Brittle Minerals Research Articles

Assessment of the Multiphase Mechanical Properties of the Longmaxi Formation Shale Using Nanoindentation Tests.

Mechanical properties are some of the most important parameters for understanding well drilling and hydraulic fracturing designs in unconventional reservoir development. As an effective tool, nanoindentation has been used to determine the mechanical properties of rocks at the nanoscale. In this study, the Longmaxi Formation shale samples from the Yibin area of China were collected and analyzed to obtain the multiphase mechanical properties. The mineral compositions and organic geochemistry of the shale samples were studied using X-ray diffraction, energy-dispersive X-ray spectrometry, and a carbon/sulfur analyzer. The pore structures of the shale samples at the micro- and nanoscales were characterized by field-emission scanning electron microscopy. The mechanical parameters of the shale samples, such as the hardness and elastic modulus, were investigated using the nanoindentation method to identify three mineral phases: brittle minerals, soft matters, and complex minerals at the interfaces between brittle minerals and soft matters. The uncertainty characteristics of the mechanical parameters of the three mineral phases were evaluated using the Weibull model, and the factors interfering with the mechanical parameters were analyzed for the different shale samples. The results showed that the brittle minerals had the largest recovered elastic deformations and the smallest residual deformations, while the soft matters had the largest residual deformations and the smallest recovered elastic deformations. The analysis results of the coefficients of variation and the Weibull modulus both confirmed that the scatter of the hardness was higher than that of the elastic modulus because of the uncertain contact area, and the hardness and elastic modulus of the soft matters had the highest uncertainty among the three mineral phases. The elastic modulus increased nonlinearly with increasing hardness according to a power function for the whole shale sample. The elastic modulus and hardness both had a favorable linear relationship with the total organic carbon (TOC) content, illustrating that the TOC content was one of the significant factors that affected the mechanical parameters of the shale samples.

Synergetic effects of matrix components and diagenetic processes on pore properties in the Lower Cambrian shale in Sichuan Basin, South China

Synergetic effects of matrix components and diagenetic processes on pore properties were investigated from macroscale to nanoscale. Methods were combined to explore shales selected from well J1 in South China, in terms of elements measurement, organic geochemistry analysis, X-ray diffraction, nuclear magnetic resonance and direct imaging observation (2D wide horizon and 3D reconstruction). Three lithofacies were identified and porosity/permeability decrease in the order: authigenic siliceous shale > detrital clastic-rich shale > carbonate-rich shale. Further, different porous properties among lithofacies can be detected via image stitching technology, to achieve compatibility of large vision and high resolution. In this way, pore parameters are intuitively linked to related matrixes, which facilitates the elaboration of internal relationship between properties of pores and diagenetic mechanisms of matrixes. Authigenic siliceous shale is characterized by small pore size distribution (30–60 nm), high SEM-based surface porosity (6.04%) and good connectivity rate (60.66%), which is attributed to authigenic quartzes providing pre-existing frameworks and abundant OM contributing pore volumes. Detrital clastic-rich shale is characterized by broad pore size distribution (20–600 nm), moderate SEM-based surface porosity (3.09%) and fair connectivity rate (27.84%), since abundant detrital grain and lean OM trigger the domination of inorganic-related pore which are isolated with large diameters and small amounts. Carbonate-rich shale is characterized by small pore size distribution (10–100 nm), low SEM-based surface porosity (0.77%) and poor connectivity rate (12.86%), due to strong compaction, intensive cementation and limited OM migration. Even with similar geological history, diverse pore types and porous properties can be detected among the three lithofacies, which may attribute to different matrix components and their diagenetic processes. At over-mature stage, brittle minerals (quartz and calcite/dolomite) control OM distribution and prevent secondary pores from compaction. Whereas, ductile matrixes (clay and OM) provide the dominant pore volumes. Especially, OM can facilitate connectivity with porous attribute and migrating ability.

Pore structure and sorption capacity investigations of Ediacaran and Lower Silurian gas shales from the Upper Yangtze platform, China

The shale gas potential of Ediacaran and Lower Silurian shales from the Upper Yangtze platform is assessed in this study with a focus on the contributions of clay minerals and organic matter to sorption capacity. For this purpose, a multidisciplinary assessment was carried out using petrophysical, mineralogical, petrographic and geochemical methods. In terms of TOC contents (4.2%), brittle mineral contents (68.6%) and maximum gas storage capacities (0.054–0.251 mmol/g) Ediacaran shales from this study show comparable properties to other producing shale gas systems although the thermal maturity is extremely high (VRr = 3.6%). When compared to lower Silurian shales from the same region, it is evident that (1) deeper maximum burial and (2) a lack of silica-associated preservation of the pores resulted in a relatively lower mesopore volume, higher micropore volume fraction and lower overall porosity (Ediacaran shales: 1.4–4.6%; Silurian shales: 6.2–7.4%). Gas production is therefore retarded by poor interconnectivity of the pore system, which was qualitatively demonstrated by comparing experimental gas uptake kinetics. TOC content exhibits a prominent control on sorption capacity and micropore volume for both shales. However, different contributions of clay minerals to sorption capacity were identified. This can partly be attributed to different clay types but is likely also related to burial-induced recrystallisation and different origins of illite. Additionally, it was shown that variations in sorption capacity due to incorrect estimates of clay mineral contribution are in the same range as variations due to differences in thermal maturity.Article highlightsPore structure and gas storage characteristics are evaluated for the first time for Ediacaran Shales from the Upper Yangtze platformDue to a lower free gas storage capacity and diffusivity, the Ediacaran shale can be regarded as a less favorable shale gas prospect when compared to the Silurian shaleClay mineral contribution to sorption capacity is evaluated taking clay mineralogy into considerationMaturity-related changes of organic matter sorption capacity have been discussed on the basis of a compiled data set

Evaluation of Favorable Shale Gas Intervals in Dawuba Formation of Ziyun Area, South Qian Depression

A series of qualitative descriptions and quantitative analyses was used to determine the lithofacies characteristics and recognize the favorable shale intervals of Dawuba Formation in the Ziyun area of South Qian Depression. The qualitative descriptions include core description and scanning electron microscope (SEM) observation. The quantitative analyses include X-ray diffraction, total organic content analysis, vitrinite reflectance, maceral composition, porosity, and permeability, as well as gas and element composition. The Dawuba Formation could be divided into four members. In general, the shale in the first and third members showed similarly high organic matter content with most samples in range of 2–2.5% and higher brittle mineral content with a content of quartz 40.53% and 33.21%, respectively, compared with the other two members, as well as high gas content. However, the first member shale samples exhibited much higher porosity and permeability than the third member shale samples. Furthermore, the shale gas in the first member was chiefly composed of methane (average: 83.63%), while that in the third member mainly consisted of nitrogen (average: 79.92%). Hence, the first member should be regarded as the most favorable target.

Pore structure and heterogeneity of shale gas reservoirs and its effect on gas storage capacity in the Qiongzhusi Formation

Fine characterization of pore systems and heterogeneity of shale reservoirs are significant contents of shale gas reservoir physical property research. The research on micro-control factors of low productivity in the Qiongzhusi Formation (Fm.) is still controversial. The lower Cambrian Qiongzhusi Fm. in the Qujing, Yunnan was taken as the object to investigate the influence of mineral compositions on the physical properties of the reservoir and the heterogeneity of shale, using the algorithm to improve the characterization ability of Atomic Force Microscopy (AFM). The results showed that: (1) The pores are mainly wedge-shaped pores and V-shaped pores. The pore diameter of the main pore segment ranges from 5 to 10 nm. Mesopores are mainly developed in the Qiongzhusi Fm. shale in Well QD1, with the average pore diameter of 6.08 nm. (2) Microscopic pore structure and shale surface properties show strong heterogeneity, which complicates the micro-migration of shale gas and increases the difficulty of identifying high-quality reservoirs. (3) The increase of clay mineral content intensifies the compaction and then destroys the pores. Conversely, brittle minerals can protect pores. The support and protection of brittle minerals to pores space depend on their content, mechanical properties and diagenesis. (4) Compression damage to pores, large microscopic roughness and surface fluctuations and strong pore structure heterogeneity are the reasons for the poor gas storage capacity of the Qiongzhusi Fm., which will lead to poor productivity in the Qiongzhusi Fm.

Key factors controlling shale oil enrichment in saline lacustrine rift basin: implications from two shale oil wells in Dongpu Depression, Bohai Bay Basin

Comparative analyses of petroleum generation potential, reservoir volume, frackability, and oil mobility were conducted on 102 shale cores from the Dongpu Depression. Results show the shale has high organic matter contents composed of oil-prone type I and type II kerogens within the oil window. Various types of pores and fractures exist in the shale, with a porosity of up to 14.9%. The shale has high brittle mineral contents, extensive fractures, and high potential for oil mobility due to high seepage capacity and overpressure. Although the petroleum generation potential of the shale at Well PS18-8 is relatively greater than that at Well PS18-1, oil content of the latter is greater due to the greater TOC. The porosity and fracture density observed in Well PS18-1 are greater and more conducive to shale oil enrichment. Although the shales in Wells PS18-1 and PS18-8 have similar brittle mineral contents, the former is more favorable for anthropogenic fracturing due to a higher preexisting fracture density. Besides, the shale at Well PS18-1 has a higher seepage capacity and overpressure and therefore a higher oil mobility. The fracture density and overpressure play key roles in shale oil enrichment.

Porosity evolution in organic matter-rich shales (Qingshankou Fm.; Songliao Basin, NE China): Implications for shale oil retention

The Songliao Basin is an important hydrocarbon province of Northeastern China and has a long history of conventional oil and gas production. Nowadays, unconventional resource estimation becomes increasingly important and particularly shale oil/gas production from mature source rocks in the Central Depression may be of great relevance for the regional exploration strategy. The Qingshankou Formation is a promising source rock reservoir. However, for shale oil/gas production to be successful, a refined understanding of the distribution of hydrocarbons within the low-permeable rocks, as well as the timing of hydrocarbon expulsion and its linkage to changing pore characteristics, are key requirements. Therefore, a multi-analytical geochemical and porosity study was conducted on a set of 12 oil-prone, alginite-rich samples from a depth range from 200 to 2500 m in order to understand the linkage of burial compaction, hydrocarbon generation and pore evolution. Macro- (>50 nm) and mesopores (2–50 nm) were investigated by a combination of nitrogen gas adsorption/desorption measurements at 77 K and broad ion beam – scanning electron microscopy. The adsorption tests were conducted on samples before and after solvent-extraction in order to study the effects of soluble organic matter (OM) on pore distributions. These data were compared with organic petrographical observations, bulk geochemical parameters before and after extraction, as well as extract yields from the bulk source rock in order to link structural and geochemical changes in the interval from immature to late peak oil mature (0.4–0.9 %Rr). Total SEM-visible macro- and mesoporosity (φBIB) is generally limited, but a weak trend of total φBIB with quartz contents indicates the importance of brittle minerals for pore preservation. A depth trend towards smaller average pore sizes, higher pore aspect ratios, as well as preferentially bedding-parallel pore orientations reflects burial compaction. An observable deficiency of pores <100 nm in φBIB size distributions, as well as differential BJH mesopore volumes of up to 20% (original vs. extracted), show that pore occlusion by high-molecular weight bitumen changes the apparent pore characteristics particularly for samples from the oil window. A maximum of extractable organic matter (EOM), soluble S2bitumen, S1 normalized to total organic carbon (TOC) contents, as well as saturated vs. aromatic compound ratios determined from source rock extracts, is reached at a vitrinite reflectance of 0.8 %Rr. An interpreted maximum of shale oil in place at ~0.8 %Rr is followed by a decline in these parameters for the samples at slightly higher thermal maturity (0.9 %Rr). This may indicate the onset of hydrocarbon expulsion at >0.8 %Rr, corresponding with a decrease in the differential BJH volume and less visible pre-oil solid bitumen. The gas storage and retention behavior of the investigated shales may abruptly change with ongoing thermal maturation, as matrix pores are liberated from occluding hydrocarbons.

Quantitative Characterization and Determination of the Main Factors That Control Fracture Development in the Lower Paleozoic Shale in Southeastern Chongqing, China

The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.

Quantitative Characterization of Shale Porosities of Different Origins by Integrating Pore Genesis and Logging Analysis

Abstract In this paper, a new and pragmatic technique has been developed to identify pore types and characterize porosities of shales with various origins. By comparing the genesis of pore types (i.e., organic, brittle, and clay mineral porosities) in shales, the corresponding pore volumes per unit mass are determined as a function of the total porosity, density, and the content of each pore type from core samples. Subsequently, a new inverse framework was proposed and successfully applied to quantify different types of porosities in the Silurian Longmaxi formation shale in the Zhaotong area. The pore volume per unit mass of organic matter is calculated to be around 0.185–0.190 cm3/g, which is 10–21 and 8–19 times more than that of brittle mineral and clay mineral, respectively, indicating that pore space of organic matter contributes greater to the total porosity than that of the clay and brittle minerals. Using single well data, the porosity in organic matter is found to follow the same pattern as the total porosity in the vertical direction. Such an identified porosity type leads to more accurate sweet spots as well as more appropriate drilling locations for horizontal wells in shale reservoirs.

Hierarchical Microstructure of Tooth Enameloid in Two Lamniform Shark Species, Carcharias taurus and Isurus oxyrinchus.

Shark tooth enameloid is a hard tissue made up of nanoscale fluorapatite crystallites arranged in a unique hierarchical pattern. This microstructural design results in a macroscopic material that is stiff, strong, and tough, despite consisting almost completely of brittle mineral. In this contribution, we characterize and compare the enameloid microstructure of two modern lamniform sharks, Isurus oxyrinchus (shortfin mako shark) and Carcharias taurus (spotted ragged-tooth shark), based on scanning electron microscopy images. The hierarchical microstructure of shark enameloid is discussed in comparison with amniote enamel. Striking similarities in the microstructures of the two hard tissues are found. Identical structural motifs have developed on different levels of the hierarchy in response to similar biomechanical requirements in enameloid and enamel. Analyzing these structural patterns allows the identification of general microstructural design principles and their biomechanical function, thus paving the way for the design of bioinspired composite materials with superior properties such as high strength combined with high fracture resistance.

Brittleness evaluation of the Lower Silurian marine shale reservoirs: A case study of Longmaxi shale in Fenggang block, southern China

The brittleness of shale determines the fracturability of the shale reservoir and has a great influence on the exploration and development of shale gas. Therefore, prediction of brittleness and evaluation of fracturability of shale are very important in finding favorable areas for shale gas. We have used the mineral composition, rock-mechanics experiment, logging evaluation, 2D seismic data inversion, and fracture development degree to evaluate and analyze the vertical and plane brittleness characteristics and the main controlling factors of Longmaxi shale. In addition, we established the fracturability index of shale based on the brittleness index, critical strain energy release rate, and fracture toughness. The results indicate that the brittle mineral content of Longmaxi shale in the Fenggang block is between 69% and 90%, and shale samples are prone to brittle deformation by microscopic observation. Brittle deformation has a positive effect on the porosity and percolation ability of shale; the fractures in the upper and lower parts of the Longmaxi shale are relatively developed, and the degree of core fracture development is consistent with rock brittleness. The brittleness distribution of 2D seismic data inversion is in good agreement with the brittleness predicted by well logging; shale reservoirs with low fracture toughness, low critical strain energy release rate, and high brittleness index have high fracturability. Therefore, the research on shale brittleness and fracturability of Lower Silurian Longmaxi shale is expected to have important guiding significance for shale-gas exploration and development in southern China.

Reservoir characterization of the Upper Cretaceous shale for the Changling Sag, Southern Songliao Basin, NE China

The Upper Cretaceous Qingshankou (K2n) and Nenjiang (K2q) shale oil from the Changling Sag (CLS) of the Southern Songliao Basin located in NE China was studied to determine shale reservoir properties and evaluate oil potential. Multiple methods, including optical microscope, scanning electron microscope and energy dispersive spectrum analysis (SEM-EDS), nitrogen adsorption/desorption analysis, and X-ray diffraction, were conducted to determine the shale reservoirs. The mineral analyses present in this shale consist of blende, albite, chlorite, pyrite, and calcite, whose textures are crystals, schistose, particle, or clusters. The mineral results show a significant percentage of brittle minerals (40.5–61.7%), and clay minerals are dominated by illite and montmorillonite. The mineralogical brittleness indexes of the Upper Cretaceous K2n and K2q shales are 0.15–0.67 and 0.18–0.42, respectively. The K2q shale has a relatively low brittle mineral content, which may not be favorable for shale oil seepage. Higher brittleness in K2n shales is more favorable for hydraulic fracturing with larger width and smaller initial stress. Although the reservoir has an ultra-low permeability, they have a relatively good porosity. The organic matter is residual in bioclastic pyritization, calcareous bioclastic grain pores, animal skeletal grain, and algous cystocarp. Moreover, the K2n and K2q shales contain a substantial number of pores, micro-fractures, and oil. The mineral intragranular pores, dissolution pores, organic pores, and micro-fractures are significantly developed. Hysteresis loop is acquired in open pores, which have mainly inkbottle-shaped pores and few parallel-plate pores or cylindrical pores. Our findings may fill significant gaps in understanding and predicting pore systems and mineralogical brittleness indexes, with respect to the lacustrine shale oil in China.

The differential evolution of nanopores in discrete OM and organic-clay composites for shale: insights from stress manipulation

The organic carbon in shale has been proved to be a significant hydrocarbon source in two occurrences of discrete OM and organic-clay composites. Although the evolution of OM-hosted pores has been investigated by extensive research, the evolution of nanopores in discrete OM and organic-clay composites under additional stress is still unclear. In this study, two potential shales in China (Silurian Longmaxi (S1l) formation and Cambrian Niutitang (Є1n) formation) were selected to examine the nanopores characteristics developed in discrete OM and organic-clay composites using field emission scanning electron microscopy (FE-SEM) and statistical analyses. The microstructure observations of the two shales show that during the growth of nanopores, the expansion of nanopores in composites was restricted by the narrow space between clay layers, and then, the nanopores would be protected from collapse or compaction by clay layers with further maturity. The discrete OM particles constantly suffer squeezing from brittle minerals and lead to the nanopores thrown into compressive environment, resulting in a preferred orientation along the edge of mineral particles and narrowing shape, as well as diminishing the pore sizes. For organic-clay composites, the development of nanopores is determined by additional stress state. Under compressive stress, no pores could be observed, while the nanopores are widely distributed in tensile environment or stress-free state. Consequently, the nanopores development in discrete OM and organic-clay composites are significantly affected by additional stress. The research findings imply the potential hydrocarbon source in the nanopores located in organic-clay composites and would bring inspiration for the exploitation of shale gas.

Geochemical characteristics and factors controlling the deep lithologic reservoirs in Puwei Sag, Dongpu Depression—A Case Study of Well PS20

As the advances of the exploration in shallow layers, petroleum exploration moves toward deep layers has become an inevitable choice. However, the hydrocarbon accumulation mechanism in deep reservoir remains a big challenge for the petroleum exploration. The Dongpu Depression is one of the major petroliferous basins in the Bohai Bay Basin of eastern China. The deep Well PS 20 discovered in the Puwei Sag of Dongpu Depression suggests that there is great prospecting in deep reservoir. In this paper, the geochemical characteristics and main controlling factors of deep lithologic reservoir in the third member of the Paleogene Shahejie Formation (Es3) of Well PS20 are analyzed by means of geological and organic geochemical methods including rock-eval, Gas Chromatgraphy-Mass Spectrometry (GC/MS) and Scanning Electron Microscopy (SEM). The results show that the main source rocks are situated in the middle and lower parts of the third member in the Shahejie Formation of Eocene (Es3M-Es3L). They are characterized by higher organic content (>0.6%), good organic matter (most of them are type II, and some are type I) and higher maturity (Ro>1.0%). The oil-source comparison shows that the origin of the Es3U is a mixture from both local and off-sited sources. The origin of the hydrocarbon in Es3M is from a remote source, and the hydrocarbons in Es3L are indigenous. The reservoir properties reveal that the deep strata have been compacted with increasing depth, and higher oil content only occurred in less tight strata with good physical properties. The deep hydrocarbon reservoir in Puwei Sag is controlled by the source rock quality, reservoir properties and sand body thickness. The reservoir physical properties control the oil and gas accumulation and distribution. Sand thickness controls the oil content and hydrocarbon accumulation. The ultra-thin and thin layers in the lower member of Es3 are characterized by multiple layers (>85% of total layer numbers）and thick reservoir layers (>65% of total thickness). The oil saturation in the ultra-thin and thin layers is much higher than that in the thick layers. Numerous micro-fractures produced by abnormally high pressure should be responsible for the higher oil content. Besides, both sandstone and mudstone layers have high brittle mineral content, which also provides advantageous condition for micro-fractures. The results expand the space for petroleum exploration, strengthen the confidence of the new field to achieve a major breakthrough, and shed light on the long-term development of exploration risk in the new field of Dongpu Depression.

Effects of foliation on deformation and failure mechanism of silty slates

The objective of this study was to investigate the deformation and failure mechanism of silty slates loaded with an angle 0 and 90° between foliation and direction of uniaxial force. For this purpose, mineral composition and microstructure test, uniaxial compression test, acoustic emission (AE) monitoring and scanning electron microscope (SEM) were carried out on Silurian silty slates. Based on the results of tests to analyze the deformation and failure mechanism as well as the macroscopic failure modes, microscopic rupture surface characteristics and acoustic emission characteristics of silty slates under different test conditions. Tests results showed that the silty slates are mainly composed of clay minerals (chlorite and illite) and brittle minerals (quartz and feldspar) and in a layered structure. Under the two angles, silty slates specimens exhibited large differences in the deformation and damage process, macroscopic failure modes, microscopic rupture surface characteristics, deformation and failure mechanism. Besides, there are large differences in the relative parameters of AE event. These results reveal that there is a certain relationship between mechanisms of microscopic fractures and macro failure of silty slates.

Pore Properties of the Lacustrine Shale in the Upper Part of the Sha-4 Member of the Paleogene Shahejie Formation in the Dongying Depression in East China

Lacustrine shales hold a huge potential oil resource in China. Pore properties (pore volume, diameter, specific surface area, and fractal dimensions) and their relationships with geological factors (mineralogy, insoluble organic carbon, burial depth, and vitrinite reflectance) are critical for evaluating shale oil resource. However, the factors controlling pores for lacustrine shale oil remain unclear, as the relationships between pore properties of Soxhlet-extracted samples and geological factors have not been studied using multivariate analytical methods. In this paper, the samples from the lacustrine shale in the upper part of the Sha-4 Member of the Paleogene Shahejie Formation in the Dongying Depression were tested with a set of experiments including Soxhlet (solvent) extraction, X-ray diffraction mineral analysis, insoluble organic carbon, vitrinite reflectance, and low-pressure CO2 and N2 adsorption experiments. The micromesopore volume varies from 0.003 cm3/g to 0.045 cm3/g. The relationships of pore properties with geological factors were studied with partial least square regression analysis (PLSR analysis, a powerful multivariate regression analysis). The results of the PLSR analyses indicate that clay minerals and carbonates are two key factors affecting the pore properties of the lacustrine shale. Compared with marine shales, more clay minerals in the lacustrine shale make them become more important for controlling pores than organic matter. The PLSR results also illustrate that the shale with higher pore volume contains more clay minerals and fewer carbonates and thus is unfavorable for hydraulic fracturing. Therefore, the shale with high micromesopore volume may be unfavorable for shale oil production. The shale with the modest micromesopore volume (~0.036 cm3/g), relatively high content of brittle minerals (~71 wt%), and low clay mineral content (~29 wt%) is conducive to both oil storage and hydraulic fracturing for the development of the Es4U shale oil in the Dongying Depression in East China.

Factors Controlling Shale Reservoirs and Development Potential Evaluation: A Case Study

The shale of the Lower Silurian Longmaxi Formation is an important gas-producing layer for shale gas development in southern China. This set of shale reservoir characteristics and shale gas development potential provide an important foundation for shale gas development. This study takes wellblock XN111 in the Sichuan Basin, China, as an example and uses X-ray diffraction (XRD), scanning electron microscopy (SEM), isothermal adsorption, and other techniques to analyze the shale reservoir characteristics of the Lower Silurian Longmaxi Formation. The results show that the Lower Silurian Longmaxi Formation was deposited in a deep-water shelf environment. During this period, carbonaceous shale and siliceous shale characterized by a high brittle mineral content ( quartz &gt; 40 wt . % , carbonate mineral &gt; 10 wt . % ) and a low clay mineral content (&lt;30 wt.%, mainly illite) were widely deposited throughout the region. The total organic carbon (TOC) content reaches up to 6.07 wt.%, with an average of 2.66 wt.%. The vitrinite reflectance is 1.6–2.28%, with an average of 2.05%. The methane adsorption capacity is 0.84–4.69 m3/t, with an average of 2.92 m3/t. Pores and fractures are developed in the shale reservoirs. The main reservoir space is composed of connected mesopores with an average porosity of 4.78%. The characteristics and development potential of the shale reservoirs in the Lower Silurian Longmaxi Formation are controlled by the following factors: (1) the widespread deep-water shelf deposition in wellblock XN111 was a favorable environment for the development of high-quality shale reservoirs with a cumulative thickness of up to 50 m; (2) the high TOC content enabled the shale reservoir to have a high free gas content and a high adsorptive gas storage capacity; and (3) the shale’s high maturity or over maturity is conducive to the development of pores and fractures in the organic matter, which effectively improves the storage capacity of the shale reservoirs. The reservoir characteristic index was constructed using the high-quality shale’s thickness, gas content, TOC, fracture density, and clay content. Using production data from shale gas wells in adjacent blocks, a mathematical relationship was established between the Estimated Ultimate Recovery (EUR) of a single well and the Reservoir Characteristics Index (Rci). The EUR of a single well in wellblock XN111 was estimated.

Reservoir Characteristics and Genetic Mechanisms of the Mesozoic Granite Buried Hills in the Deep‐water of the Qiongdongnan Basin, Northern South China Sea

AbstractDue to its structure, rock and mineral composition, fluid and other factors, the granite Buried Hill Reservoir is highly heterogeneous with a complex longitudinal structure and a reservoir space made up of a combination of dissolution pores and fractures. This paper is based on current understanding of tectonic evolution in the northern part of the South China Sea, in conjunction with the seismic phase characteristics. It is determined that the meshed fault system was formed by three stages of movement ‐ tectonic compression orogeny during the Indochinese epoch, strike‐slip compression‐tension during the Yanshanian Period, early fracture extension activation during the Himalayan – which controlled the distribution of the Buried Hill Reservoir. Drilling revealed two types of buried hills, faulted anticline and fault horst, their longitudinal structure and the reservoir space type being significantly different. The mineral composition, reservoir space and diagenetic characteristics of the reservoir rocks and minerals were analyzed by lithogeochemistry, microsection and logging etc., it thus being determined that the Mesozoic rocks of the Songnan Low Uplift in the Qiongdongnan Basin are mainly composed of syenogranite, granodiorite, monzogranite, which is the material basis for the development of the Buried Hill Reservoir. The content of felsic and other brittle minerals is more than 70%, making it easy for it to be transformed into fractures. At the same time, the weathering resistance of granodiorite and monzogranite is weaker than that of syenogranite, which is easily weathered and destroyed, forming a thick sand gravel weathering zone. With increasing depth of burial, weathering and dissolution gradually weaken, the deep acidic fluid improving the reservoir property of internal fractures and expanding the vertical distribution range of the reservoirs. The research results lay a foundation for the exploration of Buried Hill in the deep‐water area of the Qiongdongnan Basin.

Microfracture characterization of shale constrained by mineralogy and bedding

The characterizations of microfractures within shale, and their associations with bedding, mineralogy and loading, are important for shale gas exploration and related engineering safety. Research on this topic has been limited due to technological constraints; however, advances in scanning electron microscope (SEM) techniques allow for highly detailed observations of microfractures. In this study, Longmaxi Shale samples were obtained from the locality of Shizhu, China. The samples were cored parallel and perpendicular to the bedding, and SEM observations were performed in specimens before and after compression triaxial testing. SEM images were used for qualitative and quantitative characterization of the microfractures. These tests provided some important insights into the effects of bedding, mineralogy and loading on the characteristics of microfractures. It was observed that the size of the microfractures follows a power law distribution regardless of loading. The parameters of the power law suggest that microfractures associated with brittle minerals increase in complexity after triaxial compression testing. The characteristics of quartz and carbonates show a similar changing pattern, while clay and feldspar show the reverse due to crystal foliation. Microfracture orientations tend to be parallel to subparallel to bedding for all samples, regardless of loading and loading direction. The transition from tensile failure to shearing failure dominates the failure patterns for all minerals under external loading. The mineral's strength and crystal foliation appear to dominate the microfracture mechanisms. Microfractures through carbonates showed complex networks due to the weak grains, and those through quartz tended to develop evenly, indicating that these minerals are prone to forming fracture networks. Microfractures through clays were predominantly parallel and subparallel to the bedding, and those through feldspars showed a tendency towards foliation, indicating that stress direction has crucial effects on these minerals. These results provide microscale information on shale fracturing processes, which can benefit current efforts for hydraulic fracturing enhancement.

Adsorption Characteristics and Controlling Factors of CH4 on Coal-Measure Shale, Hedong Coalfield

Adsorbed gas is one of the crucial occurrences in shale gas reservoirs; thus, it is of great significance to ascertain the adsorption capacity of shale and the adsorption characteristics of CH4. In this investigation, the Taiyuan–Shanxi Formations’ coal-measure shale gas reservoir of the Carboniferous–Permian era in the Hedong Coalfield was treated as the research target. Our results exhibit that the shale samples were characterized by a high total organic carbon (TOC) and over to high-over maturity, with an average TOC of 2.45% and average Ro of 2.59%. The mineral composition was dominated by clay (62% on average) and quartz (22.45% on average), and clay was mainly composed of kaolinite and illite. The Langmuir model showed a perfect fitting degree to the experimental data: VL was in the range of 0.01 cm3/g to 0.77 cm3/g and PL was in the range of 0.23–8.58 MPa. In addition, the fitting degree depicted a linear negative correlation versus TOC, while mineral composition did not exhibit a significant effect on the fitting degree, which was caused by the complex pore structure of organic matter, and the applicability of the monolayer adsorption theory was lower than that of CH4 adsorption on the mineral’s pore surface. An apparent linear positive correlation of VL versus the TOC value was recorded; furthermore, the normalized VL increased with the growth of the total content of clay mineral (TCCM), decreased with the growth of the total content of brittle mineral (TCBM), while there was no obvious correlation of normalized VL versus kaolinite, illite and quartz content. The huge amount of micropores and complex internal structure led to organic matter possessing a strong adsorption capacity for CH4, and clay minerals also promoted adsorption due to the development of interlayer pores and intergranular pores.