Niutitang Formation Research Articles

Analysis of Developmental Characteristics and Dominant Factors of Pore-Fracture Systems in Lower Cambrian Marine Shale Reservoirs: A Case Study of the Niutitang Formation, Fenggang Block, Southern China.

Due to breakthroughs in the Lower Silurian Longmaxi Formation in the Sichuan Basin and multiple strata around the basin, the northern part of Guizhou adjacent to the Sichuan Basin has become a key area for shale gas exploration. Compared with the Longmaxi Formation, the Niutitang Formation displays greater TOC (total organic carbon) content, depositional thickness and distribution area, but the details remain undetermined. In the study area, the Lower Cambrian Niutitang Formation typically has high TOC content, maturity and brittle mineral content. The study area has experienced multiple periods of tectonic movement, which have great influence on the fracture and pore characteristics. The fractures are mainly structural fractures and have obvious zoning. The primary types of pores are intraparticle pores, organic matter pores, and interparticle pores. Further, macropores and mesopores less than 50 nm contribute most of the pore volume, while pores less than 2 nm contribute most of the specific surface area. Many factors affect the pore-fracture system, such as tectonism, TOC content, mineral composition, and sedimentary environment. Tectonic movements produce fractures based on the changing stress field, but the degree of fracture development does not agree well with the degree of pore development. The TOC content has good positive correlations with the development of fractures and micropores, especially for nanoporosity, while clay minerals show a negative correlation with the development of fractures but a strong positive correlation with the growth of micropores. Quartz displays a positive correlation with the development of fractures but no good correlation with pore development. Finally, the lithofacies, lithologies and mineral compositions under the control of sedimentary environments are internal factors that can impact the development of pore-fracture systems.

Multi-Angle Investigation of the Fractal Characteristics of Nanoscale Pores in the Lower Cambrian Niutitang Shale and Their Implications for CH₄ Adsorption.

Shale gas has received widespread interest due to its successful commercial development in China. Pore structures in shale can directly control its gas storage and migration properties. In this study, field emission scanning electron microscopy (FE-SEM), low-pressure N₂/CO₂ adsorption and highpressure methane adsorption were used to investigate the nanoscale pore structures of the Lower Cambrian Niutitang Formation in the southeastern Upper Yangtze platform. The fractal parameters of the pore structures were also calculated using the Frenkel-Halsey-Hill (FHH) model. The relationships between the fractal dimensions and TOC content, mineral composition and pore structure parameters were also discussed. The results show that organic matter and clay minerals are primary factors affecting the nanoscale pore development. Slit-shaped pores and ink-bottle-shaped pores are the predominant pore types in the Niutitang shale. The Brunauer-Emmett-Teller (BET) surface areas vary from 4.91 m²/g to 34.33 m²/g, and the pore volumes range from 0.689 m³/100 g to 2.964 m³/100 g. Two fractal dimensions (D1 and D₂) of the Niutitang shale were obtained using the FHH model, with D1 ranging from 2.605 to 2.684, and D₂ ranging from 2.681 to 2.865. D1 adequately characterizes the surface roughness of the pore structures, while D₂ represents the complexity of the pore types. Inter-particle (InterP) pores commonly have greater shape complexities than OM pores and intra-particle (IntraP) pores, based on analyses using Image-Pro Plus software. In addition, the TOC content and clay minerals have great effects on the fractal dimension D1. Meanwhile, the fractal dimension D1 increases with increasing BET surface area, but there is no definite relationship between the fractal dimensions and pore volumes. Both the fractal dimensions D1 and D₂ are negatively correlated with pore sizes. Further investigation indicates that the fractal dimension D1 exhibits a strong positive relationship with the methane adsorption capacity indicating that Niutitang shales with greater values of the fractal dimension D1 have higher methane adsorption capacities.

Simulation experiment and mathematical model analysis for shale gas diffusion in nano-scale pores

Gas diffusion in nano-scale porous media of deeply burried shale includes bulk diffusion (Fick and Knudsen diffusions) and surface diffusion. To reveal the migration mechanism of this process, the influence of temperature and pressure on diffusion coefficient needs to be quantitatively evaluated. A case study was made with the Cambrian Niutitang Formation in the Maoping area, Zigui, western Hubei, South China. Gas diffusion was simulated by isobaric diffusion experiments under different temperature and pressure conditions. The results indicated that:(1) The diffusion coefficient DF decreases with increasing of pressure (when the pressure is higher than 30 MPa, DF tends to be constant), and increases with increasing of temperature; (2) In the high temperature-pressure setting, DF is affected significantly by pressure and generally tends to decrease. Moreover, the impacts of temperature, pressure, porosity and lithology were quantitatively calculated, and a mathematical model of gas diffusion was established, which had comparable results with simulation experiment. The following conclusions were thus drawn:(1)Higher temperature will cause stronger molecular kinetic energy, resulting in increasing bulk and surface diffusion coefficients, while higher pressure will slightly strengthen the Fick and surface diffusions, but significantly limit the Knudsen diffusion, and cause lower total diffusion coefficient; (2) Larger pore size leads to stronger bulk diffusion, but weaker surface diffusion. Finally, according to the studies of a specific research block, high pressure setting is conducive to the preservation of nano-scale porous gas reservoir in shale, while the uplift accompanied by pressure release is the main stage of shale gas loss.

The distribution characteristics of brittle minerals in the Lower Cambrian Niutitang Formation in northern Guizhou

To explore the influence of brittle mineral composition and morphological characteristics within the shale matrix on the generation of complex fractures and reservoir reconstruction, the microstructure and fractal characteristics of minerals within the shale matrix of the Lower Cambrian Niutitang formation in northern Guizhou were investigated using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectrometer (SEM-EDS), and micro-computed tomography (Micro-CT). Based on image processing technology and digital core modelling, a three-dimensional shale digital core model was established. The distribution and morphological characteristics of brittle minerals within the shale matrix were studied from the fractal perspective. The model for evaluating the mineral particles and pores within the shale matrix was established. The results show that the siliceous mineral composition is high, and it is mainly derived from the interaction of continental detrital quartz particles and biogenetic effects; the density of the brittle minerals is greater than other mineral components mainly distributed in the range of equivalent diameters of 200–800 μm; the distribution complexity of the brittle minerals was quantitatively analysed from the fractal perspective; the fractal dimension partly reflects the generating ability of complex fractures; the distribution angle and complexity of brittle mineral particles within the shale matrix partly determine the fracture morphology characteristics. The physical properties of shale reservoirs were studied from three aspects: complex fracture generation ability, pore connectivity and heterogeneity, and the evaluation model of mineral particles and pores was established using the value of fractal dimension, ratio of organic matter pores, tortuosity of pores, relative deviation of pore size distribution, and relative deviation of specific surface area. The model provides a basis for further study on the relationship between reservoir reconstruction potential and physical properties of shale reservoirs.

Effect of organic maturity on shale gas genesis and pores development: A case study on marine shale in the upper Yangtze region, South China

Abstract The marine shale in southern China has undergone complex tectonic evolution with a high thermal evolution degree. Excessive thermal evolution brings certain risks to shale gas exploration and development. With the advancement of experimental methods, the evolution process of shale reservoirs can be better understood from the micro-nanoscale. This work takes the Ordovician-Silurian Wufeng and the first member of Longmaxi Formation in the Sichuan Basin and Lower Cambrian Niutitang Formation in Outer Margin of the Sichuan Basin to study the impact of maturity upon the genesis of shale gas and development features of the reservoir. A series of geochemical research methods, including TOC, gas component and gas isotope, were adopted to study the impact of different thermal evolution stages of organic matter upon the genesis of shale gas. The nanoscale micro-imaging technique, such as FIB-SEM and FIB-HIM, was used to analyze the development of OM-hosted pores. As shown from the results, when R o = 1.2–3.5%, the marine shale gas is dominated by methane and other hydrocarbon gases, since the mixture of cracking gas from liquid hydrocarbons and kerogen-cracking gas cause the carbon isotope reversal. Besides, the pyrobitumen pores characterized by the strong connectivity and storage capacity were primarily developed. When R o > 3.5%, the organic matter is at the graphitization stage. The shale gas is mainly composed of nitrogen at this stage. The nitrogen is originated from the atmosphere and the thermal evolution process, and the OM-hosted pores (pyrobitumen and kerogen pores) characterized by the bad connectivity and storage capacity are developed. Finally, the main component of shale gas, the genesis of shale gas and the pattern of OM-hosted pores under different thermal evolution stages of organic matter are summarized, which provide technical support for the exploration and development of shale gas.

Relationship between geological structure and marine shale gas preservation conditions in the western Middle Yangtze Block

Lower Paleozoic dark shale is developed in the western Middle Yangtze Block, which lays a material foundation for the enrichment and accumulation of marine shale gas. In order to ascertain the control action of geological structures on the differential preservation of shale gas and reveal the key factors in shale gas preservation, this paper firstly analyzed the structure characteristics of this area, carried out structure pattern recognition and structural belt division, and studied structural deformation mode and intensity. Based on this, the relationships between different structure styles and shale gas preservation conditions were analyzed. Finally, combined with the structural deformation and the lithofacies paleogeographic characteristics of marine shale, the favorable exploration zones of shale gas were proposed. And the following research results were obtained. First, the western Middle Yangtze Block can be divided into four structural deformation belts, and three types of piggyback structural patterns have been identified, including restricted type, weakly reformed type and strongly reformed type. Second, the restricted type is located in the northwestern part of Hunan and Hubei Provinces. In this pattern, piggyback structure is incomplete and thrust belt and Compressive fold belt are developed. Third, the weakly and strongly reformed types are located in the western parts of Hunan and Hubei, and Wulingshan area, respectively. They both have complete piggyback structures, but the former has lower deformation intensity and has never undergone the late superimposed reformation. Fourth, there are three structural transfer belts in the western Middle Yangtze Block, i.e. the structural transfer belt between the East Sichuan fault–fold belt and West Hunan–Hubei fault–fold belt, the structural transfer belt between West Hunan–Hubei fault–fold belt and Wulingshan fault–fold belt, and the structural transfer belt between the outcrop and the hinterland of Middle Yangtze Block. The first one is structurally transformed at the Qiyueshan fault. The East Sichuan fault–fold belt on the west is an ejective fold with low fault density and formation denudation intensity, where shale gas is enriched in anticlines and slopes; while the West Hunan–Hubei fault–fold belt on the east is a trough-like fold with strong faulting and high formation denudation intensity, where shale gas is enriched in residual synclines. In conclusion, shale gas preservation conditions of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in this area are the best in Zigui syncline, thrust–detachment zone and western margin of Qiyueshan fault. The favorable exploration areas of shale gas of Lower Cambrian Niutitang Formation are distributed in the western flank of Yichang Slope, Kaixian thrust zone, compressive fold zone and thrust–detachment zone.

NanoSIMS sulfur isotope studies of pyrite from the Early Paleozoic marine shale: Implications for the sedimentary environment

The presence of pyrite has been reported previously in the Longmaxi and Niutitang Formations in southern China, and can serve as an important indicator of the sedimentary environment. In order to systematically investigate the differences between the paleoenvironments of samples from the two formations, complementary experiments were conducted on six samples from the same well (three from each of formations). The methods included nano-scale secondary ion mass spectrometry (NanoSIMS) for in situ sulfur isotope analysis, isotope ratio mass spectrometry (IRMS) for comparison with the data from NanoSIMS, field emission scanning electron microscopy (FE-SEM) to observe the pyrite forms, inductively coupled plasma mass spectrometry (ICP-MS) for the analysis of trace elements, LECO carbon/sulfur analyzer for determining the total organic carbon (TOC) content, and X-ray diffraction (XRD) to identify the mineral compositions. The results suggest that NanoSIMS can analyze submicron pyrite grains visually, which could help to differentiate euhedra and framboids during the analyses. The sulfur isotope ratios inside the framboidal pyrite analyzed by NanoSIMS suggest that the formation of the framboidal pyrite occurs from the center to the surface, as against the assumed process wherein iron non-sulfide spheroids are pseudomorphically replaced by pyrite. The size distribution of the framboidal pyrite and the redox-sensitive trace elements of the bulk rock jointly indicate that the bottom water redox conditions of the Niutitang Formation were reduced more compared those of the Longmaxi Formation. The sulfur isotopic characteristics of the framboidal pyrite imply that the two formations were deposited in an open sedimentary system (a depositional system where sulfate radical ions could diffuse into the system from the external environments); in this regard, the Niutitang environment was found to be progressively closing while the Longmaxi environment was found to be gradually opening.

Evaluation of the Pore Structure Variation During Hydraulic Fracturing in Marine Shale Reservoirs

Abstract Hydraulic fracturing becomes a necessary method to exploit shale gas, and the imbibition behavior of fracturing fluid can cause the pore structure variation. At present, the evaluation of this variation has less been investigated, and repeated imbibition was conducted to research the variation of pore structure. First imbibition was conducted as simulating the pore structure variation during hydraulic fracturing, and repeated imbibition was carried out to appraise this kind of variation. Two significant parameters were proposed to estimate the pore structure variation, which are the differential value of initial imbibition rate and the utmost differential value of normalized imbibed volume. Initial imbibition rate is closely related to pore connectivity, and normalized imbibed volume reflects the pore volume. Reservoirs with a higher value of these two parameters have advantage for developing its resources. Sichuan Longmaxi formation has the highest value in both parameters among all formations, which showed that Sichuan Longmaxi formation has advantage for exploiting its resources by hydraulic fracturing. Baojing Longmaxi formation, Sichuan Niutitang formation, and Cengong Niutitang formation have a lower value in both two parameters which indicated that these formations have less potential to develop its resources by hydraulic fracturing. These two parameters are mainly influenced by wettability and initial permeability, and have no obvious relationship with clay content. Overall, our research is conducive to screening the ideal formation for exploiting shale gas by hydraulic fracturing.

Sedimentary facies characteristics and organic matter enrichment mechanism of lower Cambrian Niutitang Formation in South China

The purpose of this study was to examine the sedimentary facies characteristics of lower Cambrian Niutitang Formation (∈1n) in South China, to reveal the mechanism of organic matter enrichment, and to guide exploration of shale gas. Macro investigation and experimental analyses were used to assess the lithology in detail, total organic matter mass fraction w(TOC), mineral composition, and trace element characteristics of ∈1n. The influencing factors of organic matter enrichment were discussed extensively, and a sedimentary facies mode was suggested. In the early stage of ∈1n, the locations of Well E’yangye 1, Well Ciye 1, Well Changye 1, and Well Anye 1 respectively develop, platform inner sag, outer shelf, Jiangnan slope belt, and South China detention basin. In the late stage of ∈1n, the sedimentary facies evolve with decreasing sea level. The study area presents a complete three-step basin in the Early Cambrian. In the early stage of ∈1n, the first step is the Yangtze carbonate platform, the second step is the outer shelf and slope, and the third step is the deep-water basin. From the Yangtze carbonate platform to the deep-water basin, w(TOC) and the mass fraction of quartz gradually increase, the mass fraction of carbonate mineral decreases, and the mass fraction of clay mineral is higher in the second step. The sea level fluctuation results in a higher w(TOC) vertically in the lower ∈1n shale, and the paleogeographic (provenance) conditions lead to better horizontal development of organic matter in the outer shelf, slope and detention basin. Trace elements are abundant in the lower ∈1n, and w(TOC) is correlated positively with many trace elements. In the outer shelf, slope, and adjacent areas, hydrothermal activity and upwelling current bring nutrient-rich material and promote organic matter enrichment under a strong reducing condition. Deep-shelf, slope and deep-water basin are the best facies for the formation and preservation of organic matter, especially deep-water basin facies. It remains necessary to strengthen the exploration of shale gas in the deep-water basin of ∈1n in central Hunan, China.

Shale adsorption characteristics of the Lower Cambrian Niutitang Formation in northern Guizhou based on surface free energy and isosteric heat data

Given the potential of shale gas as an energy source, exploration and development efforts have been underway around the world. To better understand the adsorption mechanism of the Lower Cambrian Niutitang Formation shale in the northern Guizhou region, China, core samples were obtained from two potential gas reservoirs in the region. Petrophysical and adsorption experiments were performed on these samples. The thermodynamic characteristics of shale adsorption are discussed based on the surface free energy and isostatic heat of adsorption. The effect of adsorption on gas flow in shale pore fractures is discussed. The results point to the promising potential of the exploration and development of the Niutitang Formation shale. The adsorption capacity of the shale decreases as the temperature increases. As the pressure increases, the adsorption capacity first increases and then decreases. The temperature, pressure, and shale-specific surface area all affect the surface free energy of the shale, which in turn affects the shale adsorption capacity of CH4. This change in the gas adsorption capacity is consistent with the trend exhibited by the surface free energy. Therefore, the adsorption of shale is controlled by its surface free energy. The isothermal adsorption of shale displays a positive correlation with the isosteric heat of adsorption. The average isosteric heat of FC1 shale is 28.68 kJ/mol; this shale only physically adsorbs methane. The average isosteric heat of TM1 shale is 42.90 kJ/mol. Physical adsorption may be accompanied by chemical adsorption. The isothermal adsorption curves at different temperatures can be predicted by the isosteric heat of adsorption. The gas adsorption layer affects the gas flow regimes by changing the effective pore size of the shale. The methodologies and results presented herein will be beneficial to more accurately understanding the storage mechanism of shale gas and estimating the scale of shale gas resources.

Investigation on the evolution of shale mineral composition under the fluid–solid coupling

To understand the mechanism of the fluid–shale interaction that influences the physical properties of shale during fluid–solid coupling, in this study, we investigated the changes in the mineral composition of the shale and ion concentration of the fracturing fluid during the fluid–solid coupling at the Niutitang formation in Northwest Hunan, China. The results show that the shale contains more brittle minerals but fewer clay minerals and the mineral composition of the shale in weakly alkaline fracturing fluid changes obviously. Brittle minerals decrease initially and then increase afterward under the fluid–solid coupling, which is related to the pH value and ion concentration of the fracturing fluid. These new findings can provide technical support for the prediction of the physical parameters of shale after the fluid–solid coupling and for the design of a hydraulic fracturing scheme.

Quartz types, origins and organic matter-hosted pore systems in the lower cambrian Niutitang Formation, middle yangtze platform, China

Quartz cementation is critical to shale reservoir properties, including porosity and geomechanical properties. Due to the small size of constituting components, our understanding on quartz origins in shales is still unclear. Recently, the application of high resolution scanning electron microscope (SEM) combined with cathodoluminescene (CL) technique has enabled major advances in our understanding of quartz cementation in shales. A suite of the Lower Cambrian Niutitiang shale samples were selected to study quartz types, origins and the effect of quartz cementation on porosity development by integrating geochemical analysis, nitrogen adsorption, SEM and SEM-CL imaging analysis.The results show that quartz primarily exists as five forms: silt-size/sand-size detrital quartz, siliceous skeletons, overgrowth nucleated around detrital quartz, matrix-dispersed microcrystalline quartz, and aggregates of euhedral quartz. Detrital quartz is characterized by angular shape, and bright luminescene. Quartz overgrowth can be easily identified by the characteristic non to low luminescene, indicative of authigenic origin. Matrix-dispersed microcrystalline quartz is typically less than 5 μm, and co-existed with clay minerals. Quartz aggregates are primarily composed of equate-sized euhedral quartz. Dissolution and re-precipitation of siliceous skeletons provides the major silica source for authigenic quartz (defined as quartz formed during diagenetic processes) in the Niutitang Formation. The distribution of secondary organic matter, primarily including residual bitumen and pyrobitumen, is always associated with aggregates of euhedral quartz, because the micrometer-scale and well-connected interparticle pore spaces between euhedral quartz crystals provide important storage sites for secondary organic matter, and abundant organic matter-hosted pores (organic pores) are generated during the thermal cracking of residual oil and bitumen to gas.

Elemental Geochemical Characterization of Sedimentary Conditions and Organic Matter Enrichment for Lower Cambrian Shale Formations in Northern Guizhou, South China

The black shale deposited in the Niutitang Formation and its adjacent strata is considered to be a favorable source rock in northern Guizhou of south China and has become a target horizon for shale gas exploration in recent years. Based on SQ-1 and CY-1 core samples, the organic matter properties and geochemical elements were obtained through experimental analysis. Provenance, paleoredox, paleoclimate, paleoproductivity and deposition conditions were analyzed, and the sedimentary effects on organic matter enrichment were discussed. The results show that total organic carbon (TOC) is between 0.22–10.10 wt.% in SQ-1, with an average of 2.60 wt.%, and TOC is between 0.23–7.7 wt.% in CY-1, with an average of 1.45 wt.%. The geochemical data of the samples indicate that the black shale of the Niutitang Formation and adjacent strata are deposited in the tectonic background of the passive continental margin. The provenance shows moderate weathering, with hot and humid paleoenvironmental characteristics and fast deposition rate. Using multiple ancient redox indicators, it is concluded that the formation has undergone changes in the oxidizing environment and anaerobic environment during deposition. According to the (La/Yb)N value (the average value of SQ-1 is 1.23 and the average value of CY-1 is 1.26), it shows a faster deposition rate of the two wells and shortens the residence time of organic matter in the microbial degradation zone. The Babio indicates that the bottom has a high paleoproductivity when deposited. Considering the influencing factors, the paleoproductivity mainly controls the organic matter enrichment, followed by ancient redox conditions and the deposition rate. The research results provide a reference for deepening sedimentary understanding and shale gas exploration in the study area.

In siturare earth element analysis of a lower Cambrian phosphate nodule by LA-ICP-MS

AbstractRare earth elements (REE) in marine minerals have been widely used as proxies for the redox status of depositional and/or diagenetic environments. Phosphate nodules, which are thought to grow within decimetres below the sediment–water interface and to be able to scavenge REE from the ambient pore water, are potential archives of subtle changes in REE compositions. Whether their REE signals represent specific redox conditions or they can be used to track the overlying water chemistry is worth exploring. Throughin situlaser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS), we investigate the REE compositions of a drill-core-preserved phosphate nodule from the lower Cambrian Niutitang Formation in the Daotuo area, northeastern Guizhou Province, South China. REE distributions of the nodule show concentric layers with systematic decreases in Ce anomalies (Ce/Ce*) from the core to the rim. The lowest Ce/Ce* appears in the outer rim where REE concentrations are relatively high. These results are interpreted to reflect REE exchange with pore water at a very early stage or bathymetric variation during apatite precipitation. The origin of the shale-normalized middle REE (MREE) enrichment in our sample is less constrained. Possible driving factors include preferential MREE substitution for Ca in the apatite lattice, degradation of organic matter and deposition beneath a ferruginous zone. Although speculative, the last possibility is consistent with the chemically stratified model for early Cambrian oceans, in which dynamic fluctuations of the chemocline provided an ideal depositional context for phosphogenesis.

Fractal characterization of pore structure and its influence on salt ion diffusion behavior in marine shale reservoirs

Hydraulic fracturing becomes a significant way to develop fossil hydrogen energy from marine shale. Some marine shale formations have a higher salt ion concentration in flowback fluid, about which the mechanism is still unclear. In this study, both the fractal characterization of pore structure and its influence on salt ion diffusion was researched. The fractal dimension D1 can reflect the roughness of pore surface, and the fractal dimension D2 can reflect the complexity of pore structure. The fractal dimension D1 of Sichuan Niutitang Formation is only 2.0069 and the ones of other shale samples are between 2.5291 and 2.6341, which means that the pore surface of Sichuan Niutitang Formation is far smoother than the ones of other samples. The fractal dimension D2 ranges from 2.7396 to 2.9546, which indicates that all samples have complex pore structure. The salt ion diffusion rate is mainly controlled by specific surface area, median pore-throat radius and clay content. A higher specific surface area provides more surface for the interaction between liquid and matrix, and lots of salt ion was released into imbibed liquid. A larger median pore-throat radius benefits to salt ion diffusing out of sample, thus a higher salt ion diffusion rate appears. The liquid can cause the clay-swelling which has the potential to reduce the salt ion diffusion channel, thus the higher the clay content, the lower the salt ion diffusion rate. Our research is conducive to understanding the fractal characterization of marine shale and its impact on high salt ion concentration in flowback fluid.

Oceanic chemistry recorded by cherts during the early Cambrian Explosion, South China

The early Cambrian ocean was marked by significant redox changes, but the detailed redox evolution of seawater and its relationship to biological expansion are not fully understood. Widespread chert successions are present in lower Cambrian deposits on the Yangtze Block, South China, which have great potential to record oceanic chemistry. Here, we analyze rare earth elements (REE) and Si and Fe isotopes along three regional chert sections to constrain the origin of lower Cambrian cherts and the redox conditions of the early Cambrian ocean. The REE patterns and Si isotopes illustrate that lower Cambrian cherts at the Muyang (MY; Yanjiahe Formation) and Zunyi (ZY; Niutitang Formation) locales originated from the replacement of precursor carbonates and black shales, respectively, whereas cherts at Chuanyanping (CYP; Liuchapo Formation) locale originated from direct chemical precipitation from seawater and/or hydrothermal fluids. All cherts, except for several lower CYP cherts with hydrothermal contributions, primarily preserved seawater chemical signals. The Ce/Ce* and Y/Ho ratios indicate that these cherts were deposited near a suboxic/anoxic interface, where seawater may have been ferruginous overall, while more subtle redox changes, namely, ferruginous (less reducing) and ferruginous (more reducing), were identified by Fe isotopes. Based on new data and previous results, we propose that the early Cambrian ocean on the Yangtze Block was characterized by a stratified redox framework. Before ~535 Ma in the Cambrian, oxic seawater occurred only along the inner shelf (MSC section), below which seawater was primarily ferruginous (less reducing). During ~535–526 Ma, oxic seawater gradually expanded to the outer shelf (MY section) and a metastable ferruginous (more reducing) zone dynamically existed at shelf–slope locations. Temporal and spatial comparisons revealed that the water column on the Yangtze Block may have experienced a progressive deepening of the redoxcline during the early Cambrian (~535–526 Ma). The metastable redox zone may have been an analogue of the modern oxygen minimal zone (OMZ), which is associated with biological activity and high productivity. The occurrence and spatial fluctuation of the metastable ferruginous (more reducing) zone may have been associated with the evolution of small shelly fossils from assemblage 1 (SSF1) to assemblage 3 (SSF3) and their spatial expansion from shelf to slope. Therefore, accompanied by oceanic oxygenation, the occurrence and spatial fluctuation of an OMZ-like metastable zone may have regulated the biological diversification and distribution during the early Cambrian (~535–526 Ma).

Multifractal Analysis of Acoustic Emissions during Hydraulic Fracturing Experiments under Uniaxial Loading Conditions: A Niutitang Shale Example

Fracture characterization is essential for estimating the stimulated reservoir volume and guiding subsequent hydraulic fracturing stimulations in shale reservoirs. Laboratory fracturing experiments can help provide theoretical and technical guidance for field operations. In this study, hydraulic fracturing experiments on the shale samples from Niutitang Formation in Hunan Province (China) under a uniaxial loading condition are conducted. The multifractal method is used to analyze the acoustic emission (AE) signals and characterize fracture initiation and propagation. The hydraulic fracturing process can be divided into three stages based on the characteristics of AE signals: the initial stage, the quite stage, and the fracturing stage. The multifractal analysis results showed that: (1) the value of the spectrum width, Δα, continues to increase as the energy accumulates until the fracturing stage starts; and (2) the difference in the multifractal spectrum values, Δf, reflects the relationship between small and large signal frequencies and can quantify the fracture scale, i.e., the lower the Δf, the larger the fracture scale and vice versa. The results were further verified using a time-frequency analysis of the AE signals and micro-CT scanning of the samples. This study demonstrates that the multifractal method is feasible for quantitatively characterizing hydraulic fractures and can aid field hydraulic fracturing operations.

The effect of clay-swelling induced cracks on imbibition behavior of marine shale reservoirs

Due to the low matrix porosity and permeability of gas shales, the horizontal well drilling and hydraulic fracturing technologies become a necessity to exploit the unconventional shale resources. While a large volume of fracturing fluid is injected into the shale formation to create cracks, some shale reservoirs have a low flowback rate, about which the mechanism is still unclear. In order to clarify the associated fluid retention mechanism, the spontaneous imbibition experiments were conducted, and interpreted with theoretical analyses. Shale imbibition process can be divided into three stages, including initial imbibition, transition and late imbibition stages. Longmaxi Formation shale from Sichuan Basin usually has clay-swelling induced cracks, while Niutitang Formation shale from Guizhou province commonly has no such cracks. Shale samples with induced cracks show both higher initial and late imbibition rates. Induced cracks increase new pathways for liquid entering into samples, which lead to a higher initial imbibition rate. Liquid in induced cracks has a large contact area with matrix and liquid moves quickly from cracks into matrix, which causes a higher late imbibition rate. As a contrast, shale samples without induced cracks have a lower initial imbibition rate and a lower late imbibition rate. In addition, the ultimate normalized imbibed volume is higher than the pore volume in shale samples with induced cracks. Furthermore, models were established to interpret the imbibed water saturation with the density of clay-swelling induced cracks. Theoretical analyses indicate normalized imbibed volume increases with the density of induced cracks, and the imbibed water volume can be higher than pore volume with enough density of induced cracks. As a result, clay-swelling induced cracks could account for the retention of a large volume of fracturing fluid in gas shales, and our research contributes to explaining the mechanism of fracturing liquid retention in shale gas reservoirs.

Characteristics of pore structures from the Lower Paleozoic shale gas reservoirs in northern Guizhou, South China

Pore structure in shale is an important factor influencing the storage capacity of a shale gas reservoir and the effect of shale gas development. Taking the core and outcrop shale samples from the Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation as the object of the research, combined with the scanning electron microscope, high pressure mercury injection, CO2 and N2 gas adsorption experiments, we investigated the porosity, pore types, pore structure, and pore size distribution of the Lower Paleozoic shale gas reservoirs in northern Guizhou, South China. The results indicated that there are developed various genetic types of micro-nano pores in the Lower Paleozoic marine shale in northern Guizhou, which provide the main reservoir spaces for shale gas. The porosity of the Lower Paleozoic shale in northern Guizhou is 1.30%–13.37% in range, and the average is 4.56%. The inter-granular pore, organic matter pore and intra-granular pore are most common. Micropore (<2 nm) and mesopore (2–50 nm) are the main reservoir storage space for shale gas. The specific surface area of micropore plus mesopore in the Lower Paleozoic shale accounts for 94.3% of the total specific surface area, and their pore volume accounts for 59.1% of the total pore volume in northern Guizhou. The pore size of the Lower Paleozoic shale is mainly distributed in 0.4–0.7 nm, 3–10 nm and 0.05–200 μm. The pore structure is the parallel-plate pore with slit shape, two-ends-opened tube pore and ink-bottle shape pore. There are significant differences in the porosity and pore size between the Longmaxi Formation shale and the Niutitang Formation shale. The average porosity and average pore size of the Longmaxi Formation shale is 6.4% and 4.71 nm, while the Niutitang Formation shale is 5.3% and 4.23 nm, respectively. The pore structure of the Lower Paleozoic shale gas reservoir is mainly influenced by organic matter abundance, thermal maturity, mineral composition of shale. The porosity and micropore volume of shale show an increasing trend, with the increase of total organic carbon content and quartz content. The porosity and pore volume of shale have a decreasing trend, with the increase of thermal maturity level and clay mineral content.

The effect of clay-swelling induced cracks on shale permeability during liquid imbibition and diffusion

Shale gas wells with high production rates usually have large amounts of hydraulic liquid retention, which can cause aqueous phase trapping (APT). However, the APT can be auto-relieved after a well shut-in for a certain period. To clarify its mechanism, the experiments of shale permeability during liquid imbibition and diffusion were conducted. Then, theoretical analysis was interpreted. Samples include the Longmaxi Formation (LMX Fm) from Sichuan province and the Niutitang Formations (NTT Fm) from Sichuan and Guizhou provinces. Permeability during imbibition in Sichuan LMX Fm has decreasing and increasing stages. Increase in permeability is caused by new cracks induced by clay-swelling. Only the decreasing stage of permeability occurs during imbibition in NTT Fm, tight sand, and tight volcanic rock because imbibed liquid reduces the gas-flow channel. The relative permeability of gas recovers during liquid diffusion among all samples. The permeability recovers quickly initially, which is caused by liquid migrating from large pores and cracks into small pores. The large pores and cracks have more contribution to permeability than the small pores. Subsequently, permeability nearly keeps constant because liquid migrates among the small pores, which have less influence on permeability. Sichuan LMX Fm has induced cracks, and its gas relative permeability can exceed 1 after liquid diffusion. In theoretical analysis, four types of gas-flow channels are considered, namely, organic pores, inorganic pores, organic cracks, and inorganic cracks. Initial cracks are mainly water wet. Induced cracks include both organic cracks and inorganic cracks. The ratio between inorganic induced cracks and total induced cracks influences permeability. The lower the ratio is, the more evident the permeability increase is during imbibition. The denser the cracks are, the higher the permeability recovery is during liquid diffusion. Overall, the induced cracks can contribute to the relief of APT in shale gas reservoirs.