Silurian Longmaxi Shale Research Articles

Variations in pore structure of marine shale from the same horizon of the Longmaxi Formation with changing position in a small-scale anticline: Implications for the influence of structural deformation

Nanopores are well developed in shale, and are influenced by many rock properties, such as mineral composition, TOC (Total Organic Carbon) and thermal maturity. However, the influence of structural deformation on pore properties of shale is still not well understood. In this study, 19 samples from the same horizon of the lower Silurian Longmaxi shale, but from different positions in a small-scale anticline of Wuping Village, Chongqing (China), were collected. Low pressure N2 and CO2 adsorption experiments were performed to study the influence of structural deformation on the pore structure. The results show that the mineral composition, TOC and thermal maturity of all samples are similar, but the specific surface areas and pore volumes exhibit great differences in different positions of this anticline. The mean N2-BJH pore volumes, the mean N2-BET specific surface areas, the mean CO2 D-R pore volumes, as well as the mean CO2 D-R specific surface areas exhibit decreasing trends for the samples from the core part to the sub-core part, and to the limb part of the studied anticline, respectively. Structural deformation has shown a considerable influence on the micropores and mesopores, especially for the pores lower than 10 nm because that tectonic stress increases from the core part to the sub-core part and the limb part of the anticline. Moreover, the structural curving degree characterized using curvature shows good correlation with the pore volume and specific surface area. Compared to the limb part, the core part of the anticline with high curvature is located in the tension environment, and it is beneficial for pore development and gas accumulation. Our study implies that moderate structural deformation can provide more specific surface area for absorbed gas and pore volume for free gas at the core part of the structure on a larger scale.

Factors Affecting Shale Gas Chemistry and Stable Isotope and Noble Gas Isotope Composition and Distribution: A Case Study of Lower Silurian Longmaxi Shale Gas, Sichuan Basin

The Weiyuan (WY) and Changning (CN) fields are the largest shale gas fields in the Sichuan Basin. Though the shale gases in both fields are sourced from the Longmaxi Formation, this study found notable differences between them in molecular composition, carbon isotopic composition, and noble gas abundance and isotopic composition. CO2 (av. 0.52%) and N2 (av. 0.94%) were higher in Weiyuan than in Changning by an average of 0.45% and 0.70%, respectively. The δ13C1 (−26.9% to −29.7%) and δ13C2 (−32.0% to −34.9%) ratios in the Changning shale gases were about 8% and 6% heavier than those in Weiyuan, respectively. Both shale gases had similar 3He/4He ratios but different 40Ar/36Ar ratios. These geochemical differences indicated complex geological conditions and shed light on the evolution of the Lonmaxi shale gas in the Sichuan Basin. In this study, we highlight the possible impacts on the geochemical characteristics of gas due to tectonic activity, thermal evolution, and migration. By combining previous gas geochemical data and the geological background of these natural gas fields, we concluded that four factors account for the differences in the Longmaxi Formation shale gas in the Sichuan Basin: a) A different ratio of oil cracking gas and kerogen cracking gas mixed in the closed system at the high over-mature stage. b) The Longmaxi shales in WY and CN have had differential geothermal histories, especially in terms of the effects from the Emeishan Large Igneous Province (LIP), which have led to the discrepancy in evolution of the shales in the two areas. c) The heterogeneity of the Lower Silurian Longmaxi shales is another important factor, according to the noble gas data. d) Although shale gas is generated in closed systems, natural gas loss throughout geological history cannot be avoided, which also accounts for gas geochemical differences. This research offers some useful information regarding the theory of shale gas generation and evolution.

Characterization of Anisotropic Fracture Properties of Silurian Longmaxi Shale

Fracking is widely applied to enhance shale gas mining, and insight into the fracture behaviors of shale rocks is important. To characterize the fracture properties of Lower Silurian Longmaxi shale, a chevron-notched deep beam specimen, which inherits the advantages of notched deep beam and chevron-notched specimens, is introduced, and several three-point bend tests were conducted on the Longmaxi shale specimens in three principal fracture orientations: the divider, short-transverse, and arrester orientations; the anisotropy in the critical shape coefficient, fracture toughness and fracture energy were then examined and highlighted. The results demonstrate that the critical shape coefficient depends on the anisotropic elasticity of the investigated shale, and a maximum error in the calculated fracture toughness of 9.2% could occur if a critical shape coefficient for isotropic rock is adopted. For the tested shale, the mode I fracture toughness in the divider orientation is close to that in the arrester orientation, with both values being approximately 1.5 times that in the short-transverse orientation; in addition, a progressive decrease in fracture energy from the arrester orientation to the short-transverse orientation is apparent. The fracture surface morphology was observed by scanning electron microscopy, and obvious material deterioration was found near the fracture surfaces for the arrester and divider samples.

Evidence for Biogenic Silica Occurrence in the Lower Silurian Longmaxi Shale in Southeastern Chongqing, China

The gas shale in the Lower Silurian Longmaxi Formation contains a considerable amount of biogenic silica. Various originated silicas in shale, derived from different depositional environment, are commonly associated with different degrees of organic matter enrichment, resulting in different mechanical and physical properties of shale reservoirs. Thin section identification, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, and X-ray fluorescence (XRF) spectroscopy were used to investigate the Lower Silurian Longmaxi shale from Well Yuye 1 in southeastern Chongqing, China to obtain a better understanding of the origin of silica in the Longmaxi Shale. The results show ubiquitous cryptocrystalline silicas with poorly crystalline morphology, which differs from that of the detrital silica, authigenic silica, and hydrothermal silica, proving that the cryptocrystalline silicas may have a biogenic origin. Major element and mineral composition analysis indicate no correlations between K2O/Al2O3 and SiO2/Al2O3 and between illite and SiO2, and negative correlations between TiO2 and SiO2/Al2O3, between illite and quartz and excess Si, and between Al2O3 and excess Si, and all samples being located in the area of non-hydrothermal origin in the Al-Fe-Mn diagram, excluding silicas of terrigenous detrital origin, clay mineral transformed origin, and hydrothermal origin. Moreover, the fact that almost all samples plot above the illite Si/Al line in the cross-plot of Si versus Al and the mean values of Al/(Al + Fe + Mn) and Si/(Si + Al + Fe + Ca) are close to the values of biogenic silica prove that the silicas are primarily of biogenic origin. Positive correlations between TOC and quartz and excess Si and numerous siliceous organisms are observed, indicating that the silicas are associated with siliceous organisms. The postmortem siliceous organisms underwent silica diagenesis via a dissolution-precipitation mechanism following the sequence of opal-A → opal-CT → cryptocrystalline biogenic silica as the burial depth and temperature increased.

Organic matter pores structure and evolution in shales based on the he ion microscopy (HIM): A case study from the Triassic Yanchang, Lower Silurian Longmaxi and Lower Cambrian Niutitang shales in China

To illustrate organic matter (OM) pores structure and evolution in shales with different maturities, the Triassic Yanchang shale (0.5% < Vitrinite reflectance (Ro) < 1.2%) in the Ordos Basin, the Lower Silurian Longmaxi shale (2.0% < Ro < 2.8%) and the Lower Cambrian Niutitang shale (3.0% < Ro < 4.2%) in the Yangtze Platform have been selected in this study. OM pores were analyzed by helium ion microscopy (HIM). The results show that OM pores are mainly developed in pyrobitumen. A large number of OM pores were developed in pyrobitumen of the Longmaxi shale samples. However, no OM pores were basically found in solid kerogen. OM pores number and size in pyrobitumen of the Yanchang shale samples are much smaller than that in the Longmaxi shale samples. OM pores are all basically undeveloped in pyrobitumen of the Niutitang shale samples. The structure that small size OM pores nested in large size OM pores is favorable to gas enrichment and seepage. Thermal maturity has more huge impact on OM pores structure than TOC content in shale. Low thermal maturity (Ro < 1.5%) will lead to insufficient thermal evolution of OM, resulting in small size and quantity of OM pores in shale. Excessive thermal maturity that ranges from 3.0% to 4.0% will leads to the disappearance of OM pores in pyrobitumen. Appropriate thermal maturity that ranges from 1.5% to 3.0% ensures a large number of OM pores in shale.

Pore size distributions contributed by OM, clay and other minerals in over-mature marine shale: A case study of the Longmaxi shale from Southeast Chongqing, China

Nano-scale pores play an important role in determining the adsorption capacity of shale gas. To quantitatively evaluate the pore size distributions contributed by organic matter (OM), clay and other minerals in an over-mature shale reservoir, the Lower Silurian Longmaxi shale from Southeast Chongqing was selected as a case study and was analyzed by the pore size distributions contributed by OM, clay and other minerals (PSDCOCO) method. Based on the results of total organic carbon (TOC), carbon element content in OM, X-ray diffraction (XRD) and low temperature nitrogen adsorption/desorption (LTNA) experiments, the pore size distributions contributed by a unit mass of OM, clay and other minerals were calculated and the differences between the results of the LTNA and PSDCOCO methods were determined for the four verified samples. The average value of the difference between the experimental and calculated pore volumes is 0.154 × 10−3 cm3/g. For the Longmaxi shale from the Py1 well, the contributions of OM and other minerals to the pore volume (pore width <200 nm) are primarily distributed in the pore width ranges from 2 to 20 nm and <2 nm, respectively. The nano-scale pore size distributions (<200 nm) contributed by the clay minerals have two relatively high peak values in the pore width ranges of 20–100 nm and 2–5 nm. The pore volumes for pore widths less than 200 nm contributed by a unit mass of OM, clay and other minerals were 115.776 × 10−3 cm3/g, 10.489 × 10−3 cm3/g and 0.006 × 10−3 cm3/g, respectively. In the shale reservoir, the contributions of OM, clay and other minerals to the pore volume with pore widths less than 200 nm were 45.75%, 54.17% and 0.08%, respectively, based on the limited data available.

Reservoir Characteristics of the Lower Silurian Longmaxi Shale in Zhaotong Region, Southern China

The lower Silurian Longmaxi Formation hosts a highly productive shale gas play in the Zhaotong region of southern China. According to core observation, X-ray diffraction analyses, and scanning electron microscopy (SEM) observations, the shale comprises primarily quartz, carbonate minerals, and clay minerals, with minor amounts of plagioclase, K-feldspar, and pyrite. The clay mineral content ranges from 15.0% to 46.1%, with an average of 29.3% in the Zhaotong region. Organic geochemical analyses show that the Longmaxi Formation has good potential for shale gas resources by calculating total organic carbon, vitrinite reflectance, and gas content. Scanning electron microscope images demonstrate that reservoir pore types in the Longmaxi shale include organic pores, interparticle pores, intercrystalline pores, intraparticle pores, and fractures. Reservoir distribution is controlled by lithofacies, mineral composition, and geochemical factors. In addition, we investigated the relationships between reservoir parameters and production from 15 individual wells in the Zhaotong region by correlation coefficients. As a result, the brittleness index, total organic carbon (TOC), porosity, and gas content were used to define high-quality reservoirs in the Longmaxi shale. Based on these criteria, we mapped the thickness and distribution of high-quality reservoirs in the Longmaxi Formation and selected highlighted several key sites for future exploration and development.

Influence of differential structural deformation on shale reservoirs: a case study of the lower Silurian Longmaxi Shale in north Guizhou, Southern China

AbstractSouthern China is affected by multi-stage tectonic activities, with strong fold deformation, complex fault systems and poor shale gas preservation conditions. Here, we used shale samples from the lower Silurian Longmaxi shale in the complex tectonic area of Southern China, to study the relationship between differential structural deformation, and pore structure and adsorption capacity. According to the deformation mechanism of the shale, it is further divided into brittle-slip rheological deformation (BD) and ductile-slip rheological deformation (DD). The results show that all micro-fractures can be observed under scanning electron microscopy in deformed shale samples, but in shale samples with different types of rheological deformation, the micro-fractures have large differences in scale, fracture length and lateral connectivity. The micro-fractures developed in DD shales are small in scale and short in fracture length, but have strong local connectivity. In contrast, brittle minerals are more developed in BD shales, and interlayer shearing has formed micro-fractures with large fracture length and good lateral connectivity, which is beneficial for later fracturing. In these two types of deformed shales, pores in organic matter are rare, and sporadic organic pores have small pore size and poor connectivity. The total pore volume (1.8–2.4 × 10−2 cm3 g–1) of BD shale samples is higher than that of DD shale samples (0.8–1.6 × 10−2 cm3 g–1). There is a positive correlation between total pore volume and quartz content. In addition, the specific surface area (12–18 m2 g–1) of DD shale samples is larger than that of BD shale samples (6–12 m2 g–1).

Influences of Primary Moisture on Methane Adsorption within Lower Silurian Longmaxi Shales in the Sichuan Basin, China

Moisture significantly affects the adsorption capability of gas shales for methane (CH4), the main component of shale gas. Primary moisture, i.e., the moisture that exists in in situ shale reservoi...

Reservoir characteristics and genetic mechanisms of gas-bearing shales with different laminae and laminae combinations: A case study of Member 1 of the Lower Silurian Longmaxi shale in Sichuan Basin, SW China

Based on thin-section, argon-ion polished large-area imaging and nano-CT scanning data, the reservoir characteristics and genetic mechanisms of the Lower Silurian Longmaxi shale layers with different laminae and laminae combinations in the Sichuan Basin were examined. It is found that the shale has two kinds of laminae, clayey lamina and silty lamina, which are different in single lamina thickness, composition, pore type and structure, plane porosity and pore size distribution. The clayey laminae are about 100 μm thick each, over 15% in organic matter content, over 70% in quartz content, and higher in organic pore ratio and plane porosity. They have abundant bedding fractures and organic matter and organic pores connecting with each other to form a network. In contrast, the silty laminae are about 50 μm thick each, 5% to 15% in organic matter content, over 50% in carbonate content, higher in inorganic pore ratio, undeveloped in bedding fracture, and have organic matter and organic pores disconnected from each other. The formation of mud lamina and silt lamina may be related to the flourish of silicon-rich organisms. The mud lamina is formed during the intermittent period, and silt lamina is formed during the bloom period of silicon-rich organisms. The mud laminae and silt laminae can combine into three types of assemblages: strip-shaped silt, gradating sand-mud and sand-mud thin interlayers. The strip-shaped silt assemblage has the highest porosity and horizontal/vertical permeability ratio, followed by the gradating sand-mud assemblage and sand-mud thin interlayer assemblage. The difference in the content ratio of the mud laminae to silt laminae results in the difference in the horizontal/vertical permeability ratio.

Investigation of formation and evolution of organic matter pores in marine shale by helium ion microscope: An example from the Lower Silurian Longmaxi Shale, South China

Using a helium ion microscope (HIM), we carefully observed and analyzed organic matter (OM) pores in several Longmaxi shale samples collected from the Upper Yangtze Platform, South China. The results indicate that the formation process of OM pore essentially involves gradual expansion and connection of defects/space within OM. This process, which is divided into the four stages of formation, expansion, connection, and consolidation, can involve pore scales ranging from a few nanometers to over a micrometer under HIM. More specifically, spotted spherical pores first form within OM, then expand until they connect to adjacent pores. Within large areas of such connectivity, “pit” structures eventually form, preliminarily defined as OM pores that are significantly larger than surrounding pores, essentially being greater than 100 nm in size. As the final stage of OM pore evolution, the shape and size of “pit” structures can be limited by the framework of mineral particles around or within the OM. Additionally, we found that surrounding pores are smaller when “pit” structures develop, perhaps because removal of side chains of OM tends to occur on the inner surface of the formed pores, leading them to rapidly expand, or because OM particles have differing compositions or structures, some of which may be unfavorable to pore development.

Experimental investigation of water vapor adsorption isotherm on gas-producing Longmaxi shale: Mathematical modeling and implication for water distribution in shale reservoirs

Mathematical characterization of water vapor adsorption (WVA) isotherms on organic-rich shale is important for both modeling processes such as water flow and mass transport, and accurate evaluation of moisture content in shale. Although several theoretical and empirical models have been proposed to describe WVA on porous materials (e.g, coal, food and carbon black), the applicability of these proposed models for over-mature gas shale (e.g., >2.0%Ro) is not well understood, which requires both experimental data and a theoretical description of downhole shale. In this work, WVA isotherms were measured at 50℃ (323.1 K) using a dynamic vapor sorption apparatus for four over-mature Lower Silurian Longmaxi shales, the leading gas-producing shale reservoir in the Sichuan Basin, China. To describe water adsorption behavior mathematically, five models, including the Guggenheim-Anderson-de Boer (GAB), double log polynomial (DLP), Oswin, Freundlich, and Frenkel-Halsey-Hill (FHH), for WVA isotherm are evaluated for their ability to match the experimental WVA isotherm data.In general, all the models are suitable for fitting the experimental data, with R2 values larger than 0.95. However, the transformations of non-linear isotherm equations to their linear forms could implicitly alter the error structure and may violate the error variance and normality assumptions. To address this deficiency, six more statistical parameters (AAD, MSE, SEE, RSS, ARE and χ2) were used to evaluate the goodness-of-fit results for different models. Comparative studies show that the four-parameter DLP model is the optimal to predict the WVA isotherms on Longmaxi shale, with the smallest fitting errors, rather than the most versatile GAB model documented in the literature. Considering the WVA behavior, the distribution of water (free and adsorbed) in hydrophobic and hydrophilic pores in shales was also discussed, which can provide a reference point for theoretical analysis of water flow and retention.

Tensile and Shear Mechanical Characteristics of Longmaxi Shale Laminae Dependent on the Mineral Composition and Morphology

Laminae are well developed in shale and generally influence fracture propagation during hydraulic fracturing. Hence, comprehensively understanding the tension and shear behaviors of shale laminae is crucial. There have been limited systematic studies thus far on the tensile and shear strength as well as fracture morphology of shale laminae. In this study, the Lower Silurian Longmaxi Shale (China) was investigated via Brazilian tensile and angle-varied plate shear tests. Five lamina types were tested, i.e., calcite (Cal), pyrite (Py), organic-enriched (Oc), the interface between Cal and Oc (Cal-Oc), and the interface between Py and Oc (Py-Oc) laminae. Results showed that the tensile strength was in the range 0.43–8.22 MPa, mainly in the order of Cal &gt; Py &gt; Cal-Oc &gt; Py-Oc &gt; Oc. The modes of fracture morphology were highly related to the occurrence, continuity, and mineralogy fillings of laminae. Shear strength parameters were within the range 22.50–29.64 MPa for cohesion and 37.29–43.60° for internal friction angle. Fracture surface roughness was strongly related to its cohesion. Calcite laminae considerably influenced the tensile fracturing of shale, suggesting that the geometry and properties of calcite lamina should receive more attention during the design of shale gas exploration.

Factors affecting the nanopore structure and methane adsorption capacity of organic-rich marine shales in Zhaotong area, Southern Sichuan Basin

As a national shale-gas demonstration zone in China, the Zhaotong area has great gas resource potential. However, the nanopore structure characteristics, methane adsorption capacity, and their affecting factors of the Lower Silurian Longmaxi Shale in this area remain unclear. To address these puzzles, we conducted a series of experiments, such as X-ray diffraction, field emission scanning electron microscopy, low-pressure [Formula: see text] adsorption, and high-pressure methane adsorption, and we calculated the relevant characteristic parameters, such as pore volume (PV), specific surface area (SSA), fractal dimension, and Langmuir parameters by using the nonlocal density functional theory method, Frenkel-Halsey-Hill theory, and Langmuir model, respectively. The results indicate that the nanopores of the Lower Longmaxi Shale in the Zhaotong area are composed of micropores and mesopores, which mainly exist as organic matter (OM) pores. The pore surface exhibits a high degree of heterogeneity as indicated by the fractal dimensions ranging from 2.845 to 2.866. The nanopore structure characteristics (i.e., SSA and PV) and methane adsorption capacity are mainly controlled by the total organic carbon (TOC) content. In addition, the mineralogical composition (i.e., the quartz and clay content) also contributes significantly to the micropore PV and gas content. The external provenance has a significant effect on the mineralogical composition, TOC content, and methane adsorption capacity. With the increasing influence of the external provenance, the biogenic quartz content decreases and the relationship between the quartz content and TOC content becomes more discrete, which indicates the change of depositional environment, and the clay content increases, which can dilute the OM concentration during the deposition and enhance the compaction potential, and it can eventually result in less gas content. The results of this study reveal the nanopore system characteristics of the Longmaxi Shale in the Zhaotong area and provide further insight into the influence of external provenance on reservoir characteristics and gas content variability of the Lower Longmaxi Shale in the southern Sichuan Basin.

Modeling High-Pressure Methane Adsorption on Shales with a Simplified Local Density Model.

Shale gas has attracted increasing attention as a potential alternative gas in recent years. Because a large fraction of gas in shale formation is in an adsorbed state, knowledge of the supercritical methane adsorption behavior on shales is fundamental for gas-in-place predictions and optimum gas recovery. A practical model with rigorous physical significance is necessary to describe the methane adsorption behavior at high pressures and high temperatures on shales. In this study, methane adsorption experiments were carried out on three Lower Silurian Longmaxi shale samples from the Sichuan Basin, South China, at pressures of up to 30 MPa and temperatures of 40, 60, 80, and 100 °C. The simplified local density/Elliott–Suresh–Donohue model was adopted to fit the experimental data in this study and the published methane adsorption data. The results demonstrate that this model is suitable to represent the adsorption data from the experiments and literature for a wide range of temperatures and pressures, and the average absolute deviation is within 10%. The methane adsorption capacity of the Longmaxi shale exhibited a strong linear positive correlation with the total organic carbon content and a linear negative correlation with increasing temperature. The rate of decrease in the methane adsorption capacity with swing temperature increased with the total organic carbon content, indicating that the organic matter is sensitive to temperature.

Organic geochemical and mineralogical characterization of the lower Silurian Longmaxi shale in the southeastern Chongqing area of China: Implications for organic matter accumulation

Abundant in organic matter, the lower Silurian Longmaxi shale in the Upper Yangtze Platform, South China, has been demonstrated to play important roles in the petroleum system as both a good source rock and a shale gas reservoir. In order to better understand the heterogeneous organic matter accumulation of such shales in the southeastern Chongqing area, the vertical variations of total organic carbon (TOC) contents, mineral concentrations and biomarker distributions were systematically analyzed in this study. The Longmaxi Formation consists of fine-grained siliciclastic rocks deposited in a restricted marine shelf environment, and it can be further subdivided into two members. The lower member is enriched in TOC, whereas a diminished TOC content is documented in the upper member. Laterally, the TOC content gradually drops southeast-ward in the study area. The mineralogical compositions, dominated by quartz and clay, and, to a lesser extent, feldspar and carbonate, show good correlations with TOC contents vertically, especially in the lower member. Saturated and aromatic hydrocarbon biomarkers indicate a dominant organic matter origin from lower hydrobionts (algae and bacteria) in a reducing marine environment. Negative correlation relationships between TOC contents and terrigenous clastic minerals indicate the predominant role of dilution of terrigenous clastic materials in controlling the organic matter accumulation. The sea level drop combined with the continuously active Xuefeng Uplift contributed to more terrigenous clastic inputs into the study area, which resulted in a significant reduction of TOC content in the upper Longmaxi member.

Deformation of organic matter and its effect on pores in mud rocks

Pores within organic matter (OM) are strongly linked to hydrocarbon generation and primary migration in fine-grained source rocks and are very important for evaluating hydrocarbon storage and flow in shale reservoirs. Thus, it is critical to clarify the features of OM-hosted pores and their evolution in organic-rich mud rocks. The OM in mud rocks, including kerogen and bitumen, is deformed when the original equilibrium stress conditions are altered. The OM deformation at the nano- or microscale has rarely been discussed because of the lack of unequivocal evidence. This research begins with examining the subsurface evolution of kerogen and bitumen, with emphasis on various features of pores hosted by bitumen. Evidence of OM deformation is documented in scanning electron microscope images of seven overmature samples from the lower Silurian Longmaxi Shale, Sichuan Basin. To aid in the understanding and analysis, OM deformation is classified into three types: type I deformation induced by one additional force, type II deformation induced by two additional forces acting on two locations of single OM particles, and type III deformation induced by three or more deforming forces acting at three or more locations of single OM particles. Thus, type I deformation is generally less complex than type II and III deformation. The OM particles subjected to type I deformation were analyzed quantitatively for parameters such as pore size, geometry, and long-axis orientation of elliptical pores. Deformation of OM enhances the robust heterogeneity of OM-hosted pores.

Nanopores of the productive and non-productive sections within the Lower Silurian Longmaxi Shale Reservoir in the Southern Area of Sichuan Basin, China

This study focuses on nanopores of the productive and non-productive sections within the Lower Silurian Longmaxi shale reservoir in the southern area of Sichuan Basin. 4 rock samples from the productive section and 12 rock samples from the non-productive section were collected from a new exploration well in Changning Area. TOC, XRD and gas adsorption measurements were conducted on these samples. The productive section contains more organic matters, biogenic quartz, but less carbonates and feldspar and is much more porous relative to the non-productive section. Organic matters and clays have important impacts on the development of nanoporosity within the Lower Silurian Longmaxi shale reservoir in study area. Nanoporosity of the productive section is affected mainly by the organic matter contents, and hence implying the predominance of organic-matter pores within this section. Clay minerals have the primary controlling effects on the occurrence of nanopores within the non-productive section, and then indicating the predominance of mineral matrix pores in it. This study should be helpful for us to make a better understanding of in-situ shale gas resources and to optimize targets in this region.

Ediacaran, Cambrian, Ordovician, Silurian and Permian shales of the Upper Yangtze Platform, South China: Deposition, thermal maturity and shale gas potential

For many years, the Yangtze Platform has been successfully explored and exploited for petroleum. The Lower Silurian Longmaxi Shale proved to contain commercial quantities of gas, but several other formations might also have a high potential for shale-hosted gas exploration. This paper comprises of geochemical, petrographical, petrophysical and mineralogical information on the major Proterozoic and Paleozoic gas shales in the Upper Yangtze area, which provides insights on the depositional environment, the thermal maturity of organic matter, shale gas storage capacity and the fracability of these rocks. The total organic carbon (TOC) content varies from 0.1 to 22.5% with an average value of 2.7% and most of these shales were deposited in an oxygen-depleted marine environment. Equivalent vitrinite reflectance (VRr) values range from 2.20 to 4.25%, indicating that all samples are in the gas generation window and lost a large amount of primary organic carbon. Porosity varies between 1.5 and 13.6% with an average value of 5.8%, while the excess sorption capacity ranges from 0.10 to 0.22 mmol/g rock. Minerology data show that quartz is the predominant mineral except for the Upper Permian Longtan Formation, in which clay minerals account for 29 to 84%. New data were combined with those from other publications and compared to information on other, well-studied shale-hosted gas systems.

Mechanism of fracture damage induced by fracturing fluid flowback in shale gas reservoirs

In this paper, the Lower Silurian Longmaxi shale samples and the backflow fracturing fluid in the Changning Block of the Sichuan Basin were selected to investigate the damage mechanism of retained fracturing fluid to fractures in shale gas reservoirs. Thus, experiments were conducted on fracturing fluid backflow and gas-driving fracturing fluids. The changes of liquid permeability of shale samples, solid particle size distribution and turbidity of the backflow fracturing fluid were monitored. The gas permeability before and after fracturing fluid gas drive was compared, and the damage degree and mechanism of the backflow fracturing fluid to the fractures in shale samples were analyzed. And the following research results were obtained. First, the damage rate of shale permeability after the fracturing fluid backflow is between 53.1% and 97.6%, and the range of the solid particle size of the flowback fluid is significantly reduced. The main reservoir damage modes include phase trapping damage caused by liquid phase retention, blockage caused by the solid phase residue, particle migration induced by gas-carrying liquid and salt precipitation. Second, in the stage of gas phase flow, the damage rate of permeability drops to 23.1–80.2%, and the damage caused by liquid phase retention is relieved, but the damage caused by the blockage of solid phase residue and the salt precipitation of flowback on the facture surface is inevitable. Third, based on the damage mechanism of fracturing fluid backflow in shale gas wells to fractures, considering the treatment difficulty of the flowback and its damage to reservoir fractures, it is recommended to give a full play to the fracturing capacity of fracturing fluid and optimize the properties and dosages of fracturing fluid so as to reduce the flowback of fracturing fluid as much as possible.