Search Results (19)

The enrichment of organic matter in marine shale is a complex process involving tectonic–sedimentary interactions. The tectonic setting exerts critical control over sediment provenance, marine biota, and subaqueous environmental conditions in shale deposition. To unravel the mechanisms and differential controls of organic matter accumulation in marine shales across distinct tectonic regimes, this study systematically examines the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation shales in the Upper Yangtze Block, SW China. Through comprehensive geochemical analyses encompassing total organic carbon (TOC) contents, as well as major and trace elements conducted on 31 shale samples from the Niutitang Formation and 30 samples from the Longmaxi Formation, we characterized their depositional environmental features and compared the distinctions between them. The results indicate that both the Cambrian Niutitang Formation and Silurian Longmaxi Formation shales exhibit high TOC contents, which range from 1.04% to 8.83% (average 4.73%) and from 0.29% to 6.14% (average 3.35%), respectively. Paleoenvironmental proxies demonstrate that the Cambrian Niutitang shales developed under suboxic–anoxic to even sulfidic conditions, with moderate water restriction and high paleoproductivity levels, while the Silurian Longmaxi Formation was deposited under suboxic–anoxic environments with strong water restriction and low-to-moderate paleoproductivity. Organic matter enrichment in the Cambrian Niutitang Formation followed a “productivity + preservation model”, whereas the Silurian Longmaxi Formation primarily adhered to a “preservation-dominated model”. The differentiation in organic enrichment mechanisms between these two marine sequences is attributed to the distinct tectonic settings during their deposition. During the Early Cambrian, the Upper Yangtze Block was in a rift trough tectonic setting influenced by upwelling currents, which triggered algal blooms and subsequent bacterial sulfate reduction (BSR) coupled with marine anoxia and sulfidation. In contrast, the Early Silurian period featured a semi-restricted marine basin with weaker upwelling activity, where organic matter enrichment was predominantly controlled by a restricted, reducing water column. Our findings demonstrate that tectonic settings exert fundamental controls on nutrient availability for algal communities and water column retention levels, serving as critical determinants for organic enrichment processes in marine shale systems. Full article

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

Fractal analysis was used to characterize the organic matter nanopore structure in tectonically deformed shales, providing insights into the heterogeneity and complexity of the pore network. Shale samples from different tectonic deformation styles (undeformed, brittle deformed, and ductile deformed) in the Lower Cambrian Niutitang Formation in western Hunan, South China, were collected. By comprehensively applying techniques such as low-temperature gaseous (CO₂ and N₂) adsorption (LTGA), scanning electron microscopy (SEM), and ImageJ analysis, we accurately obtained key parameters of the pore structure. The results show ductile deformation reduces fractal dimension (D_M) by ~0.2 compared to brittle deformed shale, reflecting the homogenization of organic nanopore structures. Brittle deformation leads to a more complex pore network, while ductile deformation reduces the complexity of the organic nanopore structure. The fractal dimensions are affected by various factors, with micropore development being crucial for undeformed shale, clay and pore length–width ratio dominating in brittle deformed shale, and all-scale pores being key for ductile deformed shale. This study provides the first comparative analysis of fractal dimensions across undeformed, brittle deformed, and ductile deformed shales, revealing distinct pore structure modifications linked to deformation styles. These findings not only enhance our understanding of the influence mechanism of tectonic deformation on shale pore structure and fractal characteristics but also provide a theoretical basis for optimizing shale gas exploration and production strategies. These findings offer a framework for predicting gas storage and flow dynamics in tectonically complex shale reservoirs. For instance, in areas with different tectonic deformation styles, we can better evaluate the gas storage capacity and production potential of shale reservoirs according to the obtained fractal characteristics, which is of great significance for efficient shale gas development. Full article

To analyze the pore structure and fractal characteristics of marine shale in the lower Cambrian Niutitang Formation in northwestern Hunan Province, China, the pore characteristics of shale were characterized using total organic carbon (TOC) content, field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), low temperature nitrogen adsorption (LT-N₂GA) and methane adsorption experiments. The pore surface and pore space fractal dimensions of samples were calculated, respectively. The influencing factors of fractal dimensions and their impact on the adsorption of shale reservoirs were discussed. The results indicate the Niutitang Formation shale mainly develops four types of pores: organic pores, intragranular pores, intergranular pores and microcracks. The pores have a large specific surface area (SSA), primarily consisting of mesopores. The fractal dimensions are calculated using the FHH model and the XS model. The fractal dimensions (D₂ and D_f) are greater than D₁, indicating that the pore surface with larger pore size is rougher, and the pore structure of shale is complex. The pore volume (PV), SSA, and TOC show positive correlations with the fractal dimensions but negative correlations with APS. There is no obvious correlation between fractal dimensions and quartz content, while clay minerals show a negative correlation with D₂ and D_f. This is mainly because clay mineral particles are small in size and have weak resistance to compaction. The pyrite content is positively correlated with the fractal dimensions because pyrite promotes the development of organic, intergranular, and mold pores. According to Pearson correlation analysis, the main influencing factors of the pore surface fractal dimension are PV, SSA, and APS. The main influencing factors of the pore space fractal dimension are APS and the content of clay minerals. Further analysis of the influence of the fractal dimension on the adsorption capacity of shale reveals that the fractal dimensions are positively correlated with Langmuir volume, indicating that fractal dimensions can be used as a quantitative target for evaluating shale gas reservoirs. Full article

Polymetallic enrichment layers are commonly found at the base of the Lower Cambrian and extensively distributed across the Upper Yangtze Platform, yet their genetic models remain controversial. This study systematically collected samples from a typical section in the southeastern Chongqing region for mineral, organic, and inorganic analyses. It investigates the relationship between the abundance of various trace metal elements and organic matter at the base of the Niutitang Formation, as well as the vertical distribution characteristics of organic carbon isotopes and organic matter features. The results indicate that the Niutitang Formation shale exhibits a distinct three-part structure from bottom to top. Various metal elements are enriched in the lower interval, showing a close correlation between the abundance of polymetallic elements and the carbon isotopes of shale organic matter. The middle interval contains the highest TOC value and the lowest Ti/Al ratio, while the upper interval shows a significant decrease in organic matter abundance, with a clear positive correlation between the excess silicon content and Ti/Al ratio. Additionally, the mixing effect of deep-sea upwelling is the primary control on the formation of polymetallic enrichment layers in the lower interval, followed by the adsorption of organic matter under anoxic conditions. The sedimentary environment of the upper interval of the Niutitang Formation trends toward oxidation, with paleoclimate shifting toward colder and drier conditions, exhibiting aeolian sedimentary features that are unfavorable for the enrichment of trace metal elements. Consequently, upwelling is a key factor in the enrichment and mineralization of trace metal elements at the base of the Lower Cambrian in the Upper Yangtze region. Full article

Western Hunan province and its surrounding areas are significant targets for shale gas exploration and development in southern China, where the black shale of the lower Cambrian Niutitang Formation and Wunitang Formation is extensively distributed. Geochemical analysis was conducted on the lower Cambrian black shale from a new exploration well of XAD1 located at the southeast margin of the Yangtze paraplatform, followed by a discussion on gas-bearing properties using molecular dynamics simulation. The geochemical characteristics indicate that the black shale in well XAD1 was primarily deposited in a strongly reducing marine environment, with organic matter predominantly composed of type I kerogen derived from algae. Currently, it has reached a stage of high to over maturity with limited potential for liquid hydrocarbon generation. The recovery of the original hydrocarbon generation potential shows that they are excellent source rocks and have completed the main hydrocarbon generation evolution. Despite the favorable conditions for shale gas formation observed in well XAD1, the low measured gas content within the Niutitang Formation suggests that other geological factors may have contributed to a substantial loss of shale gas. Gas adsorption simulation reveals that the maximum methane adsorption capacity (15.77 m³/t) was achieved by Niutitang shale during the late Silurian period when there was an abundant source of natural gas without any influence from CO₂, H₂O or other molecules. However, due to a lack of natural gas replenishment and subsequent tectonic uplift and subsidence causing variations in temperature and pressure, the methane adsorption capacity gradually decreased (to 6.56 m³/t). Furthermore, water occurrence within the shale reservoir further reduced the methane adsorption capacity (below 2 m³/t), while tectonic activities exacerbated the loss of shale gas potential within this study area. The findings indicate that the dynamic alteration of gas-bearing properties in shale reservoirs due to tectonic movements is a crucial factor influencing the success rate of shale gas exploration in the study area, provided that there are sufficient gas resources and superior reservoir conditions. Full article

The Lower Cambrian Niutitang Formation in the Northern Guizhou harbors abundant organic-rich mud shale, constituting the most significant marine shale gas reservoir in Guizhou. In this article, the reservoir characteristics of Lower Cambrian Niutitang Formation in Northern Guizhou are analyzed in terms of lithology, mineralogy, organic geochemistry, pore structure, gas content and continuous thickness of shale, and the exploration potential of shale gas in this area is evaluated. The results indicate that the content of brittle minerals in the shale of well QX1 is 65.29% to 95.22% (average of 82.10%). The total organic carbon (TOC) content ranges from 2.06% to 12.10% (average of 5.64%). The organic matter maturity (R_o) within the range of 2.29–2.67%, and the kerogen type is identified as type I. The shale samples from the Niutitang Formation have high TOC content, suitable thermal maturity, and a favorable kerogen type, suggesting good gas generation potential. The results of scanning electron microscopy (SEM) show that intergranular pores, intragranular pores and microfractures are developed in the shale of well QX1, which can provide sufficient storage space for shale gas. The shale exhibits a continuous thickness of 105.66 m in the QX1 well, comprising a gas-bearing interval of 32.89 m at the top (with an effective continuous thickness of 18 m) and a hydrocarbon source rock layer of 75.78 m at the bottom. In comparison with other shale gas regions, Niutitang Formation shale in Northern Guizhou exhibits characteristics such as favorable gas generation conditions, greater storage conditions, excellent gas-bearing, strong frackability, and substantial continuous thickness, it has greater potential for shale gas exploration. Full article

Shale gas production is obviously higher within the Silurian Longmaxi Formation than that of the Cambrian Niutitang Formation according to the drilling test results in the northwest Hunan area. To clarify the reasons behind this variation, core samples from the two sets of shales were studied for a comprehensive comparison and analysis of their organic matter (OM) pore structure. Methods were used, including the total organic carbon content test, the vitrinite reflectance test, X-ray diffraction, and focused ion-beam scanning electron microscopy (FIB-SEM). The results show that these two shales have similar reservoir characteristics, both with abundant organic matter and high content of brittle minerals. However, the Longmaxi shale with 2.3% to 3.0%Ro presents lower thermal maturity than the Niutitang shale with over 3.0%Ro. In the case of pore structure associated with OM, a huge difference exists between the two shales. The OM pore shape of the Longmaxi shale is very regular, being mostly round and oval, while the OM pore shape of the Niutitang shale is irregular, being flat with a thin middle and thick and elongated ends. An important factor affecting OM pore evolution is thermal maturity. In turn, the thermal maturity is controlled by the tectonic evolution process, especially the maximum paleo-burial depth. In conclusion, the paleo-burial depth of the Lower Cambrian Niutitang shale in northwest Hunan is too large, which leads to the excessive evolution of organic matter in the shale, and the physical and chemical properties are similar to graphite, which leads to the disappearance of OM pores. Shale gas has no effective reservoir space and is largely dispersed in geological history. At the same time, due to the insufficient hydrocarbon-generation evolution time of the Lower Silurian Longmaxi Formation shale in this area, the shale could not form enough gaseous hydrocarbons and a large amount of effective reservoir space (OM pores with regular shape and large pore size), and finally failed to become a gas reservoir. Therefore, the exploration and development potential of the marine shale gas of the Lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation in northwest Hunan is poor. Full article

The early Cambrian witnessed profound environmental changes and biological evolution in Earth’ history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Ba_xs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low Co_EF × Mn_EF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform. Full article

A shale lithofacies scheme is commonly used to characterize source rock reservoirs of the Lower Cambrian Niutitang Formation. However, this classification ignores that individual components such as quartz may have different origins, potentially affecting reservoir quality. The main objective of this article is, therefore, to present a refined scheme for lithofacies and an image processing workflow for the detection of quartz types in the Niutitang Formation shales from the Jiumen outcrop in the Guizhou Province (Upper Yangtze Basin, SW China). In order to do so, a combination of bulk density, optical and scanning electron microscopy and image analysis was used. The shale lithology was macroscopically classified into seven major categories and nineteen subcategories. Subsequently, the shales were investigated at the microscopic level, mainly focusing on quartz types and microstructural variations. Afterwards, the workflow to calculate the weight per unit volume (1 cm³) of the quartz types was presented, i.e., firstly, by calculating the weight of mineral matter by subtraction of the measured weight of organic matter from the bulk shale; secondly, by calculating the weight of total quartz in bulk shale from the weight of mineral matter and its proportion calculated from X-ray diffraction data; thirdly, by calculating the weight of detrital quartz and non-detrital quartz with energy dispersive X-ray mapping, image processing and quartz density; finally, by calculating the weight of clay-sized quartz by subtracting of the weight of detrital and non-detrital quartz from the weight of the total quartz. The bulk quartz content was found to be dominated by clay-sized quartz, which may mainly control the mesopore volume available for gas storage and, hence, the shale gas reservoir development. Full article

There are as many as 25 kinds of minerals (including non-ferrous metals, ferrous metals, rare and dispersed elements, precious metals, non-metallic and energy minerals) enriched in uranium-polymetallic fertile beds in black rock series, which is therefore widely attracting scholars all over the world. However, there is still great controversy in terms of the metallogenic mechanism in such beds. The black rock series have been systematically sampled from the Baizhuyu deposit in northwestern Hunan Province, China based on field geological and radioactivity surveys. Major and trace elements as well as rare earth elements (REE) of uranium-polymetallic phosphorite and its wall rocks were analyzed. Furthermore, carbon and oxygen isotopes, Sm-Nd isotopes, and mineralogy of the Baizhuyu deposit were studied. The results show that dolomite is a normal marine sediment, while and uranium-polymetallic elements were pre-enriched in phosphorites and black carbonaceous argillaceous shales and slates that formed from marine sedimentation and submarine exhalative sedimentation. Hydrothermal reworking to uranium-polymetallic phosphorites is significant as a result of submarine exhalative sedimentation. The research results of this paper can support a better understanding of metallogenesis and the future exploration of uranium-polymetallic phosphorite in the Lower Cambrian Niutitang Formation in the study area. Full article

The lower Cambrian Niutitang/Qiongzhusi shale gas in the Middle-Upper Yangtz Block had been regarded as a very promising unconventional natural gas resource due to its high total organic carbon, great thickness, and large areal distribution. However, no commercial shale gas fields have yet been reported. From the northwest to the southeast there are considerable differences in the sedimentary environments, lithology, and erosive nature of the underlying interval (the floor interval) of the Niutitang shale. However, systematic research on whether and how these regional differences influence shale petrophysical properties and shale gas preservation in the Niutitang shale is lacking. A comparison of Niutitang shale reservoirs as influenced by different sedimentary and tectonic backgrounds is necessary. Samples were selected from both the overmature Niutitang shales and the floor interval. These samples cover the late Ediacaran and early Cambrian, with sedimentary environments varying from carbonate platform and carbonate platform marginal zone facies to continental shelf/slope. Previously published data on the lower Cambrian samples from Kaiyang (carbonate platform), Youyang (carbonate platform marginal zone) and Cen’gong (continental shelf/slope) sections were integrated and compared. The results indicate that the petrophysical properties of the floor interval can affect not only the preservation conditions (sealing capacity) of the shale gas, but also the petrophysical properties (pore volume, porosity, specific surface area and permeability) and methane content of the Niutitang shale. From the carbonate platform face to the continental shelf/slope the sealing capacity of the floor interval gradually improves because the latter gradually passes from high permeability dolostone (the Dengying Formation) to low permeability dense chert (the Liuchapo Formation). In addition, in contrast with several unconformities that occur in the carbonate platform face in the northern Guizhou depression, no unconformity contact occurs between the Niutitang shale and the floor interval on the continental shelf/slope developed in eastern Chongqing Province and northwestern Hunan Province. Such regional differences in floor interval could lead to significant differences in hydrocarbon expulsion behaviour and the development of organic pores within the Niutitang shale. Therefore, shale gas prospects in the Niutitang shales deposited on the continental shelf/slope should be significantly better than those of shales deposited on the carbonate platform face. Full article

The characteristics of shale micro-pore development and its main influencing factors have important theoretical guiding significance for shale gas exploration and resource evaluation. In order to clarify the micro-pore development characteristics of lower Cambrian shale and the main controlling factors of micro-pore development, we used the lower Cambrian Niutitang formation shale, in the Wenshuicun section of the Guizhou Province in southwest China. The micro-pore development characteristics of the shale in the region were studied by argon ion profile field emission scanning electron microscopy and a low-temperature liquid nitrogen adsorption and desorption experimental system. The relationship between micro-pore and kerogen maceral composition, total organic carbon (TOC) content and different mineral content was analyzed in combination with mineral and geochemical characteristics. Inorganic pores (clay mineral pores, dissolution pores and pyrite intergranular pores) and micro-fractures (clay mineral shrinkage crack, tectonic fractures and overpressure fractures) were the main type of pore developed in the shale of the Niutitang formation in the Wenshuicun section, and no organic pores had developed. The pore size of shale is usually 2–50 nm, accounting for 58.33% of shale pores, e.g. mesopores. Clay mineral content has an obvious positive correlation with macropore volume and average pore diameter, and an obvious negative correlation with micropore volume. In addition, the content of feldspar in brittle minerals has a strong negative correlation with macropore volume and average pore diameter, and a strong positive correlation with micropore volume and BET-specific surface area. TOC content and the content of different kerogen macerals have no obvious correlation with the development of shale micropores in this region. It is concluded that inorganic mineral composition is the main controlling factor of micro-pore development within lower Cambrian shale, and organic matter abundance and maceral content have little influence on the micro-pore development. This study provides a case study for the characteristics of micropores in lower Cambrian shale in China. Full article

Investigating the impacts of rock composition on pore structure is of great significance to understand shale gas occurrence and gas accumulation mechanism. Shale samples from over-mature Niutitang formation of Lower Cambrian in south China were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low pressure N₂ and CO₂ adsorption to elucidate the controls of distinct mineral composition on pore development. Two distinct lithofacies, namely siliceous shale and argillaceous shale, were ascertained based on their mineral composition. Due to the variability of mineral composition in different lithofacies, pore structure characteristics are not uniform. Pores in siliceous shales are dominated by interparticle pores and organic matter (OM) pores, among which the interparticle pores are mainly developed between authigenic quartz. Furthermore, most of these interparticle pores and cleavage-sheet intraparticle pores within clay minerals are usually filled by amorphous organic matter that is host to OM pores. Due to the lack of rigid minerals, argillaceous shale was cemented densely, resulting in few interparticle pores, while cleavage-sheet intraparticle pores within clay minerals are common. Comparing siliceous shales with argillaceous shales, specific surface areas and pore volumes are higher on the former than on the latter. The content of total organic carbon (TOC) and authigenic quartz have a great influence on micropore structures, but less on mesopore structure for siliceous shales. The rigid framework structure formed by authigenic quartz is believed to be able to prevent primary interparticle pores from mechanical compaction and facilitate the formation of organic matter-associated pores. In terms of argillaceous shales, due to the lack of authigenic quartz, interparticle pores were rarely developed and its pore structure is mainly controlled by illite content. Full article

The shale of the lower Cambrian Niutitang formation in northwestern Hunan is an ideal reservoir for shale gas. There is a close connection between borehole stability and drilling fluid in shale gas drilling. Ionic stabilizer is a new type of stratum consolidation agent that inhibits the hydration expansion of clay minerals and improves mechanical strength of the borehole. The traditional idea of pore wall protection is to use drilling fluid additives to prevent shale from interacting with water. However, ionic stabilizer can change the hydrophilic of clay minerals in shale, making the particles become hydrophobic and dense, therefore, the formation stability can be enhanced simultaneously. The material used in this paper is different from the normal ionic stabilizer, some chemical bonds that have been changed in the new material called enhanced normality ionic (ENI) stabilizer. This paper utilized the shale samples those obtained from Niutitang formation to study the connection between ENI and the mechanical properties of shale. Mechanical tests and microscopic pore tests were performed on different samples which were soaked in water and the ENI with different concentrations. It has been found through tests that ENI can inhibit the development of shale pores, and as the concentration increases, the inhibition increases. In addition, as the ENI concentration increases, the uniaxial compressive strength and Young’s modulus of the shale increase, and the ratio of stability coefficients decreases. It can be concluded that the ENI can improve the mechanical strength of carbon shale, and prevent the development of rock damage. Moreover, it can improve the ability of rock to resist damage, and enhance borehole stability initiatively. Full article