Minerals In Shale Research Articles

Shale gas potential of Ordovician marine Pingliang shale and Carboniferous–Permian transitional Taiyuan-Shanxi shales in the Ordos Basin, China

The assessment of shale gas potential for the Ordovician Pingliang Formation and Carboniferous–Permian Taiyuan and Shanxi formations in the northwest margin of Ordos Basin, China provides insight into how fluctuation in depositional environments has a significant role on lithofacies and shale gas potential. To investigate the shale gas potential, a series of measurements (i.e. Rock-Eval pyrolysis, maceral composition analyses and X-ray powder diffraction, etc.) on representative outcrop samples were conducted to characterise shale properties. The organic matter from marine Pingliang shale is predominantly type I with a strong predominance of sapropelinite, whereas the transitional Taiyuan-Shanxi shales are dominated by types II to III kerogen. Furthermore, the Pingliang shale is characterised as a ‘poor’ source rock mainly owing to the lower total organic carbon (TOC) content (average 0.79 wt%) and higher maturity [average 1.78% in vitrinite reflectance (R o)], while the transitional Taiyuan-Shanxi shales are mostly characterised as ‘fair’ source rocks, and some samples with high TOC content (more than 2.0 wt%) present good source rocks. It is also found that the sedimentary environment, as a key factor determining the organic matter and TOC content, inevitably influences the type and content of minerals in shale, and controls the shale gas potential. For example, the transitional argillaceous Taiyuan-Shanxi shales are significantly different from the siliceous Pingliang shales, specifically, total clay content for the former is more than 50 wt%, while the latter is rich in quartz content (more than 70 wt%). Additionally, the quartz and clay contents of the Taiyuan shale range widely, especially the smectite content of I–S ML. The barrier coastal facies in the Taiyuan Formation are more conducive to the enrichment and preservation of organic matter because the Shanxi shale was deposited in shallow delta facies with a greater terrestrial influence. Conclusively, the Taiyuan and Shanxi formations have relatively good exploitation potential for shale gas, especially the relatively high TOC content (average 2.45 wt%) and moderate R o value (average 1.25%). For future exploration, selecting areas with relatively large shale thickness, high brittle mineral content, stable tectonics and better preservation conditions are key to optimising favourable exploration areas for shale gas. KEY POINTS The shale gas potentials of the argillaceous Taiyuan-Shanxi shales and siliceous Pingliang shale are compared. The influence of sedimentary facies on reservoir parameters of marine and transitional shales is established. This is a first detailed comparison of the marine and transitional shale gas potential in the northwest margin of Ordos Basin, China.

Microwave-enhanced methane cracking for clean hydrogen production in shale rocks

Steam methane reforming (SMR) generates about 95% of hydrogen (H2) in the U.S. using natural gas as a main feedstock. However, this technology also generates a large amount of carbon dioxide (CO2), a major greenhouse gas causing global warming. Carbon capture and storage (CCS) technique is required, but the cost and safety of storing CO2 underground are a concern. Here we propose a new approach using microwave/electromagnetic irradiation to produce clean hydrogen from unrecovered hydrocarbons within petroleum reservoirs. Solid carbon or CO2 produced during this process will be simultaneously sequestrated underground without involving CCS. In this paper, we perform a series of experiments to investigate the in-situ hydrogen production from shale gas (methane) conversion by passing a methane stream through a packed shale rock sample heated by microwave. We found that methane conversion was significantly enhanced in the presence of Fe and Fe3O4 particles as catalysts, with a conversion of 40.5% and 100% at reaction temperature of 500 °C and 600 °C, respectively. Methane conversion is promoted at a lower reaction temperature by the catalytic effect of minerals in shale. Additionally, the influences of catalysts, shale rock, and methane flow rate are characterized.

Geochemistry and clay mineral studies of Jurassic sedimentary rocks from the Spiti region, Himachal Pradesh, North India

An attempt is made in the present study to unravel the provenance, paleoweathering and paleoclimatic conditions of the Jurassic (Spiti Formation) black shales and sandstones from the Spiti region, Tethys Himalaya, using multi proxy approach. The sandstones are subarkose in composition and texturally poorly sorted, subrounded to subangular in shape with moderate spheriocity. The range of the chemical index of alteration (CIA) is 55–90, recorded in the black shales strongly suggests moderate to strong chemical weathering conditions in the source area, which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in Jurassic period in the Spiti region. Geochemical studies reveal that shales are enriched in felsic elements (high SiO2, Al2O3, K2O) and depleted in mafic components (Fe2O3 and MgO).The various geochemical discriminant plots and elemental ratios (SiO2/Al2O3, K2O/Al2O3, Al2O3/TiO2, K2O/Na2O, etc.) indicate the rocks to be the product of weathering of felsic rocks. The paleoclimate in the source area seems to be mostly semi-humid. The plot of the samples on the A-CN-K ternary diagram indicates a granitic weathering trend. The X-ray Diffraction studies show that the prominent clay minerals in the Spiti shales are illite, smectite, chlorite, kaolinite and vermiculite along with quartz, muscovite, alkali feldspar, calcite and phosphatic phase.When plotted on the tectonic discrimination diagram, the samples indicate passive margin tectonic setting.

Effect of Water Saturation on Gas-Accessible Effective Pore Space in Gas Shales

Abstract The existence and content of water will certainly affect the effective pore space of shales and therefore is a key point for the evaluation of in-situ gas content and gas flow capacity of shale reservoirs. In order to reasonably evaluate the gas storage and flow capacities of water-bearing shale reservoirs, the effect of water on the effective pore space of shales needs to be understood. In this study, the Upper Permian Longtan shale in the southeastern Sichuan Basin, China, was selected as an example to conduct nuclear magnetic resonance cryoporometry (NMRC) measurements under different water saturation levels. The gas-accessible effective pore spaces in shales under different water saturation levels were quantified, and the effect of water saturation on gas-accessible effective pore space in shales was investigated. The results show that water plays an important role in the gas-accessible effective pore space of shales. When the Longtan shale increases from a dry state to a water saturation of 65%, 75%, and 90%, the gas-accessible effective pore volume decreases by 35%-60% (average 46.3%), 50%-70% (average 58.8%), and 65%-82% (average 75.8%), respectively. Water has an effect on the gas-accessible effective pore space regardless of pore size, and the effect is the strongest in the 4-100 nm range, which may be mainly due to the high content of clay minerals in the Longtan shale. Our studies are of important theoretical significance and application prospects for accurately evaluating the gas-accessible effective pore space of gas shales under actual geological conditions.

Lithofacies identification of shale reservoirs using a tree augmented Bayesian network: A case study of the lower Silurian Longmaxi formation in the changning block, South Sichuan basin, China

As important units for evaluating shale gas reservoirs, lithofacies exert obvious control on the abundance of shale organic matter and the enrichment degree of gas reservoirs. Therefore, it is of great significance to accurately identify the lithofacies of shale reservoirs relative to shale gas exploration and development. This paper takes the Longmaxi Formation shale in the Changning block, Sichuan Basin, China, as an example and uses a tree augmented Bayesian network for the quantitative classification and identification of lithofacies for the first time. First, based on thin section and scanning electron microscopy observations and X-ray diffraction, we can determine the shale lithofacies types of the Longmaxi Formation. Second, the logging response characteristics of different lithofacies were obtained by comparing conventional logging data. Finally, the tree augmented Bayesian network (TAN) was built according to the selected logging parameters and identified unknown lithofacies. The results are as follows: (1) Longmaxi Formation shale is rich in siliceous minerals. By comparing analysis of shale mineral composition and diagenetic characteristics, we divide the lithofacies into five types: siliceous rock, siliceous shale, argillaceous/siliceous mixed shale, calcareous/siliceous mixed shale and calcareous shale. (2) Based on the logging data, 6 logging curves (GR, AC, DEN, CNL, PE and RT) can sensitively indicate diagenesis and changes in mineral content. (3) The TAN model trained by logging information was used to identify 68 shale samples through the 5-fold cross validation method, with an accuracy rate of 95.58%. The results showed that the tree augmented Bayesian network not only can overcome the limitation of interdependence among well-logging properties but also represents an efficient and accurate new method for identifying shale lithofacies. This algorithm can be applied to lithofacies identification of shale reservoirs with only conventional well-logging data, which can help promote the development of lithofacies identification of unconventional oil and gas geology and provides a new method for shale gas reservoir evaluation.

Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review

Many sets of Paleozoic marine organic-rich shale strata have developed in South China. However, the exploration and development results of these shale formations are quite different. Based on the data of core experiment analysis, drilling, fracturing test of typical wells, the reservoir differences and controlling factors of four sets of typical marine organic-rich shale in southern China are investigated. The four sets of shale have obvious differences in reservoir characteristics. Ordovician–Silurian shale mainly develops siliceous shale, mixed shale and argillaceous shale, with large pore diameter, high porosity, moderate thermal maturity, large pore volume and specific surface area. Cambrian shale mainly develops siliceous shale and mixed shale, with small pore diameter, low porosity, high thermal maturity and smaller pore volume and specific surface area than Ordovician–Silurian shale. Devonian–Carboniferous shale has similar mineral composition to Ordovician–Silurian shale, with small pore diameter, low porosity, moderate thermal maturity and similar pore volume and specific surface area to that of Cambrian shale. Permian shale has very complex mineral composition, with large pore diameter, low to medium thermal maturity and small specific surface area. Mineral composition, thermal maturity and tectonic preservation conditions are the main factors controlling shale reservoir development. Siliceous minerals in Cambrian shale and Ordovician–Silurian shale are mainly of biological origin, which make the support capacity better than Devonian–Carboniferous shale and Permian shale (siliceous minerals are mainly of terrigenous origin and biological origin). Thermal maturity of Ordovician–Silurian shale and Devonian–Carboniferous shale is moderate, with a large number of organic pores developed. Thermal maturity of Cambrian shale and Permian shale is respectively too high and too low, the development of organic pores is significantly weaker than the two sets of shale above. There are obvious differences in tectonic preservation conditions inside and outside the Sichuan Basin. Shale reservoirs inside the Sichuan Basin are characterized by overpressure due to stable tectonic activities, while shale reservoirs outside the Sichuan Basin are generally normal–pressure. Four sets of marine shale in South China all have certain resource potentials, but the exploration and development of shale gas is still constrained by complicated geological conditions, single economic shale formation, high exploration and development costs and other aspects. It is necessary for further research on shale gas accumulation theory, exploration and development technology and related policies to promote the development of China’s shale gas industry.

Nano-scale mechanical properties of constituent minerals in shales investigated by combined nanoindentation statistical analyses and SEM-EDS-XRD techniques

The mechanical properties of the constituent minerals in shale rock are fundamental to a better knowledge of multi-scale shale behaviors. It benefits the engineering applications and predictive physics modeling of the shale formation . In this work, nanoindentation testing combined with scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) were used to obtain the in situ mechanical properties of individual mineral phases in four shale samples. Engraved cross-marks were employed to locate the indented areas on a micron scale for subsequent SEM-EDS measurements and analysis. The elastic moduli and hardness of the quartz, iron-type minerals, muscovite , clinochlore , organic matter, and the mineral assortments in the matrix were analyzed and relevant deformative behaviors were compared. Results show that the identified mineral groups exhibit a diverse set of mechanical properties at the nanoscale . The iron-type minerals have the highest elastic modulus (104.7 GPa), then followed by quartz, muscovite, clinochlore, and organic matter. Quartz shows the highest hardness (10.2 GPa). The identified minerals demonstrated different elasto-plastic characteristics to the indentation loads. Quartz exhibits the lowest plastic behavior, while phyllosilicates exhibited large plastic behavior owing to the layered structure. Organic matter showed both elastic-dominant and plastic-dominant behaviors, which may be related to the chemical compositional differences and various thermal maturity of kerogen in the shale samples. The mechanical properties of three mineral assortments in shale matrices vary significantly due to the wide variety of the mineral compositions. Radial cracks were observed on the boundary of brittle minerals and relatively weak minerals, while shear cracks were observed on strength-weak minerals such as layered silicates and organic matters. The mechanism of the induced cracks was discussed. This study obtains a basic understanding of shale behavior on nano-scale and provides reliable basic data for multi-scale modeling of shale reservoirs without the need for large-scale mechanical tests.

Thermal Evolution and Hydrocarbon Generation Capacity of Typical Palaeozoic Marine Source Rocks in the South China Craton: Constrains from Semiclosed Artificial Thermal Maturation Experiments

Abstract Palaeozoic petroleum systems in the South China craton are prospecting frontiers for unconventional oil/gas. Understanding the hydrocarbon generation/retention capacity of source rocks has critical implications for reserve evaluation and targeting, which yet remains ambiguous in this region. In this study, six representative lithological types of organic-rich, immature-low-mature source rocks (two shales, two carbonates, a coal, and a solid bitumen) taken from the South China craton were treated with semiclosed hydrous pyrolysis (300–550°C). Pyrolysis results show that shale and carbonate source rocks have great hydrocarbon generation capacities, and immature solid bitumen can also generate appreciable quantities of hydrocarbons, whereas coal has the least hydrocarbon generation capacity. Both oil and gas are important hydrocarbon products of thermal maturation of shales, whereas gas generation capacity of carbonate source rocks is relatively moderate compared to their oil generation capacity. Therefore, marine shale is an important exploration target for unconventional oil and gas, yet marine carbonate is an important target for unconventional oils. The presence of appreciable carbonate minerals in shales can facilitate oil and gas generation likely by (1) enhancing oil expulsion, (2) facilitating decarboxylation of organic acid, and (3) catalysis. Pyrolysis data also support that molecular composition of thermogenic gas is not dominantly controlled by lithofacies of source rocks but mainly affected by maturation. Outcomes of this study demonstrate that marine organic-rich rocks within the Palaeozoic strata in the South China craton are critical prospecting targets for unconventional oil and gas.

Molecule Simulation of CH4/CO2 Competitive Adsorption and CO2 Storage in Shale Montmorillonite

The main source of production in the middle and late stages of shale gas extraction is the adsorbed gas in shale, and the adsorbed gas in shale mainly comes from organic matter casein and clay minerals in shale; therefore, this paper uses sodium-based montmorillonite to characterize the clay minerals in shale and study the CH4 adsorption law in clay minerals, and this study has certain guiding significance for shale gas extraction. In addition, this paper also conducts a study on the competitive adsorption law of CH4 and CO2, and at the same time, predicts the theoretical sequestration of CO2 in shale clay minerals, which is a reference value for the study of CO2 burial in shale and is beneficial to the early realization of carbon neutral. In this paper, the slit model of sodium-based montmorillonite and the fluid model of CH4 and CO2 were constructed using Materials Studio software, and the following two aspects were studied based on the Monte Carlo method: Firstly, the microscopic adsorption behavior of CH4 in sodium-based montmorillonite was studied, and the simulations showed that the adsorption capacity of montmorillonite decreases with increasing temperature, increases and then decreases with increasing pressure, and decreases with increasing pore size. CH4 has two states of adsorption and free state in the slit. The adsorption type of CH4 in montmorillonite is physical adsorption. Secondly, the competitive adsorption of CH4 and CO2 in sodium-based montmorillonite was studied, and the simulations showed that the CO2 repulsion efficiency increased with increasing CO2 injection pressure, and the CO2/CH4 competitive adsorption ratio decreased with increasing pressure. The amount of CO2 storage decreased with increasing temperature and increased with increasing CO2 injection pressure.

Lithofacies and its controls on the organic matter-hosted pores in the Cambrian gas-rich Niutitang shale in upper Yangtze Plate, China

The organic-rich shale in the Lower Cambrian Niutitang Formation (Є1n) has great gas resources potential in south China but have no commercial gas production achieved yet. The EYY1 well is the first well located at the eastern edge of the upper Yangtze plate to obtain success in commercial gas production in the Niutitang Formation, providing a rare case for better understanding gas enrichment mechanism in the Niutitang shale. By observation of cores and thin sections, bulk rock X-ray diffraction mineral analysis, the Niutitang shale is classified into three kinds of organic-rich lithofacies: calcareous shale (CS), siliceous shale (SS) and transitional shale (TS). The organic matter (OM) and OM-hosted pore in different lithofacies were examined by using scanning electron microscope (SEM) with argon-ion polishing sample treating, accompanied by energy spectrum test and SEM image analysis. The results show that: (1) Shale lithofacies control the organic matter type and development degree of organic matter-hosted pores (OMPs). (2) CS have plenty of depositional organic matter (DOM) in which nano-micron sized OMPs are poorly developed with surface porosity ranges between 3.05% and 9.08%. Siliceous and transitional shale have more migrated organic matter (MOM), which possess more sponge-like nano-sized pores with surface porosities range of 10.7%–31.5%. The pore development degree and gas content of siliceous shale is higher than that of transitional and calcareous shale. (3) Organic matter abundance (TOC content), type, and mineral composition also have important constrain on OMPs development. TOC has a positive correlation to the OMPs, whereas TOC 4 wt% or so is a balance point between the influence of organic matter content and formation compaction on pore development, higher TOC cannot prevent the pore from collapsing due to compaction in this region. The OMPs associated with or adjacent to clay minerals in siliceous shale are more developed, which indicate clay minerals which have positive effects on the adsorption of organic matter and the formation and preservation of OMPs.

Boron in the strata and waters of the Eocene Green River and Uinta Formations, Uinta Basin, Utah

Boron is a naturally occurring element that can be found in the strata and water of the Eocene Green River and Uinta Formations of the Uinta Basin of Utah. Whereas boron is suspected to be a necessary trace nutrient for proper plant and animal growth and development, higher concentrations of boron can be detrimental to living things, but the element is not known to be carcinogenic. Utah has no limit on boron content in drinking water; however, for irrigation purposes a limit of 750 μg B/L (0.75 mg/L) has been established. The Green River Formation (GRF), deposited in ancient lakes in Wyoming, Colorado, and Utah, is well known for its oil shale and saline mineral deposits, particularly the thick, economic trona beds in Wyoming. Such evaporitic mineral deposits are characteristic of a saline lake environment that existed at the end stages of Eocene lacustrine deposition. In Utah they are found in the upper GRF, and to a minor extent in the lower Uinta Formation, where elevated boron is evident. Boron-bearing minerals are present in the Parachute Creek Member of the GRF in Utah, and they occur as secondary silicate minerals in at least 12 wells across Utah’s Uinta Basin. These minerals were first reported by the U.S. Geological Survey in the 1950s. This study determined that the boron mineral occurrences correlate stratigraphically and coincide with the areas delineated for the hypersaline events in the Parachute Creek Member of the GRF. This argues that boron was concentrated with other saline constituents in Lake Uinta and is an indicator of hypersaline conditions. Water quality analyses reporting boron content from surface and subsurface samples were compiled from public databases from the Utah Department of Environmental Quality, the U.S. Geological Survey, and the Utah Geological Survey. This new database consists of 155 samples from 38 sites from the upper part (Parachute Creek Member) of the GRF, and 58 samples from 26 sites from the lower part (Douglas Creek Member) of the GRF. The boron concentration was found to be different for the two GRF parts. The average boron content of the 58 lower GRF aquifer water analyses is 6338 μg/L, while the mean boron content for the 26 individual sites varies from 15 to 205,000 μg/L. The upper GRF aquifer contains at least twice the boron content of the lower GRF, and the average boron content of the 155 analyses of upper GRF groundwater is 18,172 μg/L. The mean boron content for the 38 individual sites varies 40 to 480,000 μg/L. Areas of highboron concentration in groundwater of the GRF tend to coincide with the location of GRF hypersaline paleodepocenters. Groundwater boron content in the Uinta Formation comes from 40 analyses from 36 sites. The average of the 40 Uinta Formation analyses is 3251 μg B/L, while the mean boron content for the 36 individual sites varies from 40 to 24,000 μg/L. For each interval studied, less boron tends to be found in analyses from sites near the outcrop and boron content tends to increase in the studied formations as they are more deeply buried. Additionally, boron content was compiled for surface waters and springs for 3955 analyses from 374 sites in the Uinta Basin. When the mean boron content of these surface water sites was examined by hydrologic drainage unit subareas, it was found that the tributaries in the northern Uinta Basin, north of the Duchesne and White Rivers, contain the lowest mean boron contents, whereas higher mean boron contents are common for the tributaries in southern part of the basin. The boron content of surface water from the southern Uinta Basin drainages also tends to increase northward toward the demarking water courses. The tributaries in the southern part of the Uinta Basin have higher boron contents due to their waters having contact with the boron-bearing, shallow-inclined strata of the upper GRF or member B of the Uinta Formation. The tributaries in northern part of the basin have lower boron contents because they are not in contact with the GRF, have less contact with the member B of the Uinta Formation, and are diluted by greater snow-melt runoff from the Uinta Mountains which bound the basin to the north.

The genetic relationship between paleoenvironment, mineral compositions and lithofacies in the Ordovician–Silurian Wufeng–Longmaxi sedimentary succession in the Sichuan Basin, SW China

• Five intervals (I–V) showing changes in minerals, lithofacies and paleoenvironment were identified. • High productivity and low detrital influx lead to the enrichment of nondetrital quartz in siliceous shale in Intervals I-II. • The enrichment of carbonate in calcareous shale in Interval III was due to high detrital influx. • High detrital influx and strong chemical weathering lead to an abundance of silt and clay minerals in silty–argillaceous shale and argillaceous shale in Intervals IV-V. Changes of sedimentary environments recorded in the Late Ordovician–Early Silurian sedimentary succession can be reflected in mineral composition and lithofacies of shales. Outcrops and cores from this set of shales in the Sichuan Basin, China, are analyzed. The results show that quartz, clay minerals, and carbonate are the main minerals. Five major lithofacies are developed. Based on the provenance proxies (Al 2 O 3 /TiO 2 , Th/Sc, and Zr/Sc ratios), chemical index of alteration (CIA), paleoproductivity proxies (Cu/Al and Ni/Al ratios), redox condition proxies (U/Th, V/Cr, and Ni/Co ratios), proxies of detrital influx (Ti and Al contents), and sedimentation rate (SR), the paleoenvironment is analyzed. The source rocks are dominated by felsic igneous rock. Comparison between five lithofacies revealed that the relatively low degree of chemical weathering, anoxic conditions, maximum paleoproductivity probably associated with the high frequency of volcanic activities, and minimum detrital influx and SR (0.60–5 m/Ma) led to the enrichment of nondetrital quartz and organic matter, which eventually formed siliceous shale. The moderate degree of chemical weathering, dysoxic–oxic conditions, decreasing paleoproductivity, and increasing detrital influx and SR (10–16.67 m/Ma) resulted in the increase in detrital quartz, clay minerals, and carbonate content, and the development of mixed shale and calcareous shale. Relatively strong chemical weathering, oxic conditions, minimum paleoproductivity, and maximum detrital influx and SR (100.74 m/Ma) favored the enrichment of silt and clay minerals, forming silty–argillaceous shale and argillaceous shale. This study provides an insight into the relationship between the environmental changes and shale mineral composition and lithofacies.

Transitional shale quality and exploration potential: A case study from the Lower Carboniferous Jiusi Formation in northwest Weining, China

The organic-rich shale of the early Carboniferous Jiusi Formation in the Weining area, Southwestern China, has large geological gas reserves, making the Jiusi Formation another target after the Ordovician-Silurian marine shale in South China. The complex superposition of multi-stage tectonic phases since the Himalayan paroxysmal phase has induced a series of controversies in the sedimentology of the Jiui Formation, as only a few drillings were executed. In order to better understand the development potential of the shale associated with the Jiui Formation, five outcrops and a well (YS302) were studied, clarifying the sedimentary evolution of the formation, while two 2D seismic sections were used to analyze the regional structural styles. The result shows that the Jiusi Formation was significantly influenced by the synsedimentary extension of the northwestern margin of the Yadu-Ziyun-Luodian Fault, with the depositional center of the Jiusi Formation extending to well YS302 along a NW direction and asymmetrically thinning in a NE–SW direction. Fifteen lithofacies and three facies associations were identified, from tidal or delta settings to lagoon environments, finally turning into platform facies at the base of the Shangsi Formation. According to the geochemical analysis of 27 core samples and 37 outcrop samples, as well as the well YS302 logging interpretation result. The transitional Jiusi shale in the Weining area has a strong heterogeneity, with high total organic content (TOC) values being mostly provided by the surrounding coal seams. XRD analyses show that the minerals of the transitional shale are dominated by a high clay content (42 wt%). The content of water-sensitive minerals in Jiusi shale is also relatively high, with clay minerals dominated by an illite/smectite mixture (52.8%). When compared with the commercially developed Ordovician-Silurian marine shale, the Jiusi transitional shale shows relatively poor quality, related to dense bioturbation and terrestrial input affecting the preservation of organic matter. Therefore, the evaluation standard for marine shale is difficult to apply to the transitional Jiusi shale. The cumulative thickness of shale in YS302 worthy of shale gas development is nearly 100 m. Meanwhile, considering that the transitional shales are often interlayered with coal, the shale gas and coalbed methane can be developed together.

A preliminary study on the geological conditions and indexes for the accumulation of shale gas in the marine-continental transitional facies sediments in the South North China Basin

At present, the Carboniferous-Permian shale gas in the South North China Basin is still in the exploration stage, and the understanding of the microscopic pore structures, mineral composition and hydrocarbon enrichment law of the marine-continental transitional shale gas reservoirs in this area is extremely limited. In this paper, taking the Carboniferous-Permian shale gas reservoir in the South North China Basin as an example, the geological conditions of shale gas accumulation have been systematically studied using a large amount of sedimentary, logging, seismic, core, geochemical, physical property, and scanning electron microscope data. The study shows that the transitional dark shale is stable and widely distributed in the South North China Basin. Among them, the average thickness of the high-quality shale in the Taiyuan Formation is 67 m, while that of the Shanxi Formation is 56 m. Carboniferous and Permian are the main strata for the distribution of shale gas resources. Among them, the dark shale developed in the Taiyuan and Shanxi Formations has the largest thickness and the highest organic matter abundance. In addition, the organic matter types of the Upper Paleozoic coal-measure source rocks in the South North China are mainly II2-III types, and a small part of them belong to II1 types. The brittle minerals in the target shale are mainly quartz, and the content of illite is the highest among the clay minerals. Porosity is negatively correlated with clay mineral content, and positively correlated with quartz and TOC content. In addition, the permeability is negatively correlated with the organic matter content. Finally, the criteria for key indicators of the transitional shale gas reservoirs in the South North China Basin were developed.

A quantitative analysis of flow properties and heterogeneity in shale rocks using computed tomography imaging and finite-element based simulation

A quantitative evaluation of flow property influenced by micro-scale heterogeneities in shale rocks is presented. Micro- and nano-Computed Tomography images of shale rock samples obtained from Mancos, Marcellus and Eagle Ford formations are digitised into entities such as shale grains, organic matter (kerogen), shale minerals, pores and micro-cracks. Numerical computation on selected layers and sub-micron divisions of the geometries is then carried out using a computationally efficient finite-element based simulation algorithms. Parameters such as pore-volume distribution, porosity, permeability and heterogeneity factor within and across layers are computed as part of pre- and post-process operations. The results of these flow properties characterisation indicated that some shale samples have micro cracks while some do not have. Meanwhile, the presence of micro-cracks in the shale samples contributed to the observed wide variation in permeability and porosity. The static characterisation revealed that different shale rock samples will have different morphological properties. The highest variance in permeability within a layer is of magnitude 5 in Eagle Ford while a magnitude of 2 was computed for Mancos perpendicular. However, magnitudes of 1 and 4 are recorded across layers for Mancos perpendicular and Eagle Ford perpendicular respectively. Contrary to existing assumption that heterogeneity at pore-scale is negligible, it is established from the aforementioned analysis that micro-scale heterogeneity can be quantified. Furthermore, through the analysis of variance and root-mean-square values, it was concluded that tortuosity is inversely related to porosity, permeability and their degree of heterogeneity.

Impact of Pyrite Oxidation on the Pore-Structure Characteristics of Shale Reservoir Rocks under the Interaction of Fracturing Fluid.

Hydraulic fracturing combined with horizontal drilling is widely used to develop shale gas resources, and huge amounts of fracturing fluid are injected into shale reservoirs. However, the fracturing fluid is ineluctably retained in reservoir rocks after fracturing, resulting in the alteration of shale pore systems and further affecting the hydrocarbons production efficiency. In this work, two types of shales with different pyrite contents, namely, pyrite rich (PR, Niutitang Formation) and pyrite poor (PP, Xiamaling Formation), were emphasized to illustrate the effect of pyrite oxidation on pore structure after fracturing operation. Slickwater fracturing fluid was used to treat the shale samples for a period of 3 days, under the condition of 100 °C and 50 MPa. The field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) were utilized to determine the surface morphology and mineral composition. The low-temperature N2 adsorption was performed to quantify the pore structure. The results showed that the pyrite oxidation induced the dissolution of both the pyrite and calcite and generated many dissolution pores for the pyrite-rich shale after slickwater treatment. The mineral dissolution led to an increase in the number of mesopores, enlarged the total specific surface area (TSSA) and total pore volume (TPV), and strengthened the pore-structure complexity. On the other hand, the pyrite-poor shale only experienced clay swelling after slickwater treatment. Its pore surface roughness and pore-structure complexity degraded with the loss of nanopores and the reductions in TSSA and TPV. The results of this study enhance the understanding of the impact of pyrite oxidation on the pore structure and provide new insight into the optimization of fracturing operation conditions based on shale’s mineral composition characteristics.

Influence of Organic-Inorganic Composition on the Adsorption of Niutitang Formation Shale in Huijunba Syncline

To study the influence of different organic-inorganic composition on the adsorption in shale, taking the Niutitang Formation shale in the Huijunba syncline in southern Hanzhong as the research object, based on acid treatment to extract kerogen and wet chemicals. Extracting mineral components by oxidation method, a series of isothermal adsorption experiments were carried out on the original rock, kerogen, and mineral components of the samples. Through the three-dimensional Langmuir model fitting and adsorption thermodynamic calculation method, the adsorption thermodynamic parameters such as the isometric heat of adsorption and the standard adsorption entropy are obtained. The research results show that organic matter is the primary factor affecting the methane adsorption of shale and clay minerals also affect the adsorption to a certain extent. When the burial depth is relatively shallow, diagenesis has a promoting effect on the total amount of adsorption. As the depth increases, the diagenesis will turn into an inhibitory effect on the total amount of adsorption. Through the component weighting method, the lower limit ratio of the adsorbed amount of organic matter to the total amount in the shale is 29.3%~46.7% when the depth reaches to 3000 m, while the upper limit ratio of the adsorption of mineral components is 66.4%~73.6%.

Pyrite Characteristics and Its Environmental Significance in Marine Shale: A Case Study from the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formation in the Southeast Sichuan Basin, SW China

Pyrite, as a characteristic mineral in organic-rich marine shale, is a significant index for the interpretation of paleoredox conditions. In this study, based on drilling cores and focused ion beam-scanning electron microscopy (FIB-SEM), the occurrence, diameter and particle size distribution of pyrites from 32 samples obtained from the Wufeng–Longmaxi Formation in the southeast Sichuan Basin were analyzed. The results show that pyrite displays various occurrences at the macro-scale and micro-scale. At the macro-scale (mm–cm), pyrite laminations, nodular pyrites and lenticular pyrites can be found from drilling cores. At the micro-scale (nm–µm), the common occurrences of pyrite are pyrite framboids, euhedral pyrites and infilled pyrite framboids. According to the formation mechanism of pyrites, pyrites can be divided into syngenetic pyrites and diagenetic pyrites. The infilled pyrite framboids are categorized as diagenetic pyrites. The mean pyrite framboid diameters (Mean, D) range from 2.94 µm to 5.33 µm (average of 4.26 µm), with most samples showing pyrite framboid diameters from 3.5 μm to 4.8 μm. Most of the diameters of the framboid microcrystals (Mean, d) are less than 0.4 µm. Therefore, according to the (Mean, D) and the (Mean, d), the pyrite framboids can be divided into three sizes: syngenetic framboids (SF, D < 4.8 µm, d ≤ 0.4 µm), early diagenetic framboids (EDF, D > 4.8 µm, d > 0.4 µm) and late diagenetic framboids (LDF, D < 4.8 µm, d > 0.4 µm). Additionally, box-and-whisker charts of the diameter, standard deviation/skewness value of the mean diameter of pyrite framboids (Mean, D) and the ratio of trace elements indicate that the sedimentary water body was a euxinic–dysoxic environment. Euxinic conditions dominated the Wufeng Formation to the lower part of the Long11-3 section, which is beneficial for the preservation of organic matter. However, the middle-upper part of the Long13-Long12 sub-member is a dysoxic sedimentary environment.

A numerical investigation for the impacts of shale matrix heterogeneity on hydraulic fracturing with a two-dimensional particle assemblage simulation model

A shale gas reservoir has complex mineral components, bedding planes and micro-cracks, thus being of inherent heterogeneity and anisotropy. The roles of bedding planes in hydraulic fracturing performance have been deeply studied, but the effects of rock inherent heterogeneity induced by mineral components are not well understood. This paper establishes a two-dimensional particle assemblage model to investigate the impacts of mineral component heterogeneity on hydraulic fracturing treatments. The impacts of the mass fraction and elastic modulus of low-brittle minerals on fracture initiation pressure, breakdown pressure, fluid injection volume and fracture permeability are comparatively investigated through numerical simulations. It has been found that the mass fraction of low-brittle minerals is closely related to the critical pore water pressure while the elastic modulus of low-brittle minerals plays a significant role in fracture permeability. With the same hydraulic fracturing treatments, the shale gas reservoir with a large mass fraction of clay minerals could decrease the critical pore water pressure, while the shale gas reservoir with a low elastic modulus of clay minerals is conducive to increasing fracture aperture. The shale matrix heterogeneity induced by the fraction coefficient and micro-mechanical properties of clay mineral has a critical role in the fracture initiation and propagation as well as the variation of fracture aperture. This understanding of shale mineral composition can benefit the effectiveness evaluation on hydraulic fracturing and the morphology of fracture networks.

U-Net model for multi-component digital rock modeling of shales based on CT and QEMSCAN images

The establishment of digital images of cores with multi-mineral components holds the premise of analyzing the spatial distribution of shale minerals and carrying out numerical simulations of their physical properties. However, it is a challenge to directly obtain 3D multi-mineral-component digital core images by the existing technical means. The image multi-threshold segmentation of grayscale core images is a common method to obtain multi-component core images, but the thresholds need to be manually adjusted, which is time-consuming and may cause large errors. In this paper, to establish shale digital core images with multi-mineral components accurately and quickly, with the shale CT image as the original image and the corresponding QEMSCAN image as the label image, a U-Net model is trained and used to perform automatic segmentation of the CT images to convert grayscale images into multi-mineral-component images. Then, the manually segmented micropores and fractures are added to form the final model. The image segmentation effect of the constructed U-Net model is analyzed, and the results are satisfactory in terms of mineral content, morphological characteristics, and spatial structure. Compared with the traditional method, our method avoids repeated artificial adjustment, makes full use of the information in the CT images and QEMSCAN images, realizes the rapid automatic segmentation, and provides a feasible way for establishing images of a core with multi-mineral components.