Development Of Organic Matter Pores Research Articles

Petrographic Insights into the Evolution of Nano-Scale Organic Matter Pores with Organic Matter Conversion

To investigate the influence of organic matter conversion on the evolution of organic matter pores, fractional conversion (TRHI) and loss of TOC (TOCL) from the organic matter conversion of Middle Jurassic Dameigou Formation shale samples were calculated using petrographic analysis. The TRHI of organic matter varies from 0.30 to 0.88 and TOCL content ranges from 0.62% and 4.09%. Relative to samples of Type III organic matter in shales, type II samples exhibit higher TRHI and TOCL values. Petrographic calculations of TRHI reveal that the fractional conversion of different kerogens differs for the same thermal maturity level. The specific surface area (SBET) ranges between 1.25 and 6.63 m2/g and micropore surface area (Smic) ranges between 4.16 and 21.27 m2/g. Correlations between pore structure parameters and TOCL content are higher than those between pore structure parameters and TOC content. The original TOC content decreases with increasing maturity level owing to hydrocarbon generation from organic matter conversion. The development of organic matter pores depends mainly on organic matter conversion, which is influenced by the richness, organic maceral compositions, and thermal maturity of the organic matter. The contents of kaolinite, illite, and mixed-layer illite/smectite (I/S) in the studied shales are 17.83%–37.05%, 5.36%–11.31%, and 5.27%–14.36%, respectively. Pore structure parameters (SBET and Smic) exhibit moderate positive correlations with illite content and I/S content, and moderate negative correlations with kaolinite content, indicating that different clay minerals have differential effects on pore structure.

Deformation and kerogen macromolecular structure transformations of the Wufeng–Longmaxi shale under the control of the detachment layer

Tectonism caused strong deformation of the organic-rich shale in the detachment layer of the Wufeng–Longmaxi Formation. In this study, the deformation of the samples collected from the detachment layers were observed using an optical microscope and field emission scanning electron microscopy. Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance, X-ray diffraction, and high-resolution transmission electron microscopy were employed to characterize the macromolecular structure of extracted kerogen samples. The research shows that tectonically deformed shale (TDS) is commonly developed in the detachment layer of the Wufeng–Longmaxi Formation, dominated by brittle–ductile and ductile deformation. In the brittle–ductile deformation stage, the aliphatic carbon (fal) decreases, manifested as the decrease of –CH or –CH2 (falH), from 6.77% to 3.12%. The Al/–OX (defined by FTIR A3000–2800/A1800–1520) gradually decreases, indicating that the brittle–ductile deformation has a stronger effect on the aliphatic structure. The aromaticity I and aromatic carbon (fa) show an increasing trend due to aromatization. In the ductile deformation stage, the fal and aliphatic carbon bonded to oxygen increase first and then decrease, falH decreases gradually, and the aliphatic side chain becomes shorter. The Al/–OX shows an increasing trend, and carbonyl decreases from 7.82% to 5.12%, indicating that the effect of ductile deformation on oxygen-containing functional groups is stronger. The fa shows an overall increasing trend, and the aromatic ring carbon and protonated aromatic carbon increase more significantly due to aromatic condensation. The change trend of the interlayer spacing is opposite in these two stages. However, both brittle–ductile and ductile deformation can improve the size and orientation of aromatic fringes to varying degrees, and the latter is more significant. The rigid mineral intrusion and organic matter (OM) chemical structure evolution are the two main reasons for the low development of OM pores in TDS.

Differential Development Mechanisms of Pore Types under the Sequence Stratigraphic Constraints of the Wufeng–Longmaxi Formation Shale from the Upper Yangtze Platform

Various types of pores, including organic and inorganic variations, exhibit distinct impacts on the storage capacity of shale gas reservoirs and play a significant role in shale gas occurrence. However, there is a limited number of studies that have quantitatively addressed the developmental characteristics of these diverse pore types and their primary controlling factors. This paper explores the development of inorganic pores, specifically interparticle pores and intraparticle pores, as well as organic matter (OM) pores within the shales of the Wufeng–Longmaxi Formation in the Upper Yangtze region. Parameters such as areal porosity, pore diameter, and pore number based on the FE-SEM and image digitization are discussed. Additionally, the influence of the sedimentary environment on the development of various pore types through integrated wavelet transform techniques and geochemical analysis are analyzed. This analysis reveals the distinctive mechanisms governing the development of pore types under the sequence stratigraphic constraints. The findings reveal that the Wufeng–Longmaxi Formation within the study area can be classified into four systems tracts (transgressive systems tracts TST1 and TST2, and highstand systems tracts HST1 and HST2). Within TST1+HST1, OM pores emerge as the predominant pore type, contributing to over 65% of the porosity. TST2 similarly displays OM pores as the dominant type, comprising over 45% of the total porosity, with an average OM areal porosity of 7.3%, notably lower than that of TST1+HST1 (12.7%). Differences in OM pore development between TST1+HST1 and TST2 shales are attributed to variations in OM abundance and type. In HST2, inorganic pores are the dominant pore type, primarily consisting of interparticle pores associated with clay minerals, contributing to more than 50% of the porosity, while OM pores remain almost undeveloped. The frequent sea level fluctuations during the sequence stratigraphic evolution caused variations in sedimentary environments across different depositional sequences. These differing depositional environments lead to varying OM content and types, mineral genesis, and content, ultimately resulting in disparities in the development of shale pore types within different sequences.

Key geological factors governing sweet spots in the Wufeng-Longmaxi shales of the Sichuan Basin, China

Production performance of the Wufeng-Longmaxi shales varies significantly among Fuling, Weirong, and Wulong fields in the Sichuan Basin. Total organic carbon (TOC) content, mineralogy, and organic matter (OM) pore characteristics are investigated to identify key factors governing sweet spots. Siliceous shales with good preservation conditions in the Fuling Field exhibit large thickness, high TOC content and thin-section porosity (TSP), and well-developed OM macropores, thus high initial production and estimated ultimate recovery (EUR). Thin carbonate-containing siliceous shales with good preservation conditions in the Weirong Field feature medium-to-high TOC and well-developed OM macropores but low TSP, leading to high initial production but low EUR. Siliceous shales with poor preservation conditions in the Wulong Field are characterized by large thickness, high TOC, low TSP and poorly-developed OM macropores, causing low initial production and EUR.Both sedimentary and preservation conditions are intrinsic decisive factors of sweet spots, as they control the mineral composition, TOC, and OM macropore development. Deep-water shales in transgressive systems tracts (TSTs) exhibit better-developed OM macropores and greater TOC compared to highstand systems tracts (HSTs). OM macropores are most prevalent in siliceous shales, followed by carbonate-containing siliceous shales and then argillaceous shales. Furthermore, good preservation conditions are conducive to retain OM macropores with low pore aspect ratio (PAR). Comparison among the three fields shows that high-TOC silicious shales with good preservation conditions are the highest in TSP and EUR. Therefore, organic richness, lithofacies, and preservation conditions are the major factors which determine OM pore development, governing the sweet spots of the Wufeng-Longmaxi shales.

Pore structure characteristics and impact factors of laminated shale oil reservoir in Chang 73 sub-member of Ordos Basin, China

This research paper focuses on the laminated shale oil reservoir in the third sub-member of the seventh member of Yanchang Formation (Chang 73 sub-member) in the Ordos Basin. The study aims to comprehensively analyze the lithofacies type, micro qualitative and quantitative pore structure parameters, and the main controlling factors of the pore structure in laminated shale. The analysis involves various techniques, including a comprehensive analysis of TOC, rock-eval, X-ray diffraction, polarized light and fluorescence microscope observation, field emission scanning electron microscope observation, and low-pressure N2 adsorption analysis. Based on the sedimentation characteristics, geochemistry, and mineral composition differences, the Chang 73 laminated shale can be classified into three lithofacies types: tuffaceous-organic matter binary laminated shale, clayey-organic matter binary laminated shale, and felsic-clayey binary laminated shale. The pore network consists primarily of organic hydrocarbon generation pressurization fractures, clay mineral felsic intergranular composite pores, and felsic plasmid intergranular pore fracture systems. Mesopores are the most developed pore type. The pore volume and specific surface area increase in the order of “tuffaceous-organic matter”, “clayey-organic matter”, and “felsic-clay”, while the heterogeneity of the pore network gradually weakens, and the roughness of the pore surface enhances. The overall development of organic matter pores is limited, with organic matter-pyrite-clay mineral composite pores being the main components of micropores. The primary intergranular pore system, associated with rigid quartz particles, dominates the mesopores and macropores, and acts as the main framework of the entire pore network. The development of feldspar dissolution pores is limited and contributes minimally to the pore network.

Effects of Quartz Precipitation on the Abundance and Preservation of Organic Matter Pores in Cambrian Marine Shale in South China

To evaluate the effects of quartz precipitation on the abundance and preservation of organic matter pores in marine shale reservoirs, the type of authigenic quartz and the source of silica, as well as the corresponding relation of the Lower Cambrian Shuijingtuo Formation shale in South China were investigated. Quartz in the Shuijingtuo shale occurs as four different types: detrital quartz, replacement of biosiliceous debris, euhedral quartz filled in interparticle pores, and microquartz dispersed in a clay matrix. Euhedral quartz (1–5 μm) and matrix-dispersed microquartz (100–400 nm) are found to be the dominant forms of authigenic quartz. The euhedral quartz accumulates along the interparticle pores, and the porous organic matter fills the interior of the space. Microquartz is mainly wrapped in porous organic matter. Two silica sources were revealed: biogenic silica and clay-derived silica. Biogenic Si is most likely the major source for authigenic quartz in the organic-rich (total organic carbon (TOC) > 2.55 wt.%) samples, which accounts for 23–57 wt.% (average 35 wt.%) of the total Si. Based on petrographic observations, we posit that the precipitation of large-sized euhedral quartz in the interparticle pores most likely originated from biogenic silica in the early stage of diagenesis and that the silica for the clay matrix-dispersed microquartz is provided by biogenic silica and clay-derived silica. The observation of SEM images indicates that the precipitation of early diagenetic euhedral quartz in the interparticle pores enhances rock stiffness, and the buttressing effect can protect the organic matter pores from compaction during the late-stage burial diagenesis. In contrast, the precipitation of late diagenetic microquartz in the clay matrix can lead to a reduction in the capacity of the accommodation space to host retained petroleum, consequently leading to a reduction in the development of organic matter pores and the generation of shale gas.

Diagenesis and Pore Formation Evolution of Continental Shale in the Da’anzhai Lower Jurassic Section in the Sichuan Basin

As an unconventional oil and gas reservoir, the diagenesis and evolution of continental shale controls the formation and occurrence of inorganic and organic pores. In order to quantitatively characterize the pore characteristics of a continental shale reservoir and their influence on the evolution of the diagenesis stage, the characteristics of organic and inorganic pore types of continental shale in the Da’anzhai section of the lower Jurassic Ziliujing Formation were identified by means of X-ray diffraction mineral composition analysis and argon ion polishing scanning electron microscope measurements and observations, and the influence control of the diagenesis stage on the pore development of the continental shale reservoir and its control were clarified. The results show the following: ① The organic matter pores in continental shale are developed in large quantities, including organic matter pores in the mineral asphalt matrix and organic matter pores in the kerogen; the pore types of inorganic minerals are very rich, the main pore types are linear pores between clay minerals, intergranular (intergranular) pores, and intragranular corrosion pores, and microcracks are also developed. ② When affected by compaction diagenesis, the inorganic pores of continental shale decrease with an increase in the burial depth and diagenesis degree. ③ The burial depth of continental shale is 2000–3000 m in the middle of diagenetic stage A, and a large number of organic matter pores and dissolved inorganic pores develop at this depth, meaning that the total porosity of shale increases significantly. The burial depth of continental shale is 3000–4000 m at diagenetic stage B, where kaolinite and other clay minerals are dehydrated and converted into illite, the brittleness of shale is increased, and the interior of the shale is subject to external stress, causing microcracks to form. In the late diagenetic stage, when the buried depth of the continental shale is more than 4000 m, the organic matter is subject to secondary cracking and hydrocarbon generation, the organic pores of shale increase in number again, and the inorganic pores decrease in number due to compaction. In conclusion, we found that the burial depth is the main control factor for the development of pores and microfractures in continental shale reservoirs; diagenesis caused by burial depth is the main factor affecting the development of pores and microfractures in continental shale reservoirs; and the shale burial depth in this case is more than 3500 m, which is in the middle of diagenetic stage B. Inorganic porosity in shale is reduced, and the number of microfractures is increased. When the shale is buried more than 4000 m deep in the late diagenetic stage, the thermal evolution of organic matter in shale is high, and methane gas is generated in large quantities, which is conducive to the formation and development of organic matter pores in continental shale.

Identifying Organic Matter (OM) Types and Characterizing OM Pores in the Wufeng-Longmaxi Shales.

Organic matter (OM) pores are considered to be an important pore type in the Ordovician Wufeng-Silurian Longmaxi Formation shales in the Sichuan Basin, China, because they have a high capacity to store natural gas. However, to the best of our knowledge, research on the characterization and quantitation of different OM pore characteristics is insufficient. In this study, detailed optical microscope and scanning electron microscope (SEM) observations and the pores/particles and cracks analysis system (PCAS) were applied to identify the OM pores and obtain quantitative information on pores such as pore size, surface porosity, form factor, and probability entropy. Moreover, CO2 and N2 adsorption experiments were performed to study the properties of pores for samples with different TOC and mineral compositions. The results show the following. (1) Pyrobitumen and kerogen can be distinguished under an optical microscope and SEM; the former can be further divided into pyrobitumen without a fixed shape and pyrobitumen with a certain shape, and the latter contains algal fragments, bacteria-like aggregates, graptolite, and micrinite. The overwhelming number of SEM-visible OM pores are mainly observed in pyrobitumen without a fixed shape, whereas pores in other OM types are complex. A PCAS analysis showed that meso-macropores are developed in pyrobitumen without a fixed shape, whereas pores in algal fragments and bacterial-like aggregates are mainly mesopores. (2) Quartz-rich brittle shale will provide more visible SEM pores compared to clay-rich ductile shale, and carbonates are unfavorable for pore development because they can block the pore as cements. Moreover, the rigid mineral framework, including that constructed by quartz recrystallization and pyrite cementation, and the pore-fluid pressure are favorable for the development of OM pores. (3) Adsorption experiments showed that pyrobitumen makes a great contribution to pore development, including micropores and meso-/macropores. Finally, we propose that the pore parameters (e.g., pore diameter, pore form factor, and deformation) of pyrobitumen without a fixed shape may characterize the enrichment condition of shale gas.

Pore Structure of Organic Matter in the Lacustrine Shale from High to Over Mature Stages: An Approach of Artificial Thermal Simulation

The pore structure of organic matter (OM) is affected by the maturity and type of OM, and these exert an important control on the development of OM pores. However, it is often challenging to distinguish between autochthonous and secondary OM. In this study, different types of OM were distinguished by means of morphology and the form of its occurrence under scanning electron microscopy (SEM) observation. Pyrolyzed samples were used to obtain pore structures of autochthonous and secondary OM from a lacustrine shale from high to over mature stages. The SEM observations indicated that the secondary OM is porous, whereas the autochthonous OM is nonporous. Further, nitrogen adsorption data showed that nanopores in the autochthonous OM are blocked in non-extracted samples and reoccurred after extraction, whereas nanopores in the secondary OM remain open from high to over mature stages. Moreover, the pore structure, i.e., pore type and pore size, differed for autochthonous and secondary OM in the lacustrine shale. For instance, nanopores in the autochthonous OM were micropores (smaller than 1.5 nm); nanopores in the secondary OM were mesopores (ranging from 5 to 20 nm) and micropores. Nanopores in the autochthonous OM were formed at reflectance values (Ro) of 1.50%, but they were slightly destroyed at the over mature stage. Nanopores in the secondary OM were formed at 2.00% Ro and were preserved. Therefore, the development of OM nanopores in the lacustrine shale was synchronously affected by the type of OM and maturity. Furthermore, nanopores in the secondary OM were more developed than those in the autochthonous OM; thus, the OM nanopores at the high mature stage belonged to the autochthonous OM, and those at the over mature stage mainly belonged to the secondary OM. Besides, the nanopores in the autochthonous OM are easily blocked. Therefore, the secondary OM is important to the development of OM nanopores in the lacustrine shale at the over mature stage.

Thermal maturity: The controlling factor of wettability, pore structure, and oil content in the lacustrine Qingshankou shale, Songliao Basin

Shale oil resources are abundant in the Qingshankou Formation of the Songliao Basin, but there are significant production differences between wells. Although there are many factors affecting shale oil production capacity, they are all fundamentally reflected in the differences of pore structure and oil-bearing; however, research on the pore structure and oil-bearing characteristics of naturally mature shale is still rarely reported. Lacustrine shales with low to high maturity developed in the Qingshankou Formation, Songliao Basin; these shales can be used as good natural mature samples to reveal the wettability, pore structure, oil content, and evolution of shale under the control of thermal maturity. In this study, geochemistry, oil content, and mineral composition information were analyzed using total organic carbon, thermal maturity, Rock-Eval pyrolysis, and X-ray diffraction; low-temperature nitrogen adsorption–desorption (N2GA), nuclear magnetic resonance (NMR), high-pressure mercury injection, and field emission scanning electron microscopy (FE-SEM) were used to analyze the shale pore structure; and spontaneous imbibition (SI) combined with NMR and contact angle measurement (CAM) experiments were performed to reveal the shale wettability. The results indicate that with the evolution of maturity from low to high, the cracking degree of organic matter (OM) and the amount of hydrocarbon generation increases, and the scale (number, size) of OM pore development continues to increase. The porosity, pore volume, and Brunauer–Emmet–Teller specific surface area of shale increased continuously due to the increased thermal evolution of OM, and a great many of micropores formed in the shale. Therefore, the average pore size decreased with increased maturity. In addition, shale oil content also improves with the continuous hydrocarbon generation of OM in the process of thermal evolution; The wettability of the shale interior and surfaces gradually changes from water wetting to oil wetting when vitrinite reflectivity (Ro) > ∼1.20% and the degree of oil wetting continues to increase as maturity increases. The crude oil generated by kerogen continues to flow from micropores to macropores, and the pore network continues to develop with increasing oil wetting degree, which is beneficial to crude oil migration and accumulation in the shale reservoir.

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.

Formation mechanism of the sealing capacity of the roof and floor strata of marine organic-rich shale and shale itself, and its influence on the characteristics of shale gas and organic matter pore development

The sealing capacities of shale, roof, and floor strata are essential for the preservation of shale gas. Sealing capacity development and its influence on shale gas composition and organic matter pore development are important but have not been addressed by existing studies. In this paper, the Upper Ordovician Wufeng-the first member of the Lower Silurian Longmaxi Fm. in the Yangtze region is selected as the study object. We dissected the J1 well in the Jiaoshiba block inside the Sichuan Basin and the Y6 well in the Youyang block outside the Sichuan Basin to analyze the gas components and carbon isotopes in recovered shale gas. In this study, two microscope techniques (i.e., FIB-SEM and FIB-HIM), porosity measurement, TOC content analysis, permeability tests, isothermal adsorption measurements, are utilized. The analytical results show that both the roof and floor strata have vertical sealing effects on the organic-rich shale due to their different physical properties compared to the shale. The shale overburden pressure increases with burial depth, and the adsorbed gas increases with increasing TOC content and thickness: all of these can lead to a decrease in permeability and an increase in self-sealing capacity. This paper determined the formation mechanism of the sealing capacity for shale, roof and floor strata. Furthermore, the control patterns of “sealing capacity strength” on the shale gas source and organic matter pore development are clarified. The main findings are as follows: when the shale, roof, and floor strata have a strong sealing capacity, only a small amount of liquid hydrocarbons can be discharged from the shale, but the natural gas formed due to liquid hydrocarbon cracking is abundant. Under such circumstances, shale gas has a mixed origin of liquid hydrocarbon cracked gas and kerogen cracked gas; the carbon isotopes are inverted; abundant pyrobitumen pores with high porosity and good connectivity are developed. When the sealing capacities are weak, the amount of liquid hydrocarbons discharged from the shale are high, and the amount of natural gas is low. The shale gas is mainly derived from kerogen cracking with weak or no carbon isotope inversion, and kerogen pores are small with poor connectivity. These findings will advance the study of shale gas formation mechanisms and provide a scientific basis for selecting shale gas exploration area.

Image-based model for dynamic apparent gas permeability in Organic-rich shales

Current shale gas apparent permeability models often fail to integrate morphological properties of multiple storage spaces and various transport mechanisms. To deal with this, we propose a comprehensive model for estimating the dynamic apparent gas permeability (DAGP) during depressurization based on a representative model. First, dual fractal characteristics of organic matter (OM) particles and inorganic matter (IOM)/OM pores along with morphological information of OM particles from scanning electron microscope (SEM) images are considered to establish the representative model conditioned to a total organic carbon (TOC) content. Next, multiple transport mechanisms and a pore size change due to a poromechanical response and desorption-induced volumetric strain are incorporated in the DAGP calculation. Results reveal that the controlling factor of the DAGP comes from pore size shrinkage owing to a poromechanical response to gas transport mechanisms during depressurization at a high Young’s modulus, while the DAGP is totally controlled by a poromechanical response at a low Young’s modulus. The critical TOC content is determined to be about 5% at which the influence of the degree of OM pore development can be neglected. This work provides a practical method to estimate the DAGP from both geostatistical and petrophysical perspectives, which effectively decreases the uncertainty during productivity prediction in shale gas reservoirs and other unconventional reservoirs.

The influence of depositional and diagenetic processes on rock electrical properties: A case study of the Longmaxi shale in the Sichuan Basin

Primary depositional and diagenetic processes exert very important influences on shale gas reservoirs. Rock electrical properties are an important basis for making reservoir prediction using the electromagnetic method (EM). However, there is still a lack of understanding about the impact of the sedimentary diagenesis process on shale electrical properties, this study focuses on the impact of diagenesis on rock properties. In this study, rock electrical properties are studied based on electromagnetic experiments. We systematically studied the lithofacies of different depositional paleoenvironments and diagenetic processes, and the influence of diagenetic evolution on the rock electrical properties was discussed, by means of X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), low-temperature nitrogen adsorption (LTNA), and trace element (TE) geochemical analysis. Based on the study of mineral composition, grain assemblages and pore systems, we identified four lithofacies in the Longmaxi Formation: siliceous shale, siliceous-argillaceous mixed shale, silty shale and argillaceous shale. Redox proxies (U/Th, V/Cr, and Ni/Co) indicate that the siliceous shale was deposited in a relatively anoxic and reducing environment, indicating a deep-water shelf depositional paleoenvironment. The siliceous-argillaceous shale, silty shale and argillaceous shale were deposited under a relatively dysoxic-oxic environment, indicating a shallow-water shelf depositional paleoenvironment. The order of resistivity values of the lithofacies within the Longmaxi Formation is 36.78 Ω m (siliceous shale), 66.81 Ω m (siliceous-argillaceous mixed shale), 79.54 Ω m (silty shale), and 107.00 Ω m (argillaceous shale), and the resistivity decreases with an increase in porosity. The siliceous shale has the most abundant authigenic quartz, which filled the primary pores forming a rigid framework during the gas window, inhibited compaction, increased the distribution of organic matter (OM), and enhanced the development of OM pores. The high TOC content and high maturity of siliceous shale at the bottom of Longmaxi Formation make the OM pores more developed. Pyrite, conductive fluid and pore network under the main control of OM pores in shale form a conductive circuit when AC voltage is input, which increases the exchange capacity of cations and leads to the phenomenon of low resistivity. The interconnected OM pore network, both depositionally and diagenetically derived, affects the electrical properties of the Longmaxi shale. This study reveals that electrical properties of shale rocks and its variations can be impacted by the depositional environment and diagenetic processes. This work provides resistivity parameters for the electromagnetic exploration of shale gas under complex terrain conditions and provides a theoretical basis for later interpretation.

The current study and development of organic matter pores in organic-rich shale

The current study and development of organic matter pores in organic-rich shale

Characteristics of Pore Structure and Gas Content of the Lower Paleozoic Shale from the Upper Yangtze Plate, South China

This study is predominantly about the differences in shale pore structure and the controlling factors of shale gas content between Lower Silurian and Lower Cambrian from the upper Yangtze plate, which are of great significance to the occurrence mechanism of shale gas. The field emission scanning electron microscopy combined with Particles (Pores) and Cracks Analysis System software, CO2/N2 adsorption and the high-pressure mercury injection porosimetry, and methane adsorption were used to investigate characteristics of overall shale pore structure and organic matter pore, heterogeneity and gas content of the Lower Paleozoic in southern Sichuan Basin and northern Guizhou province from the upper Yangtze plate. Results show that porosity and the development of organic matter pores of the Lower Silurian are better than that of the Lower Cambrian, and there are four main types of pore, including interparticle pore, intraparticle pore, organic matter pore and micro-fracture. The micropores of the Lower Cambrian shale provide major pore volume and specific surface areas. In the Lower Silurian shale, there are mesopores besides micropores. Fractal dimensions representing pore structure complexity and heterogeneity gradually increase with the increase in pore volume and specific surface areas. There is a significant positive linear relationship between total organic carbon content and micropores volume and specific surface areas of the Lower Paleozoic shale, and the correlation of the Lower Silurian is more obvious than that of the Lower Cambrian. The plane porosity of organic matter increases with the increase in total organic carbon when it is less than 5%. The plane porosity of organic matter pores is positively correlated with clay minerals content and negatively correlated with brittle minerals content. The adsorption gas content of Lower Silurian and Lower Cambrian shale are 1.51–3.86 m3/t (average, 2.31 m3/t) and 0.35–2.38 m3/t (average, 1.36 m3/t). Total organic carbon, clay minerals and porosity are the main controlling factors for the differences in shale gas content between Lower Cambrian and Lower Silurian from the upper Yangtze plate. Probability entropy and organic matter plane porosity of the Lower Silurian are higher than those of Lower Cambrian shale, but form factor and roundness is smaller.

Experimental Study on the Development Characteristics and Controlling Factors of Microscopic Organic Matter Pore and Fracture System in Shale

Research on microscopic pore and fracture system of shale is a hot spot in the field of unconventional petroleum geology. Micro- and nano-scale organic matter pores in shale play a vital role in the accumulation of hydrocarbons. Research on the types, evolution rules and controlling factors of organic matter pore-fracture system in shale reservoirs can provide scientific guidance for the prediction of shale “sweet spots”. In this paper, taking the shales from the Napo Formation, Oriente Basin, the Shahejie Formation, Zhanhua Sag, Bohai Bay Basin, and the Longmaxi and Wufeng Formations, Sichuan Basin as an example, the developmental characteristics of organic matter pore-fracture system were systematically studied using thin section, argon ion profiling scanning electron microscopy, X-ray diffraction, N2 adsorption and desorption, geochemistry experiments, and image processing technology. The types of shale organic matter pores were divided into kerogen-hosted pores, organic matter microfractures (intra-organic matter and organic matter edge microfractures), and asphalt pores (or intra-asphalt pores). The circumferences of organic pore were generally within 100 nm, and the areas of most pores were smaller than 1,000 nm2. Face rates of the organic pores were generally less than 1.5%, and the proportion of shale samples with a shape factor of 1 reached more than 50%. In addition, the deviation angles of organic matter pores at (0°, 45°) reached 90%, which showed that most of the organic matter pores tended to be oriented pores. The increase in the degree of thermal evolution provided driving force for the formation of circular pores in the organic matter. Internal factors (abundance of organic matter, kerogen types, and maturity) and external factors (diagenesis and mineral composition) controled the development of shale organic matter pores. Maturity, TOC content and inorganic minerals such as clay and pyrite content were positively correlated with the development of organic matter pores. However, brittle minerals caused a decrease in the face ratio of organic matter pores. Diagenetic compaction caused the organic matter pores being deformed or eventually disappeared. This research can provide scientific guidance for the high-efficiency exploration of hydrocarbons in shale.

Types of biogenic quartz and its coupling storage mechanism in organic-rich shales: A case study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China

Biogenic quartz in the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation (Wufeng– Longmaxi) shale layers in the Sichuan Basin and its periphery is qualitatively analyzed and quantitatively characterized by organic petrologic, mineralogic, and geochemical methods to find out the coupling effect between organic matter and quartz. (1) There are two types of biogenic quartz in the shale layers: Type I quartz is submicron quartz appearing in clusters around the organic matter. Type II quartz is in nano-scale grain size and floats in spherical shape on organic matter, with grains in point-to-point or surface-to-surface contact; this type of quartz is mainly biologic origin and slightly affected by hydrothermal activity in local parts. (2) The reservoirs in the Wufeng–Longmaxi formations is consistent in distribution with biogenic silica content in them, and mainly concentrated at the bottom of the Wufeng–Longmaxi formations, and is thinner in the Changning and Weiyuan regions, while thicker in the Fuling region. (3) The biogenic quartz in the Wufeng–Longmaxi shale worked through the entire evolution process of hydrocarbon generation. The presence of biogenic quartz can enhance the development of organic matter pores and microcracks, and can effectively preserve the organic matter pores and residual intergranular pores, forming “biological silicon intergranular pores, organic pores and micro-fractures”. This would benefit later hydraulic fracturing and result in high production/stable production of well. The coupling effect between biogenic quartz development and organic matter evolution and hydrocarbon generation is a critical factor for high-quality shale reservoir development.

Slope Belts of Paleouplifts Control the Pore Structure of Organic Matter of Marine Shale: A Comparative Study of Lower Cambrian Rocks in the Sichuan Basin

Extensive exploration of the marine shale of the Niutitang Formation in south China has been conducted. However, exploration and development results have varied considerably in different areas. For example, the Niutitang shale in Jingyan City (Southwestern Sichuan Basin) produces a large amount of gas with a long period of stable production. In contrast, most development wells in the Niutitang shale in Chongqing City do not produce gas. Scanning electron microscopy images showed that the organic matter (OM) pore development in the Niutitang shale in Jingyan is abundant, large in size, and are well connected. In contrast, OM pores in the Niutitang shale in Chongqing are rarely observed. OM pore development of the Jingyan and Chongqing shales is mainly controlled by thermal maturity as shown by equivalent vitrine reflectance determinations. The moderate thermal maturity has resulted in the development of a large number of OM pores in the Niutitang shale in Jingyan, whereas the high thermal maturity of the Niutitang shale in Chongqing has led to the destruction of most of the OM pores. Due to the existence of ancient uplift, the shale was buried shallowly in the process of hydrocarbon generation evolution, and the shale avoided excessive thermal evolution and retained appropriate thermal maturity. In the Jingyan area, due to its location near the central uplift in the Sichuan Basin, the Niutitang shale deposited nearby avoided excessive evolution, and a large number of OM pores were retained in the reservoir.

Geochemical, petrographic and reservoir characteristics of the transgressive systems tract of lower Silurian black shale in Jiaoshiba area, southwest China

In recent years, lithofacies identification and classification have been a hot topic in shale gas research. Even if some previous works are considering the distribution characteristics of lithofacies in different system tracts (third-order sequences), only few studies systematically reveal the lithofacies distribution and reservoir characteristics. In this study, we focus on transgressive systems tract (TST) section which is characterized by high TOC content and highest gas content in the Long-1 member and divided the third-order TST into five Units (parasequences) according to graptolite biozonation, gamma ray (GR) and acoustic (AC) logging data. Twenty-six samples were analyzed for total organic carbon (TOC) content, mineral and elemental composition, porosity, pore type, pore size distribution and pore connectivity. The results show that Unit 1 is predominantly composed of S-3 and S-4 lithofacies, and Units 2–5 is predominantly composed of S-4 and M-2 lithofacies. The S-4 lithofacies was associated with high primary productivity of organic matter, and anoxic bottom water conditions. Redox conditions changed from anoxic to dysoxic condition and the productivity decreased through Unit 1 to Unit 5. Compared to other units, Units 1 and 2 are characterized by higher porosity and better pore connectivity. The S-3/S-4 and M-2 lithofacies exhibit different reservoir quality, especially in pore structure. Mineralogical composition is the key factor to determine lithofacies type and affects the characteristics of pore structure. Besides, TOC plays an important role in the development of organic matter pores. Anoxic bottom water conditions and high paleo-salinity contributed to the preservation of organic matter.