Shale Organic Matter Research Articles

Molecular dynamics computations of brine-CO2/CH4-shale contact angles: Implications for CO2 sequestration and enhanced gas recovery

The rock wettabilities and water contact angles describing interactions between CO2, CH4, brine and shale formations are of great significance to CO2 sequestration and enhanced gas recovery processes. However, water contact angles on the surfaces of shale organic matter in the atmospheres of CO2 and CH4 under reservoir conditions are not well-understood. In this study, we present an investigation of water/brine contact angles as functions of temperature, pressure, salinity, ion types, and gas contents by molecular dynamics simulations, and compare results with data from literature. It is found that temperature has profound effects on water contact angles below the critical temperature at an intermediate pressure. Meanwhile, water contact angles increase with pressure before reaching 180° at high pressure and the CO2-water-shale organic matter system turns from a neutrally-wet state to a CO2-wet state at the critical pressure of CO2. We also demonstrate that salinity and ion types have minor impacts on the brine contact angles in the CO2-brine-shale system. Only a slight increase in water contact angles is observed with increasing salinity, and an increase in brine contact angles caused by the divalent cations Mg2+ and Ca2+ is larger than that by the monovalent cations Na+ at the same salinity. Additionally, an increase in the CO2 fraction of gas mixtures can increase water contact angles at the same pressure and temperature. The surfaces of shale organic matter have a stronger affinity for CO2 than CH4, which contributes to a higher CO2 adsorption capacity and improves the displacement efficiency of CH4.

Prediction of organic matter carbonization zones of the Lower Silurian Longmaxi Formation in the Middle-Upper Yangtze region, China

Selecting and evaluating marine shale gas areas in South China to determine the distribution range of organic matter carbonization zones and exclude the high-risk zones of shale gas exploration are significant tasks. Based on a large number of drilling and outcrop data, this study has carried out the identification and prediction of the organic matter carbonization areas of the Lower Silurian Longmaxi Formation in the Middle-Upper Yangtze region. This study compiled the industrial maps of the distribution of the organic matter carbonization areas in the Longmaxi Formation, as well as preliminarily obtained four achievements and understandings. Firstly, the criteria for identification of organic matter carbonization in high-over mature marine shales were put forward, in which the response characteristic of thin neck type from the logging resistivity is exclusively correlated with that of the carbonization of organic matter in shales. The above is the perfect standard utilized as it is reliable to apply this method and criterion to judge organic matter carbonization of high-over mature marine shales. Secondly, the distribution of organic matter carbonization for the Longmaxi Formation in the Middle-Upper Yangtze region has certain regularity, indicating that carbonization points are mainly distributed in four exploration areas (i.e., East Sichuan-West Hubei, Northwest Hubei, the west of South Sichuan, and the east of the Changning structure) covering an area of more than 35,000 km2. Thirdly, the organic matter carbonization of the Longmaxi Formation shows a trend of continuous distribution in Eastern Sichuan-Western Hubei and the west of Southern Sichuan, with characteristics of a wide area as well as weak-severe carbonization. Finally, regions of organic matter carbonization are relatively small, showing the characteristics of serious and weak carbonization in Northwest Hubei and the east of the Changning structure, respectively. Consequently, these four carbonization areas are restricted zones for the shale gas exploration of the Longmaxi Formation, and high-risk belts for the shale gas resource evaluation and strategically selected areas.

Influence of Pore Water on the Gas Storage of Organic-Rich Shale

Previous studies that utilize dry samples have been conducted on the nanoporosity of organic matter in shale. However, underground shales contain pore water that may greatly influence their pore structures and adsorbed gas content. To address this, a typical organic-rich, marine, gas-bearing shale in China was analyzed in this study. Water content and isothermal adsorption under different saturation conditions were determined, and the adsorbed and free gas contents were quantified. Across scales, water occurs within clays, between grains, and in fissures. In micropores and mesopores, gas is mainly adsorbed onto the surface of kerogen, as clay surfaces are water-saturated. In macropores and fractures, gas mainly exists in a free state. Adsorbed and free gas contents decrease with increasing water saturation in shale reservoirs. In high-production reservoirs, a limited amount of adsorbed gas is stored within micropores, while a relatively large amount of free gas is stored in larger pores and natural fractu...

Molecular dynamics of methane flow behavior through realistic organic nanopores under geologic shale condition: Pore size and kerogen types

Up to date, for the purpose of simplicity, graphene-based structures, like nano-porous carbons or carbon nanotubes, have been widely utilized to investigate methane flow behavior inside shale organic nanopores. However, realistic shale organic matrix is composed of kerogen molecules, possessing complex amorphous structures and apparently different surface attributes compared with graphene-based nanopore surface, which will inevitably have a great impact on surface-methane interactions and methane flow behavior. Current research works in terms of the graphene-based material fails to capture the influence of kerogen surface, and therefore cannot accurately characterize nano-confined methane flow behavior through realistic shale organic matter. Also, shale organic nanopores with different kerogen types contain different surface compositions, while its impact on methane flow capacity has not been reported yet. To bridge this knowledge gap, this paper simulates the methane flow behavior through authentic kerogen-based circular nanopores with the use of molecular dynamics (MD) for the first time. And a novel construction method was developed to generate kerogen-based organic nanopores with desirable pore size and different kerogen types for MD simulation. Main results show that a) decrease in pore size will contribute to the enhancement of adsorption capacity for nanopores and type-III kerogen > type-II kerogen > type-I kerogen in terms of methane adsorption capacity; b) ratio of average methane density confined in nanopores to bulk-gas density ranges from 1.2 to 2.6, which will decrease with the increase of the pressure and increase with decreasing pore size; c) Under shale geological condition, the conventional theoretical model for nanoconfined gas flow will underestimate that of 0.41 time for type-I kerogen-based nanopores, 0.59 time for type-II kerogen-based nanopores, and 0.88 time for type-III kerogen-based nanopores.

Assessment of the elastic response of shale using multiscale mechanical testing and homogenisation

Robust geomechanical characterisation of shale reservoirs is necessary for safe and economic resource exploitation but there is still a lack of mechanical data on well-characterised shale, partly due to the difficulties of obtaining high quality core samples for laboratory testing. The composition of shale also presents challenges when attempting to constrain the mechanical response. Multi-scale homogenisation techniques have recently been used to predict the macroscopic behaviour of shales based on quantitative mineralogical descriptions. However, there is a considerable amount of uncertainty associated with some key inputs into these homogenisation schemes. In particular, the organic matter of shale encompasses a range of scales, from nanometre to micrometre-size material, and its mechanical properties are not well understood. Here, PeakForce Quantitative Nanomechanical Mapping (PF-QNM), a recently developed form of atomic force microscopy (AFM), is combined with nanoindentation testing to characterise the mechanical response of the organic matter and clay phases of Posidonia shale from north-west Germany. The nanoscale testing revealed a clear peak in the histograms of the reduced elastic modulus, which can be attributed to kerogen in the shale matrix. Upscaling of the mechanical properties through homogenisation showed a reasonable prediction when compared with experimental data, including capturing the inherent anisotropy of the shale response. The influence of factors such as the volume fraction of silt inclusions and the applicability of different homogenisation formulations warrant further investigation.

Kinetic simulation of hydrocarbon generation and its application to in-situ conversion of shale oil

Abstract The kinetic parameters of hydrocarbon generation are determined through experimental simulation and mathematical calculation using four typical samples selected from the Cretaceous Nenjiang Formation in the northwest of Songliao Basin, Chang 7 Member of Triassic Yanchang Formation in the southwest of Ordos Basin, Paleogene in the southwest of Qaidam Basin, and Lucaogou Formation of Jimusar Sag in the east of Junggar Basin. The results show that activation energy of hydrocarbon generation of organic matter is closely related to maturity and mainly ranges between 197 kJ/mol and 227 kJ/mol. On this basis, the temperature required for organic matter in shale to convert into oil was calculated. The ideal heating temperature is between 270 °C and 300 °C, and the conversation rate can reach 90% after 50—300 days of heating at constant temperature. When the temperature rises at a constant rate, the temperature corresponding to the major hydrocarbon generation period ranges from 225 to 350 °C at the temperature rise rate of 1—150 °C/month. In order to obtain higher economic benefits, it is suggested to adopt higher temperature rise rate (60—150 °C/month). The more reliable kinetic parameters obtained can provide a basis for designing more reasonable scheme of in-situ heating conversion.

Pore characteristics of the lower Sinian Doushantuo Shale in the Mid-Yangtze Yichang area of China: Insights into a distinct shale gas reservoir in the Neoproterozoic formation

The Lower Sinian Doushantuo Shale comprises the oldest Neoproterozoic shale gas reservoir in the Middle Yangtze Yichang area of China, which makes the study of their pore characteristics critical to future shale gas exploration and exploitation. Geochemical, petrographical, and pore characteristics are studied in detail, with focused ion beam scanning electron microscopy (FIB-SEM) image analysis to further support and confirm the interpretations of the nano-scale pore structure characteristics. High total organic carbon (TOC) content, high thermal maturity, and considerable brittle mineral content indicate promising potential of the Doushantuo shale. High TOC samples suggest elevated micro-meso porosity, and H2(b) and H3 type hysteresis loops indicate inkbottle, slit-like, and flake-like pore types. Significant inorganic pores are commonly observed in dolomite, pyrite, and apatite, which are characterized by diamond or irregular shapes, with pore diameters ranging from 5 nm to 1000 nm as a result of the interaction between the minerals and the acidic organic fluids. Clustering organic matter (OM) pores are observed with pore diameters ranging from 5 nm to 50 nm; they are characterized by sponge-like, elliptical, and alveolate shapes, which indicate minor organic matter (OM) pores. More substantial organic matter (OM) pores can also be found around or between the pyrite and apatite. Fractures can be clearly observed between the organic matter and minerals with lengths of approximately 10 μm and 50 nm in width, which serve as conduits for shale gas migration. The pores in the Doushantuo Shale are dominated by inorganic pores and minor organic matter (OM) pores, which are different from the pore structure of the Wufeng-Longmaxi Shales in the Sichuan Basin. The Doushantuo Shale organic matter (OM) pores are much smaller and more densely distributed than those in the Wufeng-Longmaxi Shales. The different organic matter origins of these two shales have caused distinct organic matter (OM) pore-development mechanisms. Organic matter (OM) pores and inorganic pores could contribute to the volume of the adsorbed gas and free gas, respectively, which provide significant reservoir space in the Lower Sinian Doushantuo Shale in the Yichang area.

Molecular Investigation of CO2/CH4 Competitive Adsorption and Confinement in Realistic Shale Kerogen.

The adsorption behavior and the mechanism of a CO2/CH4 mixture in shale organic matter play significant roles to predict the carbon dioxide sequestration with enhanced gas recovery (CS-EGR) in shale reservoirs. In the present work, the adsorption performance and the mechanism of a CO2/CH4 binary mixture in realistic shale kerogen were explored by employing grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Specifically, the effects of shale organic type and maturity, temperature, pressure, and moisture content on pure CH4 and the competitive adsorption performance of a CO2/CH4 mixture were investigated. It was found that pressure and temperature have a significant influence on both the adsorption capacity and the selectivity of CO2/CH4. The simulated results also show that the adsorption capacities of CO2/CH4 increase with the maturity level of kerogen. Type II-D kerogen exhibits an obvious superiority in the adsorption capacity of CH4 and CO2 compared with other type II kerogen. In addition, the adsorption capacities of CO2 and CH4 are significantly suppressed in moist kerogen due to the strong adsorption strength of H2O molecules on the kerogen surface. Furthermore, to characterize realistic kerogen pore structure, a slit-like kerogen nanopore was constructed. It was observed that the kerogen nanopore plays an important role in determining the potential of CO2 subsurface sequestration in shale reservoirs. With the increase in nanopore size, a transition of the dominated gas adsorption mechanism from micropore filling to monolayer adsorption on the surface due to confinement effects was found. The results obtained in this study could be helpful to estimate original gas-in-place and evaluate carbon dioxide sequestration capacity in a shale matrix.

Stability of Organic Carbon Components in Shale: Implications for Carbon Cycle

Stability and mobility of organic matter in shale is significant from the perspective of carbon cycle. Shale can only be an effective sink provided that the organic carbon present is stable and immobile from the host sites and, not released easily during geological processes such as low pressure-temperature burial diagenesis and higher pressure-temperature subduction. To examine this, three Jurassic shale samples of known mineralogy and total organic carbon content, with dominantly continental source of organic matter, belonging to the Haynesville-Bossier Formation were combusted by incremental heating from temperature of 200 to 1400°C. The samples were analyzed for their carbon and nitrogen release profiles, bulk δ13C composition and C/N atomic ratio, based on which, at least four organic carbon components are identified associated with different minerals such as clay, carbonate, and silicate. They have different stability depending on their host sites and occurrences relative to the mineral phases and consequently, released at different temperature during combustion. The components identified are denoted as, C-1 (organic carbon occurring as free accumulates at the edge or mouth of pore spaces), C-2 (associated with clay minerals, adsorbed or as organomineral nanocomposites; with carbonate minerals, biomineralized and/or occluded), C-3(a) (occurring with silicate minerals, biomineralized and/or occluded) and C-3(b) (graphitized carbon). They show an increasing stability and decreasing mobility from C-1 to C-3(b). Based on the stability of the different OC components, shale is clearly an efficient sink for the long term C cycle as, except for C-1 which forms a very small fraction of the total and is released at temperature of ∼200°C, OC can be efficiently locked in shale surviving conditions of burial diagenesis and, subduction at fore arc regions in absence of infiltrating fluids. Under low fluid flux, C-3(b) can be efficiently retained as a refractory phase in the mantle when subducted. It is evident that the association and interaction of the organic matter with the different minerals play an important role in its retention in the shale.

Microscopic mechanism of methane adsorption in shale: Experimental data analysis and interaction potential simulation

The adsorbed methane in shale determines the geological reserves of shale gas and production of the well. Therefore, it is of great importance to clarify the microscopic mechanism of methane adsorption in shale for evaluating the geological condition and formulating exploitation plan. In this study, the experimental data and the simulation result were combined to research the microscopic mechanism of methane adsorption in shale. The experimental data for 99 groups of low-temperature nitrogen adsorption and methane isothermal adsorption were collected, to calculate the number of adsorption layers of methane in shale at the micro-level. The surface, microfracture and micropore structures of the organic matter in shale were constructed by graphite structure with the hydroxyl group. Thereafter, they were used to simulate the interaction potential of the organic matter on methane molecules and the adsorption layer's number by using the theory of intermolecular interaction potential field. Furthermore, the effects of different fracture widths and pore diameters on methane adsorption were analyzed and verified. The results show that the adsorption capacity of methane in shale is related to shale's structure, and the order of the adsorption capacity is surface < microfracture < micropore. Meanwhile, the multilayer adsorption can occur in microfractures with a width of 1.44–1.59 nm and micropores with a diameter of 1.49–1.83 nm, whereas the surface structure just can be the monolayer adsorption. These quantitatively results establish a foundation for the development of adsorption theory for methane in the shale.

Mechanism Analysis of Organic Matter Enrichment of Upper Ordovician-Lower Silurian Shale in the Upper Yangtze Area: Taking Jiaoye-1 Well in the Jiaoshiba Block as an Example

Organic matter is the material basis of shale hydrocarbon generation. The current organic matter content in shale is controlled by the original sedimentary organic matter abundance. Therefore, the study of the enrichment mechanism of sedimentary organic matter in shale has become an important issue to be solved. The Upper Yangtze area is the important exploration and exploitation area of marine shale gas in China. The shale of the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in the Yangtze area is the research object. Choosing redox indicator and biological productivity indicator, the study explores the enrichment mechanism of sedimentary organic matter from two aspects, sealing of water and volcanic activity. The results show that excess siliceous mineral in the shale of the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area is bioorigin. Excess siliceous mineral can be used as one of the indicators of biological productivity. On the one hand, layer phenomenon occurred since the strong water sealing during the sedimentary period of Wufeng and the lower section of the Longmaxi Formation, which results in the high content of oxygen in surface water. On the other hand, the active volcanic activity brought volcanic ash which was beneficial to biological reproduction. Both of these factors led to higher biological productivity during this period. At the same time, the strong sealing of water made the lower layer of the water more reductive, and the active volcanic activity caused climate change, enhancing the reduction of the lower layer of the water, which made the rich organic matter deposited from the surface water well preserved. In the sedimentary period of the upper section of the Longmaxi Formation 1st member in the Upper Yangtze area, on the one hand, due to the weakened sealing of water, the oxygen content of the upper water decreased. On the other hand, the volcanic activity weakened until it stopped, and the source of volcanic ash rich in nutrient elements decreased. These two aspects led to lower biological productivity during this period. At the same time, the weaker water sealing could lead to a decrease in the reduction of the lower layer of the water, and the gradual cessation of volcanic activity no longer affected the climate, causing the destruction of sedimentary organic matter by oxidation.

Evolution of organic-mineral interactions and implications for organic carbon occurrence and transformation in shale

AbstractOrganic-mineral interactions are pervasive in sedimentary environments; however, the extent of these interactions is not constant and has a significant impact on organic carbon (OC) occurrence and transformation. To understand the evolution of organic-mineral interactions and the implications for OC occurrence and transformation in fine-grained sediments, several shale samples were selected and subjected to physical and chemical sequential treatments. The samples were subjected to pyrolysis, Fourier transform infrared spectrophotometry (FTIR), and adsorption measurements to determine the organic parameters and the mineral surface area (MSA) of the shale samples. The results show that the organic fractions derived from sequential treatments have varying pyrolysis and FTIR characteristics. The correlation between the total OC content and MSA is positive, but it is split according to organic fractions with different attributes. Correlations between the different organic fractions and MSA indicate that the organic matter in shale is mainly adsorbed on mineral surfaces, while a certain portion of organic matter occurs in the pores and is adsorbed on the organic-mineral aggregates, suggesting variable interactions between the organic fractions with different attributes and minerals. From the pyrolysis and FTIR analysis, the organic fractions of different occurrence sites vary in their OC proportion, proclivity to form organic functional groups, and hydrocarbon generation potential. With increasing burial depth, the MSA and hydrogen index as well as OC loading per unit MSA are reduced, and the OC proportions of organic fractions with different attributes have regular trends. These observations indicate that the extent of organic-mineral interactions that can stabilize organic matter gradually decreases, resulting in transformation of the tightly mineral-combined OC into free OC. Our work reveals the heterogeneity in organic matter occurrence and the effect of the evolution of the organic-mineral interactions on OC occurrence and transformation, which is significant in the global carbon cycle and in petroleum systems.

Experimental study on tensile strength and acoustic emission characteristics of shale exposure to supercritical CO2

ABSTRACT The effect of supercritical CO2 on the mechanical properties of shale is a long-term process. To study the influence of supercritical CO2 on the tensile strength and acoustic emission characteristics of shale, the Brazilian splitting experiment equipped with a real-time monitoring of acoustic emission were conducted on the shale collected from Longmaxi formation in Sichuan Basin before and after supercritical CO2 treatment at different exposure times (0 day, 15 days, and 30 days) and a constant pressure (8 MPa) and temperature (40°C). The results show that the tensile strength of shale decreases and the failure deformation increases after supercritical CO2 treatment. The peak acoustic emission energy generated by the treated shale specimens in the process of Brazilian splitting experiment was significantly lower than that of the untreated, indicating that supercritical CO2 deteriorated the failure strength of the shale. The percentage of C element content has no obvious change, and the percentage of H element content decreased after supercritical CO2 treatment, which indicates that the supercritical CO2 fluid has negligible influence on organic matter in shale, while the water content of shale reduced. After supercritical CO2 treatment, the pore structure of shale measured by Poremaster-33 shows an increasing trend, the most probable pore size is shifted from 100–150 μm to 50–100 μm, and many new micro-pores (<20 μm) are generated in the shale. In addition, the formed CO2-water weak acid system in the process of supercritical CO2 treatment dissolves the carbonate and silicate in the shale measured by SEM/EDS. The main reasons for the decrease of tensile strength of shale after supercritical CO2 treatment can be attributed to the increased pore structure/volume and the dissolution of carbonate and silicate in the shale. The research results can provide reference for supercritical CO2 fluid exploitation of shale gas.

Methane Diffusion in a Flexible Kerogen Matrix.

It has been recognized that the microporosity of shale organic matter, especially that of kerogen, strongly affects the hydrocarbon recovery process from unconventional reservoirs. So far, the numerical studies on hydrocarbon transport through the microporous phase of kerogen have neglected the effect of poromechanics, that is, the adsorption-induced deformations, by considering kerogen as a frozen, nondeformable, matrix. Here, we use molecular dynamics simulations to investigate methane diffusion in an immature (i.e., with high H/C ratio) kerogen matrix, while explicitly accounting for adsorption-induced swelling and internal matricial motions, covering both phonons and nonperiodic internal deformations. However, in the usual frozen matrix approximation, diffusivity decreases with increasing fluid loading, as evidenced by a loss of free volume, accounting for adsorption-induced swelling that gives rise to an increase in free volume and, hence, in diffusivity. The obtained trend is further rationalized using a Fujita-Kishimoto free volume theory initially developed in the context of diffusion in swelling polymers. We also quantify the enhancing effect of the matrix internal motions (i.e., at fixed volume) and show that it roughly gives an order of magnitude increase in diffusivity with respect to a frozen matrix, thanks to fluctuations in the pore connectivity. We eventually discuss the possible implications of this work to explain the productivity slowdown of hydrocarbon recovery from shale immature reservoirs.

Enhanced Shale Gas Recovery: Gas Sorption Controls on Recoverable Gas and CO2 Storage Capacity

The natural gas industry has seen a revolution in recent times due to the boom in shale gas extraction. The process can be made more efficient by using CO2 injection as part of enhanced recovery. This would serve two key advantages: (1) Preferential shale adsorption of CO2 leading to CO2 storage and (2) Displacement of CH4 leading to higher recovery of natural gas. Executing this process on a field-scale level would need an understanding of the controls on gas adsorption in shale and the overall Gas-in-Place (GIP). In this study, CO2 and CH4 adsorption have been gravimetrically measured at high pressure on a sample of the Bowland shale at 80°C using a Rubotherm Magnetic Suspension Balance. To quantify the compositional effect on adsorption, mesoporous carbon, a synthetic material, has also been used in similar experiments, as an analogue of the organic matter in shale. Both sets of adsorption isotherms have been used to calculate the GIP at reservoir conditions. Good agreement between the two independent sets of data suggests that shales are largely mesoporous and that the organic matter is the main contributor to gas adsorption in shale.

Utilization of zeolite as a potential multi-functional proppant for CO2 enhanced shale gas recovery and CO2 sequestration: A molecular simulation study on the competitive adsorption of CH4 and CO2 in zeolite and organic matter

It is well known that CO2 is one of the most effective enhanced hydrocarbon recovery agents due to its thermodynamic characteristics, and extensive research and pilot studies have been conducted in recent years on how to utilize CO2 for enhanced gas recovery in shales. The common delivery method involves injecting CO2 in its liquid or supercritial form into a shale formation. In this paper, we propose a novel approach to shale gas recovery that uses zeolite as a multi-functional proppant and carrier of adsorbed CO2 to enhance shale gas recovery as well as CO2 sequestration and storage. This process involves complex thermodynamic and transport processes, among which the competitive adsorption behaviors of CO2 and CH4 into organic matter and zeolite is the most critical to the success of the proposed approach.In this paper, we carry out a systematic molecular simulation study to investigate the adsorption behaviors of methane and CO2 into organic matter (kerogen) and silica zeolite (silicalite-1). We use grand canonical Monte Carlo simulations to measure single-component adsorption isotherms and calculate the isosteric heat of adsorption at surface temperature and at elevated temperatures of up to 425 K. Moreover, we simulate the competitive adsorption of binary mixtures of CH4 and CO2 with various compositions and investigate the competition between the two gas components in kerogen and silicalite-1. Both silicalite and kerogen show a stronger affinity for CO2 than for CH4. While the adsorption capacity of kerogen is about two times that of silicalite, the isosteric heat of adsorption demonstrates that the kerogen/CO2 interaction is the strongest among all four single-component adsorption systems. These findings demonstrate the great potential of using zeolite as a proppant and CO2 carrier to displace CH4 in shale organic matter under subsurface conditions. This observation is also validated via a competitive adsorption study, in which kerogen preferentially adsorbs CO2 over CH4 under all conditions and silicalite exhibits weaker CO2/CH4 selectivity, especially when the CO2 fraction is very low in the bulk phase.These results suggest the potential applicability of using zeolite as a proppant and CO2 carrier to enhance shale gas recovery. In reservoir conditions, the CO2 desorbed from zeolite can be favorably adsorbed by kerogen due to the increase in temperature and decrease in pressure; in the meantime, it can displace the adsorbed CH4 to enhance gas production.

Porosity and storage capacity of Middle Devonian shale: A function of thermal maturity, total organic carbon, and clay content

Porosity and pore size distribution (PSD) are critical reservoir parameters. Pore surface area, pore volume, PSD, and porosity were measured using subcritical nitrogen (N2) adsorption, and helium porosimetry. A suite of 17 samples were collected from 4 wells in Pennsylvania and West Virginia to analyze the evolution of porosity with increasing thermal maturity in Middle Devonian shales of the Appalachian Basin. The thermal maturity of the tested samples covers a wide range in the hydrocarbon generation sequence from wet gas/condensate zone (vitrinite reflectance (Ro) = 1.16%) to post-mature zone (Ro = 2.79%). Shale samples from the Marcellus Shale and Mahantango Formation used in this study have total organic carbon contents from 0.41 to 7.88 wt%. Results indicate that total organic carbon (TOC) has the strongest effect on porosity and pore structure. The presence of organic matter in shale strongly enhances the storage capacity by increasing the specific surface area and pore volume, which represents sorption storage capacity and free-gas storage capacity. Differences in porosity and pore structure have a complex relationship to thermal maturity, micro texture, mineralogy, clay content, and TOC.

Geochemical characteristics and resource potential analysis of Chang 7 organic-rich black shale in the Ordos Basin

AbstractThe Ordos Basin is the largest and most important intracontinental sedimentary depression in China, and a significant amount of crude oil resources has developed within this Mesozoic formation. High-grade organic-rich shale is prevalent in the large-scale areas of the Chang 7 sedimentary stage and provides essential hydrocarbon resources for abundant oil enrichment in the Mesozoic. This research investigated the geochemical characteristics of Chang 7 shale using core samples and well logs and via laboratory tests. In addition, the microscopic components of the shale organic matter (OM), biological marker compounds, carbon isotopes, enrichment grade of trace elements, and elemental ratio were analysed systematically. Moreover, the aspects related to the shale OM source, sedimentary environment and resource potential were evaluated. Our results revealed that spherical alginate and calcium spherical alginate were predominant in the micropetrological components of the shale. Many biomarkers, including n-alkanes, steranes and terpanes, were detected in the gas chromatography – mass spectrometry spectra. An analysis of n-alkanes, regular sterane shapes (C21−/C22− and C26+C27/C28+C29), odd–even predominance index (OEP) and carbon preference index (CPI) values and carbon isotope distributions showed that OM was produced from aquatic organisms. The indicators of trace elements, such as Sr/Ba and V/V+Ni, combined with the biomarker compound in Pr/Ph and the gammacerane index showed the presence of a semi-deep – deep lake environment containing fresh–brackish water. In addition, the hydrocarbon conversion rate index and shale rock pyrolysis parameters revealed that Chang 7 has a high hydrocarbon generation ability and hydrocarbon expulsion efficiency.

Effect of Hydrothermal Activity on Organic Matter Enrichment of Shale: A Case Study of the Upper Ordovician and the Lower Silurian in the Lower Yangtze, South China

The effect of organic matter on hydrocarbon potential, storage space, and gas content of shale is well-known. Additionally, present-day content of sedimentary organic matter in shale is controlled by depositional and preservation processes. Therefore, a study of the enrichment mechanisms of sedimentary organic matter provides a scientific basis for the determination of favorable areas of shale gas. In this study the Upper Ordovician Xinkailing Fm. and the first member of the Lower Silurian Lishuwo Fm. were examined. Stratigraphic sequences were identified through conventional logs and elemental capture spectrum data. Oxygen isotope analysis was applied to recover paleotemperature of seawater in the study area. The excess silicon content was calculated and the origin of the silica was determined by the Fe-Al-Mn ternary plot. The enrichment mechanism of organic matter was analyzed by two aspects: redox conditions and paleoproductivity. As a result, the stratigraphic interval was divided into two 3rd-order sequences. Through oxygen isotope, the paleotemperature of seawater was 62.7–79.2 °C, providing evidence of the development of hydrothermal activity. Analysis of excess siliceous minerals identified two siliceous mineral origins: terrigenous and hydrothermal. It also revealed an upwards decreasing tendency in hydrothermal activity intensity. Strong hydrothermal activity during the Late Ordovician, recognized as TST1, formed a weak-oxidizing to poor-oxygen environment with high paleoproductivity, which promoted organic matter enrichment. During the Late Ordovician to the Early Silurian, identified as RST1, TST2, and RST2, weakening hydrothermal activity caused the decline of paleoproductivity and increased oxidation of bottom waters, leading to a relative decrease of organic matter content in the shale. Therefore, favorable areas of shale gas accumulation in the Upper Ordovician and Lower Silurian are determined stratigraphically as the TST1, with a high total organic carbonate content. Geographically, the hydrothermally-active area near the plate connection of the Yangtze and the Cathaysian is most favorable.

Effect of the hydrothermal activity in the Lower Yangtze region on marine shale gas enrichment: A case study of Lower Cambrian and Upper Ordovician-Lower Silurian shales in Jiangye-1 well

Abstract Finding favorable sites for the exploration of shale gas, is still one of the important areas of research that needs immediate attention. The content of organic matter in shale plays a crucial role in the hydrocarbon generation potential, reservoir space and gas-bearing capacity of shales. Therefore, studying the sedimentary environment of organic shale can provide a scientific basis for locating favorable exploration areas for shale gas. The article takes the Lower Cambrian and the Upper Ordovician-Lower Silurian shales in the Yangtze region as the research object and selects representative wells to quantitatively calculate the existence of excess silicon in shale siliceous minerals and the content of excess silicon. Then, the origin of excess silicon can be clarified by the Al, Fe and Mn elemental analysis. Finally, the sedimentary organic matter enrichment mechanism is analyzed from water oxidation-reduction environments and biological productivity. The results of the study show that the excess silicon in the Lower Cambrian and Upper Ordovician-Lower Silurian shales in the Lower Yangtze region is of hydrothermal origin. The hydrothermal activity improves biological fertility on the one hand; whereas on the other hand, it can enhance the reducing capacity of the bottom water conducive for the preservation of organic matter thereby enriching the sedimentary organic matter. The place near the junction of Yangtze plate and Cathaysian plate, where hydrothermal activities were more intense, provided favorable loci for shale gas exploration in the Lower Yangtze region. It was observed that, since the hydrothermal activity was stronger in the Early Cambrian than in the Late Ordovician-Early Silurian times, the total organic carbon (TOC) content of the Lower Cambrian shale was higher than that of the Upper Ordovician-Lower Silurian shales.