Rigid Minerals Research Articles

A modified surface to volume (SVR) method to calculate nuclear magnetic resonance (NMR) surface relaxivity: Theory and a case study in shale reservoirs

Surface relaxivity (ρ2) is a critical parameter for converting nuclear magnetic resonance (NMR) T2 data to pore size distribution (PSD). The surface-to-volume ratio (SVR) method, known for its simplicity and ease of operation, has been widely used for ρ2 calculation in unconventional reservoirs. However, previous studies often overlooked the equivalence of pore ranges characterized when directly applying the classical SVR model. Moreover, shale reservoirs generally develop layered fractures, whose ρ2 values are different from matrix pores. The logarithmic mean value of the T2 distribution (T2LM) is significantly influenced by layered fractures, therefore, relying solely on the T2LM value of a whole sample under fluid-saturated state will lead to inaccurate ρ2 values of matrix pores, particularly in laminated shales where fractures are well developed. However, insufficient attention has been paid to the effect of fractures on the ρ2 calculation. In this study, a modified SVR method based on the theory of NMR relaxation in partially fluid-saturated pores was proposed to characterize the ρ2 of shale matrix pores. Twenty-four shale core samples from the Shahejie Formation in the Jiyang Depression, China were selected, and subjected to series of NMR experiments at varying oil-bearing conditions, and low-temperature nitrogen adsorption (LTNA) analysis. The results indicate a strong linear correlation (R2 > 0.85) between the inverse T2LM (1/T2LM) and the inverse fluid saturation (1/f) when oil molecules across the entire surface layer participate in the exchange process. For a whole core sample, ρ2 values obtained using the modified SVR model are higher than those obtained using the classical SVR model, especially in samples with numerous fractures. The modified SVR method effectively reduces the impact of fractures on the characterization of ρ2 of matrix pores. For shale pore ρ2 characterization, the classical SVR model may be more suitable for pores smaller than 300 nm, with a recommended T2 range of <33 ms. Additionally, ρ2 values for different pore ranges (<25 nm, 25–100 nm, and >100 nm) within individual samples were estimated. It is found that the ρ2 values of smaller pores is greater than those of larger pores, which may be due to differences in mineralogy of the pores across various size ranges. The small pores are more associated with clay minerals while large pores are surrounded by quartz and rigid minerals. In addition, ρ2 is lower in larger pores and fractures that do not contain organic matter and clays, thus the underestimation of ρ2 by the classical SVR method can be corrected by modified SVR method. This study represents the first attempt to examine ρ2 variations across different pore ranges in shale reservoirs. The methodology presented can be applied to other formations, enhancing NMR data application in both laboratory settings and well logging.

Comparison of mechanical characteristics of different types of shales in the Ordos Basin

Shale gas is a very important unconventional energy. The mechanical properties of the three types of shale (laminated shale, sandwich shale and foliated shale) are different, and the difference in fracturing effectiveness is very significant. In this paper, the mineral composition, mechanical properties and conductivity of these three different types of shale were studied and compared by X-ray diffraction, triaxial mechanical experiments, and fracture conduction experiments. The study found that the foliated shale has the lowest content of rigid minerals (47.5%), lower elastic modulus and tensile strength (26.98 Gpa and 168.29 MPa, respectively), higher Poisson’s ratio (0.25), the smallest brittleness index (0.48), and larger fracture toughness (0.42). The laminated shale has a higher content of rigid minerals (68.50%), the lowest elastic modulus and tensile strength (25.77 Gpa and 122.46 MPa, respectively), the highest Poisson’s ratio (0.26), the highest brittleness index (0.56), and the lowest fracture toughness (0.18). The sandwich shale has the highest rigid mineral content (78.16%), the highest elastic modulus and tensile strength (35.31 Gpa and 197.37 MPa, respectively), the lowest Poisson’s ratio (0.24), a larger brittleness index (0.52), and larger fracture toughness (0.415). Furthermore, with the increase in the coring angle, the elastic modulus of all three shales increases. In addition, with the increase in closing pressure and the decrease in the sand laying concentration, the proppant embedding depth gradually increases and the conductivity decreases. This means that from the perspective of forming complex fracture networks, the fracturing effect of the foliated shale is unsatisfactory, while the fracturing effect of the laminated and sandwich shales is better. Moreover, it is recommended to prefer directional injection along vertical laminae or at high angles, which is conducive to the formation of complex fracture networks. For laminated shale with low strength, the sand laying concentration should be increased to ensure the conductivity of the fractured fracture. This study provides some technical guidance for the identification of different types of shale fracturing desserts and fracturing processes.

Pore Structure Characteristics, Genesis, and Its Controlling Effect on Gas Migration of Quaternary Mudstone Reservoir in Qaidam Basin

The Sanhu Depression in Qaidam Basin is the largest Quaternary biogenic gas exploration area with the shallowest burial depth in the world. The shallow high abundance of gas reservoirs and high-quality pure methane have become the main production areas. The development characteristics of loose mudstone reservoirs are restricted by extreme heterogeneity. Therefore, the Quaternary Qigequan Formation mudstone in Qaidam Basin is selected as the research object. Based on the study of the sedimentary background, petrological characteristics, and pore structure characteristics, by comparing the characteristics of different mudstone reservoirs, the controlling factors of the development of different mudstone reservoirs and the control of gas migration are clarified. Research shows the following: (1) The mudstone reservoirs of the Qigequan Formation mainly develop intergranular pores, clay mineral pores, and intragranular pores. The pore size distribution varies from nanometers to micrometers, and mesopores mainly contribute to the specific surface area. (2) Rigid minerals and clay minerals are the main controlling factors for the pore structure of mudstone reservoirs. The increase in the content of rigid minerals is conducive to the development of macropores or larger micropores, while the increase in the content of clay minerals is conducive to the development of mesopores and provides an important specific surface area for gas adsorption. (3) The gas migration form of pure mudstone is mainly dominated by Fick diffusion and slippage flow, which has the characteristics of self-sealing accumulation. The gas migration form of silty mudstone is the coexistence of Fick diffusion, slippage flow, and Darcy flow, which has the features of self-sealing and Darcy flow accumulation. The gas migration form of sandy mudstone is mainly Darcy flow, only with Darcy flow accumulation characteristics. The flow form of gas creates different accumulation modes of mudstone biogas. The Quaternary mudstone reservoir shows different particularity under different material components, and the exploration targets should be treated differently according to specific mudstone types.

New insights into the role of system sealing capacity in shale evolution under conditions analogous to geology: Implications for nanopore evolution

Closed and semi-closed pyrolysis of a lacustrine shale sample was conducted using the same instrument characterized by lithostatic pressure and limited reactive space in order to unravel the impacts of the system sealing capacity on organic matter occurrence and nanopore development in shales under conditions analogous to natural processes. The solid residues were subjected to organic geochemical measurements, low-pressure gas (N2 and CO2) adsorption tests, and field emission-scatter electrical microscope analysis. The results revealed that solvent extraction played positive and negative roles in the pore development of solid residues from both systems at different thermal maturities. Inorganic and organic pores were primarily filled or shielded by soluble bitumen at low and high temperatures, respectively. The occurrences of organic matter in solid residues from the two systems were different at lower temperatures but gradually evolved similarly at higher temperatures and were dominated by nanoscale interparticle organic matter. Organic pore occurrence in solid residues from the two systems was approximately comparable. However, differences in pore volume and specific surface area of solid residues from the closed and semi-closed systems increased measurably below 550 °C. At 550 °C, a strong mechanical compaction and overpressure environment caused deformation and closure of pores surrounding rigid minerals and pores associated with ductile minerals surrounding organic matter in solid residue from the closed system, while only a few pores surrounding rigid minerals from the semi-closed system deformed. Consequently, similar pore parameters of solid residues from the two systems were obtained at this temperature. Additionally, the variation in organic porosity of solid residues from the two systems has obvious stages based on mass balance calculation. The organic porosity evolution is collectively affected by system sealing capacity and thermal maturity at lower matured stages. In contrast, organic porosity evolution in solid residues is independent of system sealing capacity at high- and overmatured stages. Although the depositional environment is different, this study provides direct evidence that the difference in system sealing capacity is not the predominant factor leading to different organic pore developments in the Silurian and Cambrian shales in South China.

Impact of Minerals and Sealing Systems on the Pore Characteristics of the Qiongzhusi Formation Shale in the Southern Sichuan Basin

The characteristics, distribution, and preservation of pores are vital in controlling the storage and distribution of shale gas. The Qiongzhusi Formation shales taken from different members with similar tectonic and thermal evolutions were used to evaluate the response of pore characteristics to minerals and sealing systems using field-emission scanning electron microscopy and gas adsorption. Because of differences in mineral structure and arrangement, feldspar, organic matter (OM)–clay, OM–rutile, and OM–apatite aggregates facilitate multiple types of pores in the shale and influence the relative proportions of surface porosity for different types of pores owing to differences in mineral structure and arrangement. Rigid frameworks and pressure shadows formed by rigid minerals and OM–mineral aggregates preserved OM and pores to some extent. The sealing capacity of the floor controls the sealing system and hydrocarbon expulsion efficiency of the Qiongzhusi Formation in different members. During thermal evolution, the amount of hydrocarbons generated and expelled affected the stress equilibrium state between the pore pressure and external stress, influencing the compaction intensity of shales. The OM pore development characteristics were evolved with variation in the stress equilibrium state in different sealing systems. Once the stress equilibrium state was disrupted, the OM pores deformed, narrowed, or even closed under the influence of compaction owing to the loss of overpressure support. The pore characteristics of the Qiongzhusi Formation shales responded significantly to different sealing systems. A few OM pores are flat and slitlike in the open system, whereas numerous OM pores are round and elliptical in the semiopen system. Meanwhile, the average diameter of the OM pores in the open system was reduced by approximately 40.2% compared with that of the semiopen system. Furthermore, the pore volume and specific surface area of the mesopores for open system shales were reduced by 38.4% and 37.7%, respectively, compared to the semiopen system. These findings will improve the understanding of the distribution and preservation of pore in shale and help assess the sweet-spot members for the Qiongzhusi Formation shale gas.

Differences of Pore Features in Marine Shales between Lower Cambrian and Lower Silurian Formations of Upper Yangtze Area, South China

Lower Cambrian shale and lower Silurian shale are both typical of oil-prone kerogen and siliceous composition, but different in thermal maturities. Porosity differences were determined in marine shales between the two shales. Measurements were utilized including organic geochemistry, XRD, scanning electron microscopy (SEM) and N2 gas adsorption. Pore volume (PV) of lower Silurian shale was approximately 1.5 times higher than that of lower Cambrian shale, and pore surface area (PSA) of lower Silurian shale was almost 2.5 times higher than that of lower Cambrian shale. Lower Cambrian shale and lower Silurian shale possess similar materials, but distinctive thermal degrees. Evolution mechanisms of different types of pores, especially organic matter (OM)-hosted pores, may trigger this different pore features. Pores of rigid framework are the residue of primary interparticle pores during the burial history. Pores associated with clay flakes can be preserved well adjacent to rigid grains or secondary minerals acting as rigid frameworks or grain supporters. Dissolved pores in both lower Cambrian shale and lower Silurian shale barely contribute to the total porosity and mean little to the permeability. Both excessive OM content and over thermal maturity are detrimental to development of OM-hosted pores. Rigid particles, clay flakes, and OM commonly co-exist within shale matrix. Rigid grains act as supporters, clay flakes confine ample space, and OM first migrates into and provides secondary OM-hosted pores. In this condition, pores can be preserved owing to associating matrix with good mechanic and chemical stability. The significant differences of structural settings result into various hydrocarbon explosion efficiency and different pressure circumstance, which consequently leads to the different pore features between the two shales. For lower Cambrian shale, overpressure circumstance diminish if hydrocarbon expels outside of the shale system, and OM-hosted pores destroy through compaction. Sustaining overpressure and abundant residue hydrocarbon (migrated OM) make positive contributions to the pore properties, in terms of numbers, diameters and connectivity of the lower Silurian shale samples.

Pore system and gas adsorption potential of lacustrine Yanchang Mudstone, Ordos Basin, China

Understanding the pore system and the geological factors controlling the adsorption of gas in mudstones and shales is a key element in predicting the gas accumulation potential of an unconventional reservoir. A series of integrated methods, including basic geochemistry, X-ray diffraction, scanning electron microscopy, low-pressure gas (CO2 and Ar) physisorption, and high-pressure methane adsorption were performed on a set of mudstones from the Triassic Yanchang Formation in Ordos Basin, to characterize the pore system and gas adsorption capacity and their geological controlling factors. The results show that the studied Yanchang Mudstones are mainly siliceous rocks with high total organic carbon concentration (average of 7.63%) and low to moderate thermal maturity (vitrinite reflectance of 0.55–0.92 %). The microscopic visible pores are mainly intergranular pores between organic matter and rigid mineral grains, while organic matter pores are less well-developed. The pore volume is mainly contained in micropores (diameter < 2 nm) and mesopores (diameter = 2–50 nm), while much of the specific surface area occurs in the micropores, mainly in the organic matter pores. The occurrence of rigid minerals can inhibit the compaction of the intergranular pores. The abundant organic matter can provide potential adsorption sites for gas molecules, while clays and rigid minerals also have a limited positive effect on adsorption capacity. The adsorbed methane is mainly stored in micropores, which accounts for about 55%–90% of the total adsorbed methane content.

Mechanical Properties and Microscopic Geological Features under Uniaxial Compression in Tight Reservoir

In this study, to investigate the mechanical properties and microscopic geological features, a series of uniaxial compression, petrophysical properties, X-ray diffraction, elements, and conductivity tests were conducted. The results indicate that quartz and plagioclase are essential minerals determining Upper Triassic sandstone compositions, while quartz plays a dominant role in the Lower Permian sandstones. Due to microcracks, the relationships between porosity and permeability of Upper Triassic sandstones are better than those of Lower Permian sandstones. Observations have shown that silicon and aluminum are the main elements of those rocks. The rocks which have large porosity and permeability have a strong capacity of anticompression resistance. There are good consistencies between the mechanical behavior and the high proportion of rigid minerals. Similarly, the rocks which have abundant silicon and aluminum could better support the structural integrity of rocks. High pore structure heterogeneity and abundant clay minerals have positive influence on the mechanical resistance. Shales have distinct plastic deformation features when compared with the sandstones.

Controls of Distinct Mineral Compositions on Pore Structure in Over-Mature Shales: A Case Study of Lower Cambrian Niutitang Shales in South China

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

Comparison of pore evolution for a Mesoproterozoic marine shale and a Triassic terrestrial mudstone during artificial maturation experiments

Semi-closed pyrolysis experiments were performed on a Mesoproterozoic Hongshuizhuang marine shale sample and a Triassic Chang 7 terrestrial mudstone sample, in order to understand the mechanisms of pore evolution in shales with similar organic geochemical characteristics in different depositional environments. The pore characteristics of the solid samples from the thermal maturation experiments were investigated by low-pressure gas (N2 and CO2) sorption isotherms tests and Field Emission - Scattering Electron Microscopy (FE-SEM) imaging. Results indicate the marine shale samples develop more micropores than that of the terrestrial mudstone samples during the artificial maturation. Secondary organic pores are observed in the unheated samples (~0.76% EqRo) for the marine shale, which is earlier than that of in the terrestrial mudstone (observed in the sample at 250 °C, ~0.8% Ro). Significant development of organic pores at 350 °C for both the two shales. And the transformation of smaller pores into larger pores takes place at 450 °C for the marine shale, as well as for the terrestrial mudstone. In highly overmatured shales, the overpressure formed by organic matter cracking to gas can make surrounding interparticle space to vanish. The high content of rigid minerals in the marine shale is more conducive to the preservation of organic pores than the terrestrial mudstone with rich in ductile clay minerals. Therefore, pore increases continuously for the marine shale but decreases for the terrestrial mudstone after 500 °C. Micropores would be generated if a high lithostatic pressure exists under experimental conditions, which may be largely responsible for the disagreement of pore evolution of similar shales in other publications. Finally, a conceptual model of pore evolution for type-II marine and terrestrial shales is proposed. The study reveals that the pore evolution of shales with different sedimentary environments is divergent, and this need to be separately considered in the future work.

The Controlling Effects of Compositions on Nanopore Structure of Over-Mature Shale from the Longtan Formation in the Laochang Area, Eastern Yunnan, China

Over-mature shale has undergone complex geological processes, e.g., diagenesis, deep thermolysis, epigenesis, and metamorphism. During this series of processes, the internal components of shale are transformed coordinatively, and the special pore structure with a complex genesis in the over-mature shale is formed. The study of the special pore structure of over-mature shale and its main controlling factors can provide theoretical support for shale gas exploration and development. In this study, the over-mature shale of the Longtan Formation in the Laochang area, eastern Yunnan Province, China, was tested and analyzed to determine the total organic carbon content, kerogen microscopic composition, and its vitrinite reflectance. Moreover, the mineral composition and pore characteristics of the shale were tested using X-ray diffraction, the argon ion polishing field emission scanning electron microscopy technique, and the low temperature nitrogen adsorption experiment. The results show that the compositions of Longtan Formation shales have a low rigid clastic particles content, high clay content, and abundant metasomatic ankerite, and that the organic matter, dominated by type III, is highly mature. At the same time, the shale pore structure is characterized by a large average pore size, well-developed macropores, and a high number of micropores and mesopores. By analyzing the transformation of the mineral composition of shale during thermal evolution, the controlling factors of pore structure are determined. During the evolution of shale, the intergranular pores (mainly large pores) between rigid minerals are destroyed in large quantities. After a short period, a large number of dissolved pores with the pore size of macropores are formed in the acid-resistant minerals under the action of organic acid. These dissolved pores, together with the remaining intergranular pores of rigid minerals and micro-fractures, form most of the macropores of shale. There is a large amount of ankerite in over-mature shale formed during metasomatism, which occludes some pores of shale. However, intragranular pores and cleavage-sheets develop in ankerite, contributing some porosity to the shale reservoir. Inter-clay pores and organic matter pores constitute most shale micro and mesopores, and between which inter-clay pores take dominance.

Lithofacies and pore characterization of continental shale in the second Member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China

Abstract The shale of the second Member of the Kongdian Formation (Ek2) in the Cangdong Sag is continental shale with vitrinite reflectance values (%, Ro) ranging from 0.45% to 0.84%. This study adopted systematic microscopic observation and experimental analysis to examine lithofacies and pore structure of the Ek2 shale. Four primary lithofacies are identified based on mineral compositions and sedimentary structures, namely laminated siliceous shale, laminated argillaceous shale, laminated mixed shale and laminated calcareous shale. Scanning electronic microscopy (SEM) images reveal that organic matter pores are less abundant than inorganic mineral pores. Inorganic mineral pores are mostly developed between rigid minerals, such as quartz, feldspar, calcite and dolomite. Organic matter pores are mainly presented in high mature shales and some are under protection of mutual support from mineral grains. Results of low temperature N2 adsorption analysis show that micropores are less developed than mesopores and macropores. For shales with Ro ranging from 0.45% to 0.60%, pore volumes decrease with increased Ro; while for shales with Ro ranging from 0.60% to 0.84%, pore volumes increase with increased Ro. Total organic carbon (TOC), analcime, plagioclase and calcite contents all have positive correlations with pore volumes for shales with Ro ranging from 0.60% to 0.84%, and dissolved pores make greater contribution to pore volume and permeability than organic matter pores in Ek2 shale. The geochemical analysis results show that laminated argillaceous shale has highest average TOC content of 5.20%, followed by laminated siliceous shale of 3.94%, laminated mixed shale of 3.04% and laminated calcareous shale of 1.27%. Considering study above, it is inferred that laminated siliceous shale and laminated mixed shale with higher thermal maturity are favorable for petroleum exploration in Ek2 shale.

Investigating the spontaneous imbibition characteristics of continental Jurassic Ziliujing Formation shale from the northeastern Sichuan Basin and correlations to pore structure and composition

Abstract Shale gas has been successfully explored in the Sichuan Basin of China, and the marine Silurian Longmaxi Formation shale is the main gas-bearing target layer. However, economic shale gas production from continental shale layers within the Sichuan Basin is still minimal, and its potential requires further investigation. This study focuses on the pore structure and spontaneous imbibition behaviors of continental Jurassic Ziliujing Formation shale samples from northeastern Sichuan Basin. Mercury intrusion porosimetry (MIP) was used to obtain the pore-throat size distribution information and field emission-scanning electronic microscope (FE-SEM) was conducted to observe the pore structure directly. The slopes of spontaneous imbibition curves were used to indicate the or pore connectivity, and the wettability of our shale samples was investigated qualitatively by contact angle measurements and by comparing the directional spontaneous imbibition behaviors. The effects of shale composition and pore structure on spontaneous imbibition behaviors were also investigated in this study. Our results show that organic matter (OM) pores and OM-clay interparticle pores are quite developed in Ziliujing shale samples. The protection of pore spaces by rigid minerals is also observed for these samples. The pore connectivity is generally better than pore connectivity, which could be indicated from the higher n-decane imbibition slopes compared with water imbibition slopes. The wettability of our shale samples can be classified into weakly oil-wet, more oil-wet and more water-wet according to the directional spontaneous imbibition results. The well-connected pores could also be confirmed by the fast spreading of n-decane on the sample surfaces during contact angle measurements. In summary, the continental Jurassic Ziliujing Formation shale samples used in this study contain numerous OM pores and OM-clay interparticle pores, which provide both storage spaces and adsorption sites for shale gas and contribute to the better pore connectivity.

Pore structure characterization of different lithofacies in marine shale: A case study of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formation in the Sichuan Basin, SW China

Pore structure has distinct features in different shale lithofacies. This paper presents a case study of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formation in southeast Sichuan Basin to fully describe the characteristics of shale pore structure in different lithofacies. 16 types of shale lithofacies were classified using ternary diagram of siliceous minerals (quartz + feldspar, QF), carbonate minerals (Ca) and clay minerals (Cl). Among all, argillaceous-rich siliceous shale lithofacies (S-3), argillaceous/siliceous mixed shale lithofacies (M-2) and siliceous-rich argillaceous shale lithofacies (CM-1) are the predominant lithofacies. According to scanning electron microscope (SEM) images, combined with focused ion beam-scanning electron microscope (FIB-SEM), mercury intrusion porosimetry (MIP), and gas adsorption (N2 and CO2), the pore structure characteristics and its influencing factors were systematically analyzed. The results show that: Illite + illite/smectite minerals primarily influence the development of the inorganic pores in mixed clay and silica lithofacies (CM-1). Clay minerals are protected from compaction by the rigid minerals, which significantly favors the development of mesopores (2–50 nm) and macropores (>50 nm). The abundance of organic matter is an important determinant for the organic pores in the siliceous lithofacies (S-3), and TOC has a positive contribution to the development of micropores (<2 nm) in this lithofacies. Shale with high TOC content tends to be easily affected by compaction, which results in reduction of some organic pores. Even though the siliceous S-3 reservoir has slightly lower porosity than CM-1, it is currently the primary reservoir target due to its brittle response to hydraulic fracturing.

Pore Characteristics of the Fine‐Grained Tight Reservoirs in the Yabulai Basin, Northwestern China

This work investigated the pore structure characteristics and reservoir features of the finegrained tight reservoirs in the lower member of the Xinhe Formation (J2x1) in the Xiaohu subsag, Yabulai Basin based on core samples through various techniques. Interbedded silt/fine sandstones and mudstones are developed in the study area. Scanning electron microscopy (SEM) images were used to delineate different types of pores, including primary intergranular pores, secondary intergranular and intragranular pores, organic pores and fractures. The pore types were distinguished by pore size, pore area, location and formation process. The pore radii of the fine‐grained rocks range from 1 nm to 1.55 μm, mainly concentrated between 5 and 300 nm by low pressure N2 adsorption and MICP analyses. The pore structure parameters of pore throat size and pore throat sorting coefficient are both positively correlated with porosity, while pore throat sorting coefficient has a negative correlation with permeability. The pore structures of the studied samples are much related to the mineral type and content and grain size, followed by TOC content. In these rocks with relatively low TOC and low maturity, the rigid minerals protect pores with pressure shadow from collapse, and dissolution‐related pores contribute a lot to inorganic porosity. In contrast, these rocks with abundant TOC contain a large number of organic pores. The permeability of the fine‐grained tight reservoir is mainly dominated by larger pore throats, while a large number of small pores (mostly &lt;0.1 μm) contribute considerably to porosity. These results have deepened our understanding of the interbedded fine‐grained tight reservoirs and can be applicable to fine‐grained reservoirs in a similar setting.

Relationship between pore type and pore size of marine shale: An example from the Sinian–Cambrian formation, upper Yangtze region, South China

To clarify the size distribution of different types of shale pores, nine Sinian–Cambrian shale samples from South China were examined using ion-milling, FE-SEM, nitrogen adsorption and mercury intrusion. Image-based point counting data indicated that nanopores are predominant, whereas pores larger than 1μm are absent. Organic matter-hosted pores dominate below 130nm, whereas intraparticle pores within carbonate, probably caused by dissolution, constitute most pores larger than 400nm. The small amount of residual intraparticle and interparticle pores associated with rigid minerals mostly range from 50nm to 250nm. A positive relationship was found between the TOC and total pore volume measured by nitrogen and mercury. The mercury volume of pores larger than ~300nm increased with increasing dolomite and decreasing clay mineral content. The statistics from image-analysis and the relationship between pore volume and shale components were in good agreement, indicating that different types of pores have their own relative restricted size range. Generally, intraP pores are larger than pores associated with rigid minerals, whereas the OM pores and interP pores associated with OM or clay are the smallest. Through a comparison with other less matured shale formations, the highly matured (equivalent vitrinite reflectance above 3%) Sinian–Cambrian shale in South China was characterized in this study as those with smaller interP pores, which is probably due to a stronger compacting effect.

Characteristics and evaluation of the shale oil reservoir in upper Es 3 inter salt in Liutun sag

Shale intercalations in salt deposits of the upper part of the third member of the Shahejie (沙河街) Formation (Es3) in the Liutun (柳屯) sag have industrial oil and gas potential. This article classifies and evaluates lithology, pore systems, porosity, permeability and pore characteristics using core logs, XRD, SEM, lab and log porosity data of shale reservoirs in the salt formation. The lithology of these reservoirs is complex as parts of complex salt rhythms composed of clastic, carbonate and evaporitic rocks. Their rigid minerals contents and total organic carbon are high, the organic type is good, and the maturity of organic matter ranges from medium to low. There are strong overpressures and natural hydraulic fractures. Organic matter pores and matrix-related shale pores provide an appreciable part of the permeability. Different methods of measurement, sonic logging and equivalent cycle counting-point are used to evaluate the different types of porosity and permeability. A preliminary evaluation of shale permeability has been made by using fracture data from cores and image logging, and it is considered that the organic nets composed of organic matter pores generated in the maturation stage of shale have greatly increased oil permeability. On the above basis, this article proposes classification criteria for the upper Es3 inter-salt shale oil reservoirs to provide a foundation for evaluating other shale oil reservoirs in this area.

Anatomy of a transitional brittle–ductile shear zone developed in a low-T meta-andesite tuff: A microstructural, petrological and geochronological case study from the Bükk Mts. (NE Hungary)

Microstructures of the low-T metavolcanites deriving from the transitional brittle–ductile Lillafüred Shear Zone (LSZ, Eastern Bükk Mts., NE Hungary) reveal that the prominent mylonitic rock-structure observed at hand-specimen scale was predominantly achieved at the grain scale by (1) intense microfracturing of rigid minerals (feldspar, opaque phases), and (2) intergranular sliding associated with solution mass transfer within the prevailing, fine-grained, phyllosilicate-rich matrix. Fluid migration during shearing—indicated by widespread growth of syntectonic, secondary calcite and pervasive retrograde alteration of feldspar into “weak”, fine-grained mica aggregates—led to substantial weakening and localization of deformation into narrow zones. This is accompanied microstructurally by an evolution from a cataclasis-dominated regime towards a more ductile, mylonitic deformation. Various metamorphic petrological and geochronological parameters (illite Kübler index, chlorite “crystallinity” indices, crystallite thickness, mean lattice strain, chlorite-Al IV thermometry, K-Ar ages on <2 μm white K-mica) indicate uniformly peak metamorphic conditions around the boundary of the anchi/epizone (300–350 °C) and subsequent cooling about 76 Ma both in the heavily sheared rocks of the LSZ and the enclosing, weakly deformed rock mass. These relationships suggest that the LSZ represents a late-stage, relatively “cold” (≤250–270 °C), fluid-assisted, dextral strike-slip zone formed after 76 Ma on the retrograde path of the Eoalpine tectonometamorphic cycle.

Estimates of the rates of microstructural changes in mylonites

Abstract Microstructures developed during dislocation creep such as the dynamically recrystallized grain-size and the subgrain-size are modified in response to increasing differential stresses and shear-strain rates adjacent to rigid minerals in mylonites. In two specimens variations in quartz subgrain-size around rigid minerals has been quantified from orientation contrast SEM images. In a deformed granite from the Pogallo ductile fault zone, quartz two-dimensional subgrain-size is reduced from background values of about 30 μm to 8 μm and then 4 μm in progressively narrower channels between rigid feldspars. In pelitic mylonites from the Alpine fault zone, New Zealand, quartz bands are wrapped around rigid garnets and the subgrain-size is reduced from background values of about 20 μm to 14 μm within 2000 μm of the garnet to 11 μm adjacent to the garnet. In both specimens examined the microstructure responds to both increases and decreases in differential stresses and coupled strain-rates. The analysis of the distribution of subgrain sizes provides information for the assessment of microstructural stability and the estimation of the rates of microstructural changes. The New Zealand specimens have good regional shear strain-rate controls and minimum subgrain boundary migration rates (during reduction in differential stresses and strain-rates) of 1.2 × 10 −9 μm s −1 to 1.2 × 10 −11 μm s −1 are estimated, for bulk shear strain rates of 10 −10 s −1 and 10 12 s −1 respectively. Palaeopiezometry coupled with rheological considerations suggests that these microstructural changes correspond to minimum differential stress-rates of 5 × 10 −9 MPa s −1 to 5 × 10 −11 MPa s −1 . Similar or slower regional stress-rates during fault zone evolution, will allow continuous re-equilibration of the microstructure. Thus the time at which any given microstructure is frozen in will be critically dependent upon fault zone deformation history. In the Alpine fault zone rapid uplift and high near surface temperatures combine to generate rapid stress drops as mylonitic deformation gives way to cataclastic faulting during uplift. Mylonite microstructure is frozen in during this rapid stress drop and represents the shallowest level of major crystal-plastic deformation.

The nature of seismic reflections from deep crustal fault zones

Deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement. Perhaps the best example is the Consortium for Continental Reflection Profiling (COCORP) Wind River line in Wyoming, which shows the Wind River thrust fault to be a strong reflector from the surface to depths of about 30 km. To identify the physical properties responsible for the seismic reflections from fault zones, we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones. These rocks are characterized by extensive ductile deformation of plastic minerals, brittle deformation of the more rigid minerals, grain size reduction, and development of a strong fabric. Average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock. Seismic anisotropy of 7% or greater is present in several mylonites dependent upon their composition and fabric. Using our data on mylonite properties and models for the crustal structure near the Wind River line, we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the COCORP Wind River reflection data. When anisotropy and the finely laminated structure of the fault zone are considered, our modeling indicates fairly strong reflections from mylonite zones in most cases. We also show that elevated pore pressure in shear zones may produce strong reflections. However, a permeability of the order of 10−16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive.