Niutitang Formation Research Articles

Automatic pore structure analysis in organic-rich shale using FIB-SEM and attention U-Net

The pore structure is an important factor in determining the productivity of shale, which can help us make better operational decisions. Shale formations have an extremely complex pore structure with a wide range of pore sizes, shapes, orientations, and connectivity. Advanced imaging techniques, such as focused ion beam scanning electron microscopes (FIB-SEM), contribute significantly to the analysis and understanding of complex pore networks in shale, but they are time-consuming. Developing new methods and techniques to capture the spatial distribution and connectivity of pores at various scales is essential to understanding pore structure and its impact on fluid flow. This study aims to analyze the pore characteristics of shale reservoirs by using artificial intelligence (AI) and FIB-SEM. We applied a novel attention gate (AG) model to FIB-SEM images that automatically learns to focus on target structures of varying pores’ shape and size. AG can be fully incorporated into the U-Net model with minimal computational impact. The AG improved prediction accuracy by making the U-Net model more sensitive to essential features in the input images and achieved a precision of 97%, recall of 71%, and F1 score of 81% at a confidence threshold of 1. Experimental results show that Attention U-Net outperformed in predicting the pore structures in shale samples from Longmaxi Formation, Qingshankou Formation, Cambrian Niutitang Formation, Yanchang shale oil, and Bakken shale while obtaining optimal performance without requiring multiple convolutional neural network (CNN) models. Additionally, Attention U-Net addresses some of the limitations of the Edge-Threshold Automatic Processing (ETAP) method, including the limited ability to obtain a complete description of the pore space as well as the low segmentation accuracy and sensitivity. The proposed workflow offers a promising solution for optimizing the automatic processing of microscopic images for pores and microfractures identification. Researchers and industry professionals can improve their understanding of shale properties and applications in various fields by maximizing the benefits of our workflow.

Fracture energy evolution and fractal characteristics of shales with different quartz contents

Quartz is an important mineral component of shale. In order to explore the influence of quartz content on the mechanical properties of shale, this article conducted numerical simulation of the Lower Cambrian Niutitang Formation shale in Fenggang Block 3, Guizhou using RFPA software. The results show that the effect of quartz content on compressive strength is more significant than that on elastic modulus. The damage modes of shale with different quartz content are presented as 5 types, namely, oblique tension damage (21%, 27%), inverted N-shaped damage (33%), V-shaped damage (39%), inverted V-shaped damage (45%), and λ-shaped damage (51%). When the content of quartz is higher than a certain value, the microscopic damage of shale tends to be more complicated. The destruction acoustic emission energy and fractal dimension show significant difference with quartz content. The research results of this article will play a certain role in promoting shale gas exploitation. Highlights The influence of quartz content on the mechanical properties of shale is numerically simulated. The energy evolution law of shale destruction in the presence of quartz is analyzed. The complexity of shale destruction caused by quartz content is analyzed. It is proven by a similarity test that shale changes with different mechanical properties of quartz content.

Hg isotope evidence for oceanic oxygenation during the Cambrian Explosion

The Cambrian explosion is a critical evolutionary milestone in life history, but the mechanistic relationship between the Cambrian explosion, rising atmospheric and oceanic oxygen levels, and the primary productivity remains controversial. Here, we present new mercury (Hg) isotope data from Cambrian marine sediments of the ∼521–514 Ma Yu’anshan and Shuijingtuo Formations (Nanhua Basin, South China), which preserve the famous ∼518 Ma Chengjiang and Qingjiang Cambrian Biotas, respectively. We find evidence that, prior to Cambrian animal diversification, local terrestrial and/or atmospheric inputs are important drivers of Hg enrichment in the shallow shelf Yu’anshan and Shuijintuo Formations. Elevated Hg and total organic carbon (TOC) concentrations of coeval deeper shelf and slope sections of the Niutitang Formation are mainly attributed to upwelling of Hg associated with dissolved organic carbon (Hg-DOC) from pelagic seawater, unrelated to locally high primary productivity. During the deposition of fossiliferous strata, negative shifts of Δ199Hg and δ202Hg values in the shallow water shelf shales from the Shuijingtuo Formation, combined with similar Hg isotope characteristics in the deeper shelf and slope shales, demonstrate a significant shallow and deep-water oxygenation event took place in the Nanhua Basin. A comparable negative shift of Δ199Hg values occurs in coeval Indian Craton sediments, suggesting this oxygenation event could be regional, or possibly even global, in scale. Our newly-collected Hg isotope data provide strong evidence that rising molecular oxygen levels in both surface and deep seawater are associated with enhanced marine primary productivity and could be a critical driver of the Cambrian explosion. Finally, we also argue that Hg isotopes are an emerging and promising redox proxy for studies of Precambrian seawater, but the chemical response of Hg to atmospheric and ocean oxygenation requires further calibration.

Shale Gas Exploration and Development Potential Analysis of Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation in Northwestern Hunan, South China, Based on Organic Matter Pore Evolution Characteristics

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.

The main controlling factors of tensile strength in sight of shale reservoir under horizontal bedding: Example of the Lower Paleozoic Niutitang Formation shale from Micangshan, China

Understanding the mechanical characteristics of marine shale during fracturing is essential for shale gas development, and its core scientific problem is what factors in shale control its mechanical properties. The 12 shale samples from the Lower Paleozoic Niutitang Formation in Micangshan are tested for tensile strength and examined using X-ray diffraction, low-field nuclear magnetic resonance (NMR), EA2000 elemental analyzer, and scanning electron microscopy to explore the main controlling factors of shale tensile strength under horizontal bedding conditions. The findings are as follows. (1) The tensile strength of the shale is relatively high, ranging from 10.05 MPa to 20.34 MPa. Quartz is the largest proportion of the shale minerals, accounting for 53.2 wt%–59.0 wt%, followed by anorthose and clay minerals. Total organic carbon (TOC) concentration ranges from 1.7 wt% to 4.1 wt%. (2) NMR results indicate that the pore structure of shale is mainly mesoporous, accounting for 75.76%–88.03%, followed by macropores (12.57%–21.24%) and micropores (0.68%–4.91%). Low-pressure nitrogen adsorption and desorption results indicate that the average pore diameter of shale is 12.58–16.02 nm, which is basically consistent with NMR results. The negative correlation between fractal dimension D2 and tensile strength indicates that the higher the tensile strength of the shale, the lower the complexity of its seepage pores. (3) Micropores occur mainly in clay minerals, whereas quartz indicates positively correlation with mesoporous content. The higher the proportion of mesopores, the lower the tensile strength. This indicates that the mesopores are the main factor controlling the tensile strength, and the quartz content in minerals is a secondary factor restricting the tensile strength. TOC has little controlling action on the tensile strength. This contribution provides a theoretical basis for shale fracturing.

The importance of pore-fracture connectivity in overmature marine shale for methane occurrence and transportation

Pore-fracture connectivity is a critical factor in gas occurrence and transportation in shale plays. To investigate the limiting factors of multiscale pore-fracture connectivity, which might control methane occurrence and transportation in shale reservoirs, small-angle neutron scattering (SANS) under vacuum and high-pressure conditions incorporated with repeated mercury intrusion capillary pressure (MICP), and field emission-scanning electron microscopy (FE-SEM) imaging after Wood's metal (WM) impregnation were performed on overmature marine shale samples of the Wufeng-Longmaxi and Niutitang Formations from south China. Based on the electron micrographs of the WM-injected samples, it was found that the sealing of the pore system by brittle minerals is the root cause for a reduction in overall pore connectivity within the shale matrix. Aside from being favorable to the preservation of organic and clay mineral pores, brittle minerals however, resulted in the closure of pore network, thus, forming an isolated pore system. The novel approach of integrating repeated MICP measurement and FE-SEM imaging after WM impregnation provides a new evaluation method for pore-fracture connectivity of shale. The minimum fraction of pores within the overmature shales accessible to methane was at 38% and 78% within 100 nm. Moreover, because of confinement effects, the density of methane in a pore space less than 20 nm under high pressure was greater than that of an ideal gas under the same pressure conditions which even increased with decreasing of pore size to several times of its inert state and formed nanoscale methane clusters. The combination of contrast-matching SANS (CM-SANS) and repeated MICP measurements can effectively reveal the connectivity of pores within the matrix and the connectivity between such pores and microfractures, which helps to evaluate the distribution of closed pores and dominant transport spaces of gas in shale reservoirs. Overall, the pore-fracture connectivity in overmature marine shale controls the amount of gas that is supplied from matrix pores into fracture systems and also affects the density of the gas in pore spaces within the matrix.

First confident evidence of moulting in eodiscid trilobites from the Cambrian Stage 3 of South China

Abstract Trilobite moulting behaviour has been extensively investigated. However, exuviae in eodiscid trilobites are poorly known. Here, we report two eodiscid trilobite specimens, Tsunyidiscus niutitangensis and Tsunyidiscus sp., showing Somersault configuration from the Niutitang Formation and Mingxinsi Formation of South China, respectively (Cambrian Series 2, Stage 3). The arrangements of the exoskeletons indicate that the two specimens are the slightly disturbed and undisturbed exuviae. The impression of the lower cephalic unit (LCU) displays the rostral plate in Tsunyidiscus niutitangensis. The exuviae showing the LCU inverted anteriorly under the trunk. The opening of the facial and rostral sutures would have allowed the emergence of the post-ecdysial trilobite with the partial enrolment of exoskeleton. Moreover, our discovery indicates a Somersault configuration which employed the facial and rostral sutures to create an anterior exuvial gape that also exists in eodiscid trilobites besides redlichiid trilobites, corynexochid trilobites and ptychopariid trilobites during the Cambrian.

Pyrite Concretions in the Lower Cambrian Niutitang Formation, South China: Response to Hydrothermal Activity

Pyrite Concretions in the Lower Cambrian Niutitang Formation, South China: Response to Hydrothermal Activity

The organic petrology of vitrinite-like maceral in the Lower Paleozoic shales: Implications for the thermal maturity evaluation

Vitrinite-like maceral (VLM) is a type of autochthonous, synsedimentary, dispersed organic matter, which is ubiquitous in many Lower Paleozoic marine shales. The VLM reflectance has been used as an alternative tool to evaluate the thermal maturity of pre-Devonian marine sediments lacking a classical vitrinite, however, the existing knowledge about its optical properties and reflectance characteristics is still relatively limited. In this study, the optical properties of VLM were examined by means of organic-petrological methods applied to a series of naturally and artificially matured VLM-rich shales samples and their reflectance crossplots. The strong positive correlation between the true maximum reflectance (Rmax) and bireflectance of VLM indicates the increasing anisotropy of VLM with increasing thermal maturity. VLM shows a linear coalification path in the studied shales. It displays biaxial neutral features in the samples with Rmax ≤ 2%, but shows obvious optically biaxial negative characteristics in the samples with Rmax > 2%. The shape of VLM reflectance indicating surface (VLMRIS) gradually becomes oblate with increasing maturity. These results indicate that VLM exhibits appearance and optical properties similar to vitrinite, and its reflectance can be used as a reliable indicator for thermal maturity evaluation. The equivalent vitrinite reflectance (EqVRo) values, which were calculated based on the mean measured maximum reflectance of VLM (VLMR¯max), evidence that the Niutitang Formation shales from China and Alum shales from Denmark are post-over mature, and the Yurtus Formation shales in Tarim Basin and the Alum shales from Sweden are still within the oil window.

The marine redox evolution and the formation model for the early Cambrian Gongxi-Tianzhu barite deposits in the South China Block

Late Ediacaran to early Cambrian is marked as a vital ore-formation interval of the Earth’s history, particularly for the bedded barite deposits hosted in cherts and shales. Although plenty of work has been carried out, controversies still exist, such as how the marine paleoenvironment changed in this critical interval, and where the origin of ore-formation materials (i.e., barium ions) was. To address these questions, here, we conducted systematic geochemical research on Gongxi-Tianzhu barite deposits in the South China Block, using redox proxies (MoEF-UEF, Mo-TOC, U-TOC patterns, and elemental ratios), REY patterns, and Sr isotopes. Redox proxies were used to reconstruct the paleoenvironment evolution in the Gongxi-Tianzhu region during the early Cambrian interval. Redox proxy results revealed that the bottom water was oxygen-depleted during most of the depositional time of the Niutitang Formation. The bedded and nodular barites in the Niutitang Formation were formed in the Sulfate-Methane Transition Zone (SMTZ) during the synsedimentary and early diagenetic processes. Formation of the barite ores depended greatly on the redox state variation of the bottom water and the barium remobilization in the sediments. The ore-formation event ceased when the supply of oxygen and sulfate from the upper water stopped, and the pre-stored Ba ions in the anoxic bottom seawater were exhausted and the SMTZ rose back into the water again. REY patterns and 87Sr/86Sr values recorded coeval seawater signals without hydrothermal activities during the studied interval. Based on the elemental and isotopic evidence, a three-stage redox-diagenetic model was established for the formation of the barite deposits in South China.

LA-ICPMS trace elements and C-O isotopes constrain the origin of the Danaopo Cd-Ge-bearing Zn-Pb deposit, western Hunan, China

The Danaopo Zn-Pb deposit, hosted in algal limestone, is the second largest Zn-Pb deposit (150 Mt ores at an average grade of 3.06% Zn + Pb) in the western Hunan-eastern Guizhou metallogenic belt (WHEGMB). Here we investigate the trace elements of sphalerite and C-O isotopes and rare earth elements (REEs) of calcite in the Danaopo deposit, aiming to present insights into the mineralization potentiality of critical metals and origin of Zn-Pb deposits in the WHEGMB. In-situ trace element reveals the inhomogeneous distribution of critical metals within sphalerite, including Cd (1520 to 11570 ppm), Ge (0.73 to 236 ppm), Ga (0.04 to 26.90 ppm), and Tl (0.01 to 3.62 ppm). Cd and Ge exceed the Chinese comprehensive utilization standard. The LA-ICPMS elemental map and inter-element correlations suggest that Cd and Ge enter sphalerite lattice via the substitution of Zn2+ ↔ Cd2+ and 4Zn2+ ↔ Ge4+ + 2Fe2+ + □ (vacancy), respectively. The δ13CPDB (-3.93 to −0.12‰) values of the ore-stage calcite indicate that the carbon in hydrothermal fluids primarily originates from the dissolution of Cambrian algal limestone. Calculated δ18Ofluid (+5.23 to + 9.95‰) values on calcites lies between the oxygen isotopic compositions of the algal limestone (+18.76 to + 23.87‰) and the basinal hot brine (+4 to + 10‰), implying that the oxygen was sourced from a binary mixing of ore-host rocks and basin brine. Besides, the REE distribution pattern of Cal-Ⅰ falls within the field of ore-host rocks. In contrast, Cal-Ⅱ is plotted between the Cambrian ore-host and Proterozoic Banxi Group to Cambrian Niutitang Formation footwall. These findings suggest that REE of Cal-Ⅰ was mainly sourced from country carbonate rocks, while Cal-Ⅱ has mixed REE origins from ore-host rocks and footwall sedimentary and metamorphic basement rocks. Overall, based on detailed deposit geology, sphalerite trace elements, and calcite C-O and REE evidences, this study concludes that the Danaopo is a classic Mississippi Valley-type (MVT) Zn-Pb deposit with a low-temperature and low fO2 reductive ore-forming fluid.

Impact of tectonic deformation on shale pore structure using adsorption experiments and 3D digital core observation: A case study of the Niutitang Formation in Northern Guizhou

Tectonic deformation has an evident impact on the pore structure of organic-rich marine shale. To study the impact of tectonic deformation on the pore structure of shale, cores were taken from different tectonic regions in the Niutitang Formation. The paleotectonic stress, pore structures, mineral composition, and geochemical parameters of the shale were determined by conducting the following tests: paleotectonic stress test, core thin-section observation, focus ion beam-scanning electron microscopy (FIB-SEM) observation, vitrinite reflectivity test, low-temperature nitrogen adsorption test, total organic carbon (TOC) test, X-ray diffraction (XRD) test, and nano-CT. The results showed that the shale in the deformed region mainly developed meso-macropores, and the pores were mainly slit-type and conical. The shale in the transition and stability regions primarily developed mesopores and micropores, which were primarily elliptical, slit-type, and columnar. The pore volumes of the shale in the deformed region were 9.54 cm3/kg and 13.9 cm3/kg higher than those in the transition and stability regions, respectively. The shale in the stability and transition regions had average microporous volumes 8.67% and 7.93% higher than that in the deformed region, respectively. The shale reservoir in the stability region developed more micropores and had a higher specific surface area and microporous volume. Furthermore, the organic matter had a greater impact on the micropores than on the meso-macropores. Thermal maturity had a greater impact on the microporous volume than on the macroporous volume. The shale pore connectivity of Well TX 1 was 86.48% and that of Well FC 1 was 22.33%. The pore connectivity of the shale in the stability region was higher than that of the shale in the deformed region. The shale samples from the deformed region had larger pore volumes and primarily developed macropores. The difference in the meso-macropores is significantly greater than the difference in the microporous volume. Hence, the geological tectonism had a greater impact on the meso-macropores than on the micropores in the shale cores.

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Restrictions of Weathering on Phosphorus Migration and Enrichment of the Lower Cambrian Phosphorus-Bearing Rock Series in Eastern Guizhou.

Phosphorite deposits in eastern Guizhou belong to a typical type of marine sediments. Phosphorus-bearing layers are distributed at the bottom of the Lower Cambrian Niutitang Formation. Generally, the average content of P2O5 in phosphorite deposits is 13.01%. In this study, significant P enrichment on or near the phosphorite deposit surface was found, and the average content of P2O5 reached 24.76%. However, the migration and enrichment mechanisms of P in the supergene environment still lack in-depth research. Therefore, in this work two representative phosphorite deposits, namely, primary sedimentary and weathered phosphate ores, were analyzed. The deposits were examined by polarized light microscope observation, X-ray diffraction analysis, scanning electron microscope observation, energy spectrum analysis, and NMR analysis. Major, trace, and rare earth elements in the deposits were detected by geochemical tests. The results showed that, compared with the primary phosphate ore, the porosity of the weathered phosphate ore was significantly increased. The NMR T 2 spectrum showed that the number of micropores increased, the pore size was enlarged, and more mesopores and macropores appeared in the weathered phosphate ore. The weathered phosphate ore had a lighter color than the primary phosphate ore. Under the action of aqueous acid, the primary phosphate minerals (collophosphate and apatite) and Al-bearing rocks transformed into a typical weathered mineral, sasaite. Carbonate minerals were dissolved, and pyrite was oxidized to limonite. K, Si, and Al in silicate minerals precipitated to form secondary clay minerals, such as kaolinite. The weathered phosphate ore developed recrystallized, pisolitic, and pseudomorphic textures and nodular, oriented, and metacolloidal structures, features specific to a weathered mineral. In the supergene environment, soluble components such as carbonate minerals in the primary rock were leached out, while residual P2O5 with relatively stable chemical properties was enriched in situ. Phosphorus migration and enrichment were restricted by such factors as primary rock/ore composition, primary rock/ore texture and structure, climatic conditions, topography, geological structure, and hydrogeological conditions. A three-stage enrichment model was established after mechanical crushing of the primary rock/ore, carbonate mineral dissolution, and secondary phosphate mineral formation.

Depositional Environment and Organic Matter Enrichment in the Lower Paleozoic Shale from the Northeastern Margin of the Yangtze Platform, South China

In this study, twenty-six core shale samples were collected from the marine Lower Paleozoic shale in a well in the northeastern margin of the Yangtze Platform. Analyses of TOC content, mineral composition, major elements, along with trace and rare earth elements were conducted on the samples. The results were used to investigate the depositional conditions and their effects on organic matter accumulation and preservation. Generally, the sedimentation period of Niutitang Formation shale was in a cold and arid climate with anoxic marine environments, while the shale from Wufeng-Longmaxi Formation was formed in a warm and humid climate with oxic marine environments. In addition, the Wufeng-Longmaxi and Niutitang formations are characterized by low paleo-productivity. The organic matter enrichment for shale in this study could be simultaneously controlled by paleo-redox state and paleo-productivity. Organic matter enrichment of the Niutitang shale is mainly driven by preservation rather than productivity, while the dominant driving factor is the opposite for the Wufeng-Longmaxi shale. Additionally, palaeoclimate and terrestrial influx intensity were found to significantly impact the organic matter enrichment in the Wufeng-Longmaxi shale. The findings have implications for the understanding of the sedimentary processes, organic matter enrichment and preservation and shale gas potential of the study area.

Nanopore Characteristics of the Cambrian Niutitang Formation Organic-Rich Shales in the Middle Yangtze Region and Its Formation Controlling Factors

The shale of Cambrian Niutitang Formation is one of the most important shale gas reservoirs in the Middle Yangtze region, South China. Nanopores are important reservoir spaces for shale gas. The study of pore characteristics and its formation controlling factors is of great significance for the exploration and development of shale gas in the Niutitang Formation. In this study, two drilling core samples and two profile samples were investigated. The laser Raman, X-ray diffraction analysis, FE-SEM, and low-pressure N2 and CO2 adsorption methods are used to study the geochemical and mineral compositions of the shales, and the nanopore characteristics and its formation controlling factors of the shales are discussed. Results show that the shales have the following features: (1) rich in organic matter (OM) and the total organic carbon (TOC) contents ranging from 2.65 to 11.33%, with an average value of 6.81%. (2) The main mineral components of these shales are quartz, feldspar, clay minerals, calcite, and a small amount of pyrite. (3) The samples from the center of the Middle Yangtze region (EY1 well and BGP profile) have a higher maturity value ( Rmc R 0 % = 4.0 %) relatively to the samples from the south of the Middle Yangtze region (XD1 well and JF profile), where the Rmc R 0 % is 2.8%. (4) The maximum N2 adsorption capacity of the four groups of shale samples ranges from 5.090 to 27.333 cm3/g, with an average value of 15.571 cm3/g, while the maximum CO2 adsorption capacity ranges from 2.686 to 5.567 cm3/g, with an average value of 3.776 cm3/g. The higher maturity leads to the EY1 well having the lowest average maximum N2 adsorption capacity, while most of the OM pores in the EY1 well and BGP profile samples are smaller than 10 nm in diameter. The tectonic uplift leads to the profile samples having more meso-macropores than the core samples. The dissolution pores and granular mineral support are the key factors for the better pore structure of the high-evolution stage Niutitang Formation shale.

Controls on Organic Matter Accumulation from an Upper Slope Section on the Early Cambrian Yangtze Platform, South China

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 Baxs 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 CoEF × MnEF 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.

Changing Hydrocarbon-Producing Potential of the Cambrian Niutitang Shale: Insights from Pyrite Morphology and Geochemical Characteristics.

The characteristics of shale gas enrichment conditions at different depositional positions of organic-rich shale in the Niutitang Formation of the Lower Cambrian of the Upper Yangtze in South China vary greatly. The study of pyrite provides a basis for the restoration of the ancient environment and a reference for the prediction of organic-rich shale. In this paper, the organic-rich shale of the Cambrian Niutitang Formation in the Cengong area is analyzed by means of the optical microscope, scanning electron microscope observation, carbon and sulfur analysis, X-ray diffraction whole rock mineral analysis, sulfur isotope test, and image analysis. The morphology and distribution characteristics, genetic mechanism, water column sedimentary environment, and influence of pyrite on the preservation conditions of organic matter are discussed. This study shows that the upper, middle, and lower sections of the Niutitang Formation are rich in pyrite (framboid, euhedral pyrite, subhedral pyrite, etc.). Meanwhile, the sulfur isotopic composition of pyrite (δ34Spy) shows a tight correlation with the framboid size distribution throughout the shale deposits of the Niutang Formation, and the average particle size (9.6 μm; 6.8 μm; 5.3 μm) and distribution range of framboids (2.7-28.1 μm; 2.9-15.8 μm; 1.5-13.7 μm) in the upper, middle, and lower sections show a downward trend. In contrast, the sulfur isotopic composition of pyrite shows a tendency to become heavier from above and below (mean = 0.25‰ to 5.64‰). Together with the covariant mode of pyrite trace elements (such as Mo, U, V, Co, Ni, etc.), the results showed significant differences in the oxygen levels in the water column. They show that the transgression led to long-term anoxic sulfide conditions in the lower water column of the Niutitang Formation. In addition, the main and trace elements in pyrite jointly indicated that there was hydrothermal action at the bottom of the Niutitang Formation, which led to the destruction of the preservation environment of organic matter and the decrease of TOC, which can also explain the reason why the TOC content in the middle part (6.59%) was higher than that in the lower part (4.29%). Finally, the water column became an oxic-dysoxic condition due to the decline of sea level, and the TOC content decreased (1.79%).

Study on the influence of natural cracks on the mechanical properties and fracture mode for shale at the microscale: An example from the Lower Cambrian Niutitang Formation in northern Guizhou

A large number of natural cracks exist in shale reservoirs, and the presence of natural cracks weakens the integrity of shale, which is an important factor governing the effectiveness of shale gas extraction. In this paper, shales from the Lower Cambrian Niutitang Formation in northern Guizhou were scanned by electron microscopy, their microstructures were selected for digital image processing, and uniaxial compression numerical tests were conducted on shale models containing different natural crack dips using the rock fracture process system RFPA2D-DIP to study the effects of natural cracks on the mechanical properties and fracture patterns of shales at the microscopic scale. The study shows that the peak strength and elastic modulus of shale increase with increasing natural crack inclination angle. The fracture modes of shale at the microscopic scale can be roughly divided into four categories: similar to I-type fractures (0°), oblique I-type fractures (15°, 45°, 60°, 75°), folded line fractures (30°), and V-type fractures (90°). Natural cracks within shale are found to have a significant effect on the distribution of stress. Acoustic emission can reflect the stress change and rupture process for shales containing natural cracks with different dip angles at the microscopic scale. The presence of natural cracks has a significant effect on the AE energy and fractal dimension. The magnitude of the AE energy increases with increasing stress level and reaches a maximum value at 90°, while the value of the fractal dimension is found to zigzag upwards because the value of the fractal dimension is jointly influenced by both newborn cracks and native natural cracks.

Pyrite characteristics and shale gas significance of the Niutitang Formation in Cengong area, Guizhou Province

黄铁矿特征研究可为页岩沉积环境恢复与页岩气富集保存规律预测提供依据。以贵州岑巩地区岑页1井牛蹄塘组为研究对象,基于岩心、薄片、扫描电镜及微量元素、硫同位素地球化学,确定牛蹄塘组页岩沉积环境及页岩气富集地质条件。研究表明,牛蹄塘组页岩黄铁矿发育为草莓状、自形-半自形及他形(主要为胶状他形)三种形态,不同形态的黄铁矿形成于不同沉积环境,草莓状黄铁矿形成于缺氧还原环境,自形-半自形黄铁矿形成于贫氧环境,胶状他形黄铁矿指示沉积过程有热液活动的参与。测试井牛蹄塘组整体处于较开放的沉积水体,上段为含氧-贫氧环境,硫同位素整体变轻且发生负漂;中段为较缺氧环境,硫同位素跨度大且发生正漂;下段为还原性更强的沉积环境,硫同位素小幅度负漂。黄铁矿含量与有机质富集密切相关,与黄铁矿有关的孔隙为页岩气吸附、保存及运移提供载体,且草莓状黄铁矿指示的缺氧硫化环境更利于有机质保存。研究工作系统梳理了黄铁矿对牛蹄塘组页岩沉积环境及储层页岩气富集的指示作用,为页岩气勘探提供指示。