Shale Fabric Research Articles

Activation of natural fractures during hydraulic fracturing in elastoplastic jointed rocks

Complexity of fracture development in low permeability oil and gas reservoirs is of interest to many field operators for production enhancement. In highly jointed shales such as Marcellus and Sanmin, fluid-induced shear slip deformation along joint junctions coupled with soft nature of shale fabrics make fracturing operations more complicated. Sophistication includes widely reported occurrences of seismic tremors. Understanding of slip phenomena and fracture network development in such scenarios is never straightforward and curtailed with limited lines of evidence. In this study, we propose a globally embedded cohesive zone modelling within an elastoplastic three-dimensional framework to provide an understanding of the complex fracture development in soft shale formations. Shear strength of the global pore pressure cohesive elements is obtained as a function of field variable (stress) through parallel computation with FORTRAN language. Drucker Prager frictional behavior is included in the model with different friction coefficients to ascertain the joint slip phenomena when shear strength of the joints is reduced under the drenched effect of fracturing fluid. Results indicate that when the coefficient of friction is high, the total slip of the rock is reduced compared to the scenario with low friction coefficients. Other parametric analyses indicate that an isotropic stress regime leads to a more complex fracture development, while increased injection rates favour the opening of more compressed fracture wings. The model is validated against field data from layered shale reservoir as well as against Kristianovich-Geertsma-de Klerk (KGD) model with excellent agreements in both cases. This study is important for understanding fracture development complexity in soft jointed rocks together with associated slip phenomena.

Micro X-ray fluorescence reveals pore space details and spatially-resolved porosity of rock-based microfluidic devices.

Characterization of microscopic details of the fabric of mudstones and shales (i.e., structure and composition) is important to understand their storage and transport properties. Current characterization methods struggle to probe reliably multiple scales of interest (e.g., pore and fracture) and measure properties at the finest resolution under representative in situ conditions. Micro X-ray fluorescence (μXRF) is a high-performance imaging technique that produces elemental images at sub-10 μm spatial resolution and could offer insight into a diversity of shale properties, such as mineral composition, porosity, and in situ pressure gradients. This study designed and carried out a porosity mapping protocol using model and real-rock microfluidic devices and contrast fluids. Etched silicon micromodels with real-rock pore network patterns served as ideal models to establish a proof of concept. Measurements were performed on a novel μXRF microscope not powered by synchrotron radiation. We registered the μXRF datasets with the binary rock masks used for micromodel fabrication and applied segmentation algorithms to compare porosities. We assessed expected advantages and limitations through a sensitivity analysis and beam study. μXRF is an important new imaging technique for microfluidic applications.

Terahertz Dielectric Permittivity Anisotropy and Fabric of Oil Shales: Comparison to Acoustic, Magnetic Susceptibility, and X-ray Microtomography Results

Terahertz Dielectric Permittivity Anisotropy and Fabric of Oil Shales: Comparison to Acoustic, Magnetic Susceptibility, and X-ray Microtomography Results

Nano-scale physicochemical attributes and their impact on pore heterogeneity in shale

Heterogeneity in shale due to distribution of mineral and organic matter controls the disposition of pores in them. While the effect of vertical anisotropy is accounted for most cases of porosity estimation, the effect of bedding parallel heterogeneity is often ignored. This study dives deep into the shale fabric using bedding parallel sections, and the pore volume distribution is estimated in shales of varying thermal maturity and organic carbon content (2.6% − 38.2 %). Three among the five shales used in this study are early mature (Tmax: 439–444 °C), whereas the remaining two are over-mature, signified by very high S4Tpeak (670–685 °C). The higher thermal maturity indicates thermal alteration induced by igneous intrusion. The pore volume, functional groups, and organic matter distribution are systematically derived in different positions of the bedding parallel section using a combination of 3D imaging, spectroscopy, small-angle scattering and low-pressure gas adsorption techniques. These spot-specific properties are further compared with bulk properties to quantify the extent of heterogeneity. It is observed that the over-mature shales with higher aromaticity contain 150–200% more bulk accessible pore volume than the lesser-mature shales. It is also evident that the smaller mesopores (∼10 nm pore width) are more inaccessible compared to their coarser (∼20 nm pore width) counterparts. Higher matured shales show increased inaccessibility of mesopores up to 228 % due to bitumen deposition in pores during thermal maturation, resulting in a sharp decline in mesopore surface fractal dimension. The uniformity of the functional group and organic matter distribution in different parts of the shale depends on the maturity, and reflects the extent of variation in pore volume across the specimen. The 3D tomography-derived spatial abundance of organic matter in over-mature shales show higher deviation (±15-20%) from the average than lesser-mature shales (5–10%). This study proposes a multiscale methodology which can evolve and develop as a protocol for systematic, reservoir-scale maturity-dependent heterogeneity quantification in gas-shales.

Influence of rock fabric on salt ion diffusion behavior in upper cretaceous lacustrine shale from Songliao Basin

Some shale reservoirs have high salt ion concentration in flow-back fluid, about which the mechanism is still unclear. To learn the rock fabric of lacustrine shale and its influence on the salt ion diffusion behavior, the investigation of rock component, pore structure, spontaneous imbibition and salt ion diffusion were conducted. Rock component includes inorganic minerals and organic matter. Pore structure includes overburden porosity-permeability, scanning electron microscope (SEM), nitrogen adsorption (NA), mercury intrusion porosimetry (MIP). Comparing the pore size distribution between NA and MIP, lots of pores are connected by small throat, and the pore volume controlled by throat can be 18 times of throat volume. Based on fractal dimensions analysis, which was defined by using the fractal Frenkel-Halsey-Hill (FHH) method based on NA, lacustrine shale has a smoother pore surface and a complex pore structure. The fractal dimensions are mainly controlled by specific surface area. Imbibition liquid is a medium for salt ion diffusing out of matrix and the salt ion diffuse nearly linearly with time in log scale after imbibition becomes stable. The slope of fitting line by using late diffusion data is defined as salt ion diffusion rate. The salt ion diffusion rate is mainly controlled by clay content, TOC, pore volume and specific surface area. The interaction between clay and liquid has the potential to release lots of salt ion, and a higher specific surface area provide more area for liquid interaction with minerals, and a higher pore volume provide more channel for salt ion diffusing out of sample. Our research is conducive to understanding the rock fabric of lacustrine shale in Qingshankou Formation and its influence on salt ion diffusion behavior.

Constraints of sedimentary structures on key parameters of shale gas reservoirs: A case study of the Wufeng and Longmaxi formations of the Ordovician‐Silurian transition in Luzhou area, Sichuan Basin, southwestern China

There are various sedimentary structures developed in the black shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in Luzhou area, southern Sichuan Basin, southwestern China. In order to evaluate the controlling effects of sedimentary structures on the key parameters of shale reservoir properties, collected classification of sedimentary structures, scanning electron microscope observation, analysis of shale fabric and porosity characteristics, low‐pressure nitrogen adsorption, methane isothermal adsorption, and conventional triaxial compression test were carried out based on shale samples from Wells A‐H1, B‐H1, C‐H1 and D‐H1 in the study area. The sedimentary structures recorded in these shales are categorized into five types: homogeneous bedding, weak horizontal bedding, strong horizontal bedding, discontinuous ripple bedding, and gravel‐bearing massive bedding. In general, the palaeo‐hydrodynamics and oxygen content of water columns were weak when homogeneous bedding shale was deposited. The organic matter is the most abundant and the total organic carbon content is up to 4.01% on average. The average quartz content is 43.5%, and the brittleness index is the highest. The porosity and adsorption capacity were the highest, with an average of 4.57% and 79.5 scf/ton, respectively. Combined with its mechanical properties of low Poisson's ratio and high Young's modulus, homogeneous bedding sedimentary structure is associated with the shale units possessing the highest organic matter abundance, the strongest reservoir property, and the best fracability, which is the priority target for exploration and development of the Wufeng to lower Longmaxi Formation in the Sichuan Basin, southwestern China.

Multimodal Visualization of Vaca Muerta Shale Fabric Before and After Maturation

We use two-dimensional (2D) and three-dimensional (3D) imaging techniques to characterize sister samples of an immature Vaca Muerta shale before and after maturation in a pyrolysis cell. We use a series of electron microscopy and X-ray techniques to determine the mineralogical and structural properties of each shale sample before and after maturation. We verify successful maturation of the sample using programmed pyrolysis as well as before and after 2D and 3D images to determine changes in mineralogy, the shale fabric, and the development of microcracks. We observe new crack development in organic matter-hosted regions and along grain boundaries, as well as substantial growth of pre-existing microcracks and defects. The sum of porosity plus organic matter is constant before and after pyrolysis at 35%, indicating that the majority of pore volume creation occurs at the expense of organic matter removal.

Cryptic burrow traces in black shales – a petrographic Rorschach test or the real thing?

AbstractIn black shale studies, apparent lack of bioturbation is commonly taken as an indication of anoxic bottom waters. Yet, modern oxygen‐stressed environments show that even at suboxic levels (0.0 to 0.2 ml l−1 oxygen), microscopic eukaryotic benthic organisms live in the uppermost millimetres to centimetres of the substrate. Known as meiofauna, these organisms disturb the primary fabric as they move through the sediment. These modern examples inspired students of the rock record to report sub‐millimetre irregular–wiggly features in black shale thin sections as meiofaunal burrows. Preparation flaws in thin section manufacture can cause reduced light transmission that results in darkened regions when viewed in transmitted light, and one wonders whether micro‐burrows reported in the literature are indeed burrows, or alternatively artifact‐induced optical illusions. Examining shale thin sections of Archean to Tertiary age showed: (i) that burrow‐like features are common regardless of age; and (ii) that burrow identification varies significantly between observers. When transposing locations of presumed traces onto scanning electron microscope images of the same field of view (diamond polished surface), no evidence of fabric modification at locations of presumed burrows was observed. Microfabric within and outside ‘burrows’ was the same, and larger fabric elements traversed ‘burrows’ uninterrupted. Once both sides of the rock slice were polished, presumed burrows could no longer be observed, indicating that damage to the underside of the thin section caused the mistaken identification of micro‐burrows. Because burrows emplaced in water‐rich muds must undergo substantial compaction‐induced ‘flattening’ and deformation prior to lithification, geometries of presumed burrows that disagree with a plausible compaction regime provide an additional filter to separate potential burrows from artifact‐induced features. Recognizing meiofaunal disruption is an important aspect for accurate palaeoenvironmental interpretation of black shales. Yet, initial assessment via optical microscopy needs to be verified through scanning electron microscope‐based examination of shale fabrics.

Multi-Scale Microfluidics for Transport in Shale Fabric

We develop a microfluidic experimental platform to study solute transport in multi-scale fracture networks with a disparity of spatial scales ranging between two and five orders of magnitude. Using the experimental scaling relationship observed in Marcellus shales between fracture aperture and frequency, the microfluidic design of the fracture network spans all length scales from the micron (1 μ) to the dm (10 dm). This intentional `tyranny of scales’ in the design, a determining feature of shale fabric, introduces unique complexities during microchip fabrication, microfluidic flow-through experiments, imaging, data acquisition and interpretation. Here, we establish best practices to achieve a reliable experimental protocol, critical for reproducible studies involving multi-scale physical micromodels spanning from the Darcy- to the pore-scale (dm to μm). With this protocol, two fracture networks are created: a macrofracture network with fracture apertures between 5 and 500 μm and a microfracture network with fracture apertures between 1 and 500 μm. The latter includes the addition of 1 μm ‘microfractures’, at a bearing of 55°, to the backbone of the former. Comparative analysis of the breakthrough curves measured at corresponding locations along primary, secondary and tertiary fractures in both models allows one to assess the scale and the conditions at which microfractures may impact passive transport.

Laboratory geomechanical and petrophysical characterization of Longmaxi shale properties in Lower Silurian Formation, China

The Lower Silurian Longmaxi formation is one of the most promising shale gas reservoirs in China. A comprehensive understanding of the shale geomechanical and petrophysical properties is crucial for the successful exploration and extraction of shale gas. We select four representative locations to acquire Longmaxi formation shale samples for the laboratory experiments, to investigate the geomechanical and petrophysical properties through a series of X-ray diffraction (XRD), scanning electron microscope (SEM), uniaxial compression, triaxial compression, tensile strength, and fracture toughness measurements. Laboratory results indicate that: (1) The quartz is the dominant mineral, and phyllosilicate mineral contents vary largelly from 7.30% to 47.80% in Longmaxi shale, which enables a higher brittleness index and fracbility. SEM results show that the high gas storage potential and well micro-fractures development of Longmaxi shale rocks. (2) The phyllosilicate content is vital in determining the uniaxial compressive strength, triaxial strength and elastic properties due to its weaker mechanical properties than tectosilicate minerals; (3) Fracture toughness of Longmaxi shale are relatively higher than shale formations in the USA, which indicate a higher potential to form fracture networks during hydraulic fracturing operations. (4) The anisotropy affects Longmaxi shale mechanical properities extensively due to the high-density bedding planes, which may further influence the fracture network formation during hydraulic fracturing operations. Our results revealed significant non-linear mechanical response as a consequence of shale fabric and mineralogy, which provides necessary information for the in-situ hydraulic fracturing and wellbore stability application during shale gas development in Longmaxi shale formation.

Permeability and Porosity Evolution of Organic-Rich Shales from the Green River Formation as a Result of Maturation

Summary Source rocks (oil shale) were matured artificially via pyrolysis under geologically realistic triaxial stresses using a unique coreholder that is compatible with X-ray computed tomography (CT) scanning. This study focuses on characterization of porosity and permeability as well as the evolution of shale fabric during pyrolysis. Experiments were conducted using 1-in. diameter vertical and horizontal core samples from the Green River Formation. Prior to pyrolysis, the properties of the source rock were characterized [e.g., porosity, bulk density, mineralogy, and total organic carbon (TOC)]. Samples were then heated from room temperature to 350°C under a nitrogen environment to obtain conversion of organic matter (OM) to oil and gas via pyrolysis. Porosity and permeability after maturation were measured. Micro-CT visualization was applied to investigate the fracture network developed throughout the core. Scanning electron microscopy (SEM) images were also used to compare the shale fabric and porosity evolution (pre- to post-maturation) at greater resolution. In-situ observations reveal a decrease in the average CT number (i.e., density) within some volumetric regions of the cores after maturation. In these regions, OM (kerogen and bitumen) was converted into hydrocarbons. Changes in the source rock depend on the original TOC fraction, hydrogen index (HI), and temperature. The permeability prior to pyrolysis for vertical samples is in the undetectable to nanodarcy range. The permeability of all cores increased to the microdarcy range post-maturation. In particular, the permeability of the horizontal sample increased from 0.14 to 50 µd. This improvement in permeability occurred due to the generation of open porosity and fractures (dilation, generation, and/or drainage). Additionally, the porosity after Soxhlet extraction increased proportionally by 20% from pre- to post-pyrolysis depending on pyrolysis time and TOC fraction. Longitudinal deformation depends on the orientation of the sample with respect to the triaxial stress during pyrolysis. The deformation of vertically oriented samples with isostress conditions is larger than that of horizontally oriented samples with isostrain. The measured 3D in-situ porosity distribution indicates that OM has transformed into hydrocarbons by pyrolysis. The development of a fracture and matrix porosity system under stress provides an explanation for transport of hydrocarbon away from its point of origin.

First investigation of quartz and calcite shape fabrics in strained shales by means of X-ray tomography

We document the evolution of the 3D fabric of shale along a km-long strain gradient in the Jaca basin (Southern Pyrenees, Spain). With respect to the distance from a thrust, samples were collected in the cleavage-free domain, at the onset of the pencil-cleavage domain, within the pencil-cleavage domain and within the slaty-cleavage domain. By combining high resolution X-ray computed tomography (XCT) and energy dispersive X-ray spectroscopy (EDS), the morphology and the Shape Preferred Orientation (SPO) of thousands of quartz grains, calcite grains and pores was studied. In the least deformed samples, quartz and calcite display mean foliation parallel to bedding with comparable dispersion. In the pencil-cleavage domain, quartz foliation still follows the bedding while calcite foliation is mostly governed by cleavage. In the slaty-cleavage domain, calcite shows a much better organization, with foliation parallel to cleavage, mimicking closely the pore fabric. By contrast, the quartz shape fabric is much less defined, scattered between bedding and cleavage planes. This suggests that quartz grain act as rigid marker in the ductile matrix, while calcite grain orientation is governed by dissolution-precipitation processes.

Determination of Underground Structure and Migration of Hot Plumes Contaminating Fresh Water Using Vertical Electrical Survey (VES) and Magnetic Survey, A Case Study of Tattapani Thermal Spring, Azad Kashmir

A geophysical survey was carried out at Tattapani thermal spring Azad Kashmir to delineate structure,thickness, depth, lithology and migration of hot plumes contaminating fresh water. The study area was investigated byVertical Electrical Sounding (VES) using schlumberger array at 21 locations arranged in ten profiles to a maximumdepth of 500 m and 200 magnetic observations. The extension and tectonic setup of thermal spring was mapped bygeoelectrical litho sections, subsurface geological sections (20m, 20-100m and 100-500m) pseudo section, apparentresistivity map, geoelectrical parameters, statistical distribution of apparent resistivity, total magnetic intensity andanomaly map. The data show that Tattapani hot spring is concentrated along the fault line delineated by geoelectricallitho sections and magnetic section with value of -120 nT to -300 nT, total field intensity of 50000-50450 nT andconfirm by macro anisotropy (1.0 to 2.7). The geoelectrical lithological section portrays that study area compriseslithological fabric of dolomite (≥400 ohm.m), sandstone (150-200 ohm.m), clay (80-150 ohm.m), Shaley clay (50-80)and shale (≤ 50). The Thermal Plumes (10-70 ohm.m) were pictured by resistivity section and pseudo section ataverage depth of 30-60 m and showing migration of hot plumes in the North-Eastern direction contaminating freshwater (100-200 ohm.m). The longitudinal conductance (0.95-15 mhos), transverse resistance (20-300 ohm.m2) are seenhaving maximum value in the North-Eastern and North-Western side of the study area. The study also shows that freshground water is mostly concentrated in sandstone (150-200 ohm.m), dolomite (≥400 ohm.m) and lies above the thermalplumes and thus highly prone to contamination due to upwelling of thermal water.

Determination of Underground Structure and Migration of Hot Plumes Contaminating Fresh Water Using Vertical Electrical Survey (VES) and Magnetic Survey, A Case Study of Tattapani Thermal Spring, Azad Kashmir

A geophysical survey was carried out at Tattapani thermal spring Azad Kashmir to delineate structure,thickness, depth, lithology and migration of hot plumes contaminating fresh water. The study area was investigated byVertical Electrical Sounding (VES) using schlumberger array at 21 locations arranged in ten profiles to a maximumdepth of 500 m and 200 magnetic observations. The extension and tectonic setup of thermal spring was mapped bygeoelectrical litho sections, subsurface geological sections (20m, 20-100m and 100-500m) pseudo section, apparentresistivity map, geoelectrical parameters, statistical distribution of apparent resistivity, total magnetic intensity andanomaly map. The data show that Tattapani hot spring is concentrated along the fault line delineated by geoelectricallitho sections and magnetic section with value of -120 nT to -300 nT, total field intensity of 50000-50450 nT andconfirm by macro anisotropy (1.0 to 2.7). The geoelectrical lithological section portrays that study area compriseslithological fabric of dolomite (≥400 ohm.m), sandstone (150-200 ohm.m), clay (80-150 ohm.m), Shaley clay (50-80)and shale (≤ 50). The Thermal Plumes (10-70 ohm.m) were pictured by resistivity section and pseudo section ataverage depth of 30-60 m and showing migration of hot plumes in the North-Eastern direction contaminating freshwater (100-200 ohm.m). The longitudinal conductance (0.95-15 mhos), transverse resistance (20-300 ohm.m2) are seenhaving maximum value in the North-Eastern and North-Western side of the study area. The study also shows that freshground water is mostly concentrated in sandstone (150-200 ohm.m), dolomite (≥400 ohm.m) and lies above the thermalplumes and thus highly prone to contamination due to upwelling of thermal water.

Shale fabric and organic nanoporosity in lower Palaeozoic shales, Bornholm, Denmark

In organic-rich shales, pores form during oil and gas genesis within organic matter (OM) domains. The porosity thus differs markedly from that of conventional reservoir lithologies. Here we present the first description of shale fabric and pore types in the lower Palaeozoic shales on Bornholm, Denmark. The pores have been studied using the focused ion beam scanning electron microscope (FIB-SEM) technique, which allows for high resolution SEM images of ion polished surfaces. Shale porosity is influenced by many factors including depositional fabric, mineralogical composition, diagenesis and oil and gas generation (Schieber 2013). Here we discuss some of these factors based on a study of lower Palaeozoic shale samples from the Billegrav-2 borehole on Bornholm (Fig. 1) undertaken by Henningsen & Jensen (2017). The shales are dry gas-mature (2.3% graptolite reflectance; Petersen et al. 2013) and have been extensively used as analogies for the deeply buried Palaeozoic shales elsewhere in Denmark (Schovsbo et al. 2011; Gautier et al. 2014). The Danish lower Palaeozoic shale gas play was tested by the Vendsyssel-1 well drilled in northern Jylland in 2015. Gas was discovered within a c. 70 m thick gas-mature, organic rich succession (Ferrand et al. 2016). However, the licence was subsequently relinquished, due to a too low gas content. The present study confirms a close similarity of pore development between the shales on Bornholm and in the Vendsys sel-1 indicating a high porosity within this stratigraphic level throughout the subsurface of Denmark. However, the rather different development of porosity in the different shale units presents a hitherto neglected aspect of the Palaeozoic gas play in Denmark.

Response of Velocity Anisotropy of Shale Under Isotropic and Anisotropic Stress Fields

We investigated the responses of P-wave velocity and associated anisotropy in terms of Thomsen’s parameters to isotropic and anisotropic stress fields on Longmaxi shales cored along different directions. An array of piezoelectric ceramic transducers allows us to measure P-wave velocities along numerous different propagation directions. Anisotropic parameters, including the P-wave velocity α along a symmetry axis, Thomsen’s parameters e and δ, and the orientation of the symmetry axis, could then be extracted by fitting Thomsen’s weak anisotropy model to the experimental data. The results indicate that Longmaxi shale displays weakly intrinsic velocity anisotropy with Thomsen’s parameters e and δ being approximately 0.05 and 0.15, respectively. The isotropic stress field has only a slight effect on velocity and associated anisotropy in terms of Thomsen’s parameters. In contrast, both the magnitude and orientation of the anisotropic stress field with respect to the shale fabric are important in controlling the evolution of velocity and associated anisotropy in a changing stress field. For shale with bedding-parallel loading, velocity anisotropy is enhanced because velocities with smaller angles relative to the maximum stress increase significantly during the entire loading process, whereas those with larger angles increase slightly before the yield stress and afterwards decrease with the increasing differential stress. For shale with bedding-normal loading, anisotropy reversal is observed, and the anisotropy is progressively modified by the applied differential stress. Before reaching the yield stress, velocities with smaller angles relative to the maximum stress increase more significantly and even exceed the level of those with larger angles. After reaching the yield stress, velocities with larger angles decrease more significantly. Microstructural features such as the closure and generation of microcracks can explain the modification of the velocity anisotropy due to the applied stress anisotropy.

The influence of shale depositional fabric on the kinetics of hydrocarbon generation through control of mineral surface contact area on clay catalysis

Accurately assessing the temperature and hence the depth and timing of hydrocarbon generation is a critical step in the characterization of a petroleum system. Clay catalysis is a potentially significant modifier of hydrocarbon generation temperature, but experimental studies of clay catalysis show inconsistent or contradictory results. This study tests the hypothesis that source rock fabric itself is an influence on clay mineral catalysis as it controls the extent to which organic matter and clay minerals are physically associated. Two endmember clay-organic fabrics distinguish the source rocks studied: (1) a particulate fabric where organic matter is present as discrete, >5 μm particles and (2) a nanocomposite fabric in which amorphous organic matter is associated with clay mineral surfaces at sub-micron scale. High-resolution electron imaging and bulk geochemical characterisation confirm that samples of the Miocene Monterey Formation (California) are representative of the nanocomposite source rock endmember, whereas samples from the Permian Stuart Range Formation (South Australia) represent the particulate source rock endmember. Kinetic experiments are performed on paired whole rock and kerogen isolate samples from these two formations using open system, non-isothermal pyrolysis at three different heating rates (0.7, 2 and 5 K/min) to determine the effects of the different shale fabrics on hydrocarbon generation kinetics. Extrapolation to a modelled geological heating rate shows a 20 °C reduction in the onset temperature of hydrocarbon generation in Monterey Formation whole rock samples relative to paired kerogen isolates. This result is consistent with the Monterey Formations’s nanocomposite fabric where clay catalysis can proceed because reactive clay minerals are intimately associated with organic matter. By contrast, there is no significant difference in the modelled hydrocarbon generation temperature of paired whole rock and kerogen isolates from the Stuart Range Formation. This is consistent with its particulate fabric, where relatively large, discrete organic particles have limited contact with the mineral matrix and the clay minerals are mainly diagenetic and physically segregated within pores. While heating rate may have a control on mineral matrix effects, this result shows that the extent to which organic matter and clay minerals are physically associated could have a significant effect on the timing of hydrocarbon generation, and is a function of the depositional environment and detrital vs diagenetic origin of clay minerals in source rocks.

Swelling of Shales: A Multiscale Experimental Investigation

Shale swelling can cause a series of problems during the exploitation of shale gas. However, because of the complex fabric of shale, a thorough understanding of shale swelling has not been gained. In this work, an experimental multiscale approach is employed to characterize the swelling properties of a shale sample from Sichuan basin of China. Wetting/drying tests are performed by environmental scanning electron microscopy (ESEM), and deformation of the material is measured by analyzing the ESEM micrographs using digital image correlation (DIC) techniques. Investigations are conducted at three distinct scales, enabling the macroscopic behavior to be linked to the microscopic mechanisms (e.g., the role of inclusions, the behavior of clay particles). The macroscopic swelling of the tested shale is less than 2%. This limited swelling is mainly due to the presence of nonswelling inclusions as well as the low swelling capacity of the clay inside the shale. Microcracking is evidenced along both the wetting and ...

Organism–sediment interactions in shale-hydrocarbon reservoir facies — Three-dimensional reconstruction of complex ichnofabric geometries and pore-networks

The lithological and mineralogical characteristics of mudstones and siltstones—and their stress–strain behavior at the meter to nanometer scale—can play a critical role in the exploitation of unconventional shale reservoirs. Shale fabrics that result from bioturbation can produce extensive interconnected networks of biologically redistributed sediment grains within reservoir mudstone facies. The presence of biologically generated heterogeneities may substantially affect reservoir stimulation and thus production from shale facies. This study presents volumetric evaluation of ichnofabrics dominated by Phycosiphon-like and aff. Chondrites, and provides insights into the impact of trace fossils on the rheological and petrophysical characteristics of mudstones. It is calculated that, in addition to creating significant volumes of silty (clay-poor) zones of enhanced porosity and permeability, trace fossils create interpenetrating frameworks of brittle material that reduce communication distances from the low-permeability matrix to the higher permeability silt-rich burrows. Reducing communication distances to less than 1cm increases the potential for diffusive transport of hydrocarbon molecules from the “tight” matrix to the wellbore-connected volumes. This is because shale ichnofabrics create abundant fracture-prone planes of weakness, and increase the surface area of the interface between the hydrocarbon-rich matrix and porous burrow fills, thereby promoting fluid exchange. Understanding of the three-dimensional characteristics of ichnofabrics may form the basis of future modeling of fracture spacing and complexity that is critical to shale gas reservoir characterization.

Heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin of China

This study characterizes the vertical heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin based on core description, thin sections observation, geological chemistry data interrogation, and QEMSCAN image analysis. Lithofacies associations, mineralogy, graptolite species occurrence, rock fabrics, as well as TOC (Total Organic Carbon) and gas content, change noticeably and regularly in the Lower Silurian Longmaxi marine shale within sequence stratigraphic framework. Lithofacies associations mainly include in-situ graptolite rich and siliceous shale in the transgressive systems tract (TST), micro-offsite graptolite rich shale and shale interbedded with siltstone laminaes in the early highstand systems tract (EHST), and micro-offsite graptolite rich and carbonate rich shale in the late highstand systems tract (LHST). The TST has a greater abundance of Diplogratus than EHST and LHST, and has a lesser level of Monograptus, which is conversely higher in the EHST and LHST. The TST shale is silica rich, while the LHST shale is generally clay rich. The EHST shale is more balanced in its silica and clay content than the TST and LHST are. The sedimentary structure and fabric of shale shows that shale is homogenous in the TST, with laminaes well developed in the EHST but decreasing in the LHST. TOC and gas content are highest in the TST and decrease upward. The TST, with high TOC, high gas content, high brittle minerals (silica rich) and anoxic-suboxic to dysoxic sedimentary environment, is the most potential shale gas target interval in the study area. Our research also shows that the TOC mainly controls shale gas accumulation and that the redox conditions mainly control the preservation of organic matter. The silica also plays a significant role in shale gas content due to its contribution to hydraulic fracturing. Finally, the study proposes a depositional and heterogeneity model for the Lower Silurian Longmaxi marine shale in the study area.