant tectonic activity in the region led to the formation of multiple underwater highs, indicative of an active continental margin likely associated with the rapid uplift of paleo-uplifts. The formation of organic-rich shale was primarily influenced by two factors: favorable preservation conditions in reducing water bodies and high primary productivity driven by biological activity in weakly oxic environments. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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18 pages, 6792 KB

Open AccessArticle

Organic Matter Accumulation Model of Jurassic Lianggaoshan Shale Under Lake-Level Variations in Sichuan Basin: Insights from Environmental Conditions

by Dong Huang, Minghui Qi, Xiang Deng, Yi Huang, Haibo Wang and Xiawei Li

Minerals 2025, 15(2), 159; https://doi.org/10.3390/min15020159 - 9 Feb 2025

Cited by 5 | Viewed by 1687

Abstract

Organic matter (OM) is the primary carrier for the generation and occurrence of shale oil and gas. The combination of sequence stratigraphy and elemental geochemistry plays a crucial role in the study of organic matter enrichment mechanisms in marine shale, but it is [...] Read more.

Organic matter (OM) is the primary carrier for the generation and occurrence of shale oil and gas. The combination of sequence stratigraphy and elemental geochemistry plays a crucial role in the study of organic matter enrichment mechanisms in marine shale, but it is rarely applied to terrestrial lacustrine basins. As a product of the last large-scale lake transgression in the Sichuan Basin, the Early Jurassic Lianggaoshan Formation (LGS Fm.) developed multiple organic-rich shale intervals, which is a good example for studying the OM enrichment in lacustrine basins. Based on a high-resolution sequence stratigraphic framework, the evolutionary process of terrestrial debris input, redox conditions, and paleo-productivity during the sedimentary period of the Lianggaoshan Formation lacustrine shale at different stages of lake-level variations has been revealed. The main controlling factors for OM enrichment and the establishment of their enrichment patterns have been determined. Sequence stratigraphy studies have shown that there are three third-order lake transgression-lake regression (T-R) cycles in the LGS Formation. The total organic carbon content (TOC) is higher in the TST cycle, especially in the T-R3 cycle, and lower in the RST cycle. There are differences in the redox conditions, paleo-productivity, terrestrial detrital transport, and OM accumulation under the influence of lacustrine shale deposition in different system tracts. The results indicate that changes in lake level have a significant impact on the reducibility of bottom water and paleo-productivity of surface seawater, but have a relatively small impact on the input of terrestrial debris. In the TST cycle, the reducibility of bottom water gradually increases, and the paleo-productivity gradually increases, while in the RST cycle, the opposite is true. Within the TST cycle, the OM accumulation is mainly influenced by paleo-productivity and redox condition of bottom water, with moderate input of terrestrial debris playing a positive role. In the RST cycle, the redox condition of bottom water is the main inducing factor for OM enrichment, followed by paleo-productivity, while terrestrial input flux plays a diluting role, which is generally not conducive to OM accumulation. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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A Mechanistic Framework Linking Climate Forcing, Microbial Transformation, and Sedimentary Carbon Sinks in Deep-Time Oceans

by Jingxuan Zhang, Xi Zhang, Tingshan Zhang and Hao Huang

Minerals 2026, 16(2), 221; https://doi.org/10.3390/min16020221 - 22 Feb 2026

Viewed by 502

Abstract

The ocean constitutes the largest actively exchangeable carbon reservoir in Earth’s surface system, with the ocean–atmosphere system functioning as an integrated entity that modulates atmospheric CO₂ concentrations over geological timescales. While carbonate and organic-rich sedimentary carbon sinks have been the subject of [...] Read more.

The ocean constitutes the largest actively exchangeable carbon reservoir in Earth’s surface system, with the ocean–atmosphere system functioning as an integrated entity that modulates atmospheric CO₂ concentrations over geological timescales. While carbonate and organic-rich sedimentary carbon sinks have been the subject of extensive research, their synergistic roles in long-term carbon–climate feedback loops, as well as the degree to which microbial mediation links ocean hydrographic states to basin-scale carbon sequestration efficiency, remain poorly synthesized. Here, we develop a mechanistic framework comprising five intercoupled components: (1) driving factors (tectonic–climatic forcing and anthropogenic analogs); (2) ocean state controls (basin restriction, water column stratification, and redox conditions); (3) microbial processes (microbial carbon pump-mediated transformation of dissolved organic carbon and the modulating influence of microbial carbonate formation); (4) sedimentary carbon sinks (carbonate platforms versus organic-rich shales underpinning organo-mineral stabilization); and (5) Earth system feedback expressions (e.g., carbon isotope excursions and sustained perturbations in atmospheric CO₂ levels). This framework is validated across three contrasting sedimentary basins, including the Western Tethys rift basins, the Cambrian South China platform system, and the Toarcian Lower Saxony restricted basin, and via three falsifiable propositions. Converging evidence from these case studies corroborates three key conclusions: (1) basin restriction and diminished water mass renewal foster water column stratification and hypoxic/anoxic conditions, thereby enhancing organic carbon preservation (P1); (2) the tectonic and depositional setting of a basin modulates the relative predominance of carbonate and organic carbon sinks (P2); and (3) post-extinction anachronistic facies record amplified microbial control over carbon burial pathways (P3). By emphasizing the context dependence of carbon sequestration processes and the significance of organo-mineral stabilization alongside particulate organic carbon export, this synthesis provides a transferable analytical framework for interpreting deep-time carbon cycle transitions and for contextualizing the impacts of modern ocean warming and deoxygenation on natural carbon sinks. Full article

(This article belongs to the Special Issue Element Enrichment and Gas Accumulation in Black Rock Series)

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