Organic-rich Shale Formations Research Articles

Research on the Pass Capacity Model of Logging Tools for Complex Horizontal Wells

Shale gas, which exists in organic-rich shale or mud shale formations primarily in adsorbed or free states, poses significant development challenges due to the low permeability of shale formations. Traditional vertical wells are ineffective for shale gas extraction, making horizontal wells and large-scale hydraulic fracturing essential for efficient development. However, the complexity of the formation can lead to casing deformation, severely impacting the pass capacity of plug and perforation gun strings in the wellbore. This study analyzes the causes of casing deformation and discusses the rapid development of the cluster perforating technology in perforation operations, and discusses the challenges that might encountered during the process of wireline conveying or pumping. The irregular trajectory of horizontal wells and the high dogleg deflecting severity in the deviated sections can cause cluster perforating strings to suddenly get stuck during the downhole process, leading to a sudden increase in pumping pressure, which may result in premature seating of the fracturing bridge plug, or pull-off accidents at the weak point of the wireline. This research aims to improve engineering efficiency and prevent engineering accidents by conducting a pass capacity analysis of the perforating tool string, which is of significant engineering importance for shale gas development.

IoT-ENHANCED SHALE OIL AND GAS GREEN DEVELOPMENT: STATUS AND FUTURE

Shale oil and gas, deemed as vital alternative resources to conventional oil and gas, have spurred significant interest in information-based exploration and development technologies within the industry in recent years [1]. China boasts extensive shale oil and gas reserves, with the International Energy Agency (EIA) estimating China's shale oil technology capable of extracting approximately 4.393 billion tonnes, ranking third globally. Additionally, China holds shale gas geological resources of about 134.4 trillion cubic meters, ranking first worldwide, showcasing immense development potential. Shale oil and gas, being unconventional resources, are distributed within the pore and fracture systems of organic-rich shale formations. Compared to conventional oil and gas, shale oil and gas exhibit weaker trap control and stronger distribution continuity, typically characterized by poor reservoir physical properties. Conventional extraction methods often struggle to yield industrial oil and gas flow from shale oil and gas resources, necessitating unconventional development technologies such as reservoir monitoring, directional wells, horizontal wells, and segmental fracturing [2]. However, these technologies come with challenges and risks, including complex production operations, high production data density, and environmental concerns such as surface ecological damage and pollutant emissions, further impacting shale oil and gas production. To facilitate the upgrading process of China's shale oil and gas industry, this paper presents the current status of Internet of Things (IoT) application technologies for shale oil and gas development and proposes a research outlook based on the concept of green development. Against the backdrop of energy transformation, it offers a reference framework for the sustainable development of China's extensive shale oil and gas resources.

Study on the relationship between shale organic matter development and paleowater depth—A new understanding of the condensed section

Recent studies dispute the long-standing belief that highly organic-rich shales are predominantly linked to condensed section (CS) at the maximum flooding surface (MFS), proposing instead that they more commonly develop in shallow waters along basin margins. This hypothesis, however, has not been thoroughly investigated. Our study, focusing on the pivotal regions of the Longmaxi Formation and the Chang 7 Member for marine and continental shale exploration in China, integrates sequence stratigraphy with analyses of total organic carbon (TOC) and elemental geochemistry to pinpoint the environments conducive to the formation of organic-rich shales. The most organic-rich intervals, specifically the bottom of the Longmaxi Formation and the mid-to-lower Chang 73 sub-member, are located within the transgressive systems tract (TST), showing higher TOC levels in the middle to lower TST, compared to the CS of the MFS. The formation of high organic matter shale is related to warm, humid, and dysoxic shallow waters with minimal terrigenous influx and high primary productivity. We found a pronounced inverse relationship between organic matter concentration and water depth, with deeper waters promoting the decomposition and recycling of organic matter during sedimentation, thereby diminishing the TOC content. Moreover, volcanic activity also affected the organic enrichment in the Chang 7 Member. With high primary productivity, organic matter tends to accumulate in dysoxic slope zones. Consequently, high-organic content shale intervals are generally situated in shallow, dysoxic water environments, suggesting that the CS of the MFS may actually represent organic-poor intervals with limited source rock potential. The results of this study are of great significance for shale oil and gas exploration in China, and provide a new direction for the discovery of shale oil and gas sweet spots.

An investigation on the pyrolysis of low-maturity organic shale by different high temperature fluids heating

This study evaluates the potential of oil and gas generation from thermal pyrolysis of low-maturity organic rich shales by high temperature fluids injection. This method aims to convert the solid organic matter into hydrocarbons, thereby enhancing the quality of shale oil. The effectiveness of this technique hinges on its ability to convert retained heavy compounds and kerogen into light, moveable hydrocarbons, while also improving the oil recovery factor. Our experimental findings reveal that the decrease of total organic carbon content predominantly occurs between 400–550 °C or even higher. Different heating fluid media were tested for their pyrolysis effectiveness, among the heating media tested, CO2 and steam mixture proved the most productive. A comparison of CO2, steam and CO2-steam pyrolysis experiments were carried out. It is observed that the oil yield is the lowest in a pure CO2 atmosphere, while oil yield from the mixture of CO2-steam pyrolysis is the highest. In addition, the pore structure and mechanical properties of shales during pyrolysis process are examined.This investigation provides a theoretical basis for the large-scale upgrading of organic-rich shale formations, highlighting the critical influencing factors of thermal pyrolysis in optimizing shale oil recovery and quality.

Early Cretaceous climatic influences on organic matter enrichment in lacustrine shales: Insights from the Liupanshan Basin of northwestern China

Organic-rich petroleum source rocks are widespread in Early Cretaceous strata, but the related paleoclimatic conditions are still uncertain. Aptian-Albian lacustrine sediments from the Liwaxia and Madongshan formations in the Liupanshan Basin of northwestern China provide vital material to reveal correlations between the Early Cretaceous paleoclimate and the formation of organic-rich lacustrine shales. Outcrop samples were analyzed for total organic carbon (TOC), C/N ratio, CaCO3 content, lipid biomarkers, and compound-specific n-alkane carbon isotopes. Results show that organic matter (OM) of the Liwaxia Formation was mainly sourced from lacustrine plankton and deposited in a weakly stratified and suboxic water column under a hot and arid climate. Organic matter in the lower part of the Madongshan Formation was derived from a mixture of aquatic organisms and higher plants under a warm and humid climate; plant carbon sources were widely available, and the water column was strongly saline, stratified, and anoxic. Elevated temperatures and a less humid climate accompanied the deposition of the upper part of the Madongshan Formation. Higher plants were the primary OM source, and the carbon source became limited. The water column was hypersaline and stratified, and the bottom water was less anoxic. Abrupt changes in the OM inputs and depositional environments documented in the Madongshan Formation were ascribed to global climatic variations. A warm and humid climate during the Aptian-Albian transition enhances continental nutrient inputs and promotes the formation of reduced water conditions, which directly controls the formation of the coeval organic-rich shales in lacustrine basins in China.

Sedimentary differentiation caused by glaciation and orogeny during the Ordovician–Silurian transition in the southern Sichuan Basin, China

The Wufeng–Longmaxi shales, which were deposited during the Ordovician–Silurian transition in the southern Sichuan Basin, exhibit distinct lithofacies transitions. The shift from organic-rich siliceous shale to organic-poor mixed shale and argillaceous shale is attributed to the influences of the Hirnantian glaciation and the Kwangsian Orogeny. Nevertheless, debates persist regarding the mechanisms underlying these significant lithofacies transformations. This study employs sequence stratigraphy, petrology, and geochemical analysis of key wells to unveil crucial insights. The findings indicate: 1) the Wufeng Formation to the Long-1 Member could be divided into two third-order sequences; 2) the waterbody underwent a transition from anoxic and strongly restricted conditions to oxic and moderately restricted conditions; and 3) the trend in formation thickness changed from an E–W orientation to a NE–SW orientation. The results suggest that considerable sediment condensation and anoxia resulting from glacier melting in the early Rhuddanian played a pivotal role in the formation of organic-rich siliceous shale. During the Middle Rhuddanian–Aeronian, the accelerated uplift caused by the Kwangsian Orogeny offset the transgression induced by glacier melting, leading to shallow water conditions and increased terrigenous influx. This diluted the organic matter content in the sediment, and subsequent turbidity currents altered sediment composition, driving the lithofacies transformation into organic-poor mixed shale and argillaceous shale. The study posits that the sedimentary differentiation in the southern Sichuan Basin during the Ordovician–Silurian transition is linked to the initiation and progression of the far-field effects of the Kwangsian Orogeny.

The formation of the Paleocene lacustrine organic-rich shale in the Subei Basin, East China associated with the early late Paleocene event and marine incursions

The second member of the Funing Formation (E1f2) is a Paleocene lacustrine organic-rich shale and has been identified as a primary source rock for both conventional and unconventional oil and gas in East China. It is correlated with the Early Late Paleocene Event (ELPE), a significant temperature inflection point in the Paleocene. However, the impact of the ELPE on the lacustrine organic-rich sedimentation is still not well understood. This study pinpoints the ELPE in the lacustrine E1f2 shale of the Subei Basin, identifies the resultant marine incursions by biomarkers, correlates the ELPE, marine incursions, and the depositional process, and clarifies organic matter accumulation mechanisms in the E1f2 shale. The interval preceding the ELPE has relatively low organic matter content, having been deposited in an arid paleoclimate with saline water conditions and low productivity. Eukaryotes predominantly contributed to the organic matter accumulation. The ELPE is identified in the E1f2 shale by negative carbonate carbon and oxygen isotope excursions. Subsequently, increasing temperatures and rising global sea levels induced marine incursions into the Subei Basin, supported by the appearance of 24-n-propylcholestane and the C30/C27-30 steranes ratio. The interval following the ELPE shows relatively high organic matter content and was deposited in an arid to semihumid-semiarid climate and a saline to brackish waterbody. Marine incursions likely introduced nutrients and enhanced lacustrine productivity, as indicated by the consistency between signals of marine invasion and bioavailable nutrient elements. Different origins contribute to organic matter accumulation at this stage, with prokaryotes being more dominant than eukaryotes. The E1f2 shale provides valuable insights into lacustrine organic-rich sedimentary processes and the responses of lacustrine systems to global events.

Impact of volcanism on the formation and hydrocarbon generation of organic-rich shale in the Jiyang Depression, Bohai Bay Basin, China

Globally, most organic-rich shales are deposited with volcanic ash layers. Volcanic ash, a source for many sedimentary basins, can affect the sedimentary water environment, alter the primary productivity, and preserve the organic matter (OM) through physical, chemical, and biological reactions. With an increasing number of breakthroughs in shale oil exploration in the Bohai Bay Basin in recent years, less attention has been paid to the crucial role of volcanic impact especially its influence on the OM enrichment and hydrocarbon formation. Here, we studied the petrology, mineralogy, and geochemical characteristics of the organic-rich shale in the upper submember of the fourth member (Es41) and the lower submember of the third member (Es33) of the Shahejie Formation, aiming to better understand the volcanic impact on organic-rich shale formation. Our results show that total organic carbon is higher in the upper shale intervals rich in volcanic ash with enriched light rare earth elements and moderate Eu anomalies. This indicates that volcanism promoted OM formation before or after the eruption. The positive correlation between Eu/Eu∗ and Post-Archean Australian Shale indicates hydrothermal activity before the volcanic eruption. The plane graph of the hydrocarbon-generating intensity (S1+S2) suggests that the heat released by volcanism promoted hydrocarbon generation. Meanwhile, the nutrients carried by volcanic ash promoted biological blooms during Es41 and Es33 deposition, yielding a high primary productivity. Biological blooms consume large amounts of oxygen and form anoxic environments conducive to the burial and preservation of OM. Therefore, this study helps to further understand the organic-inorganic interactions caused by typical geological events and provides a guide for the next step of shale oil exploration and development in other lacustrine basins in China.

Palaeoclimatic influence on lake palaeoenvironment and organic matter accumulation in the Middle Jurassic Shimengou formation (Qaidam basin, NW China)

The formation and accumulation of lacustrine organic-rich shales are strongly related to palaeoclimate and palaeoenvironmental conditions. Middle Jurassic organic-rich lacustrine shales in the Qaidam Basin, NW China, are important source rocks for unconventional oil and gas exploration, but factors controlling their organic matter (OM) abundance and preservation are unclear. To understand how palaeoclimate and palaeoenvironmental conditions influenced depositional processes and elucidate the factors controlling OM accumulation of lacustrine organic-rich shales, we investigated total organic carbon (TOC), mineral compositions, and element geochemistry the Shale Member of the Middle Jurassic (J2s2) Shimengou Formation from the Yuqia Coalfield in the Qaidam Basin. From the bottom to the top, the Shale Member includes three significant stages based on their average TOC values of 5.45% (Stage I), 9.90% (Stage II), and 1.10% (Stage III). Paleoclimate proxies including C-values and Rb/Sr ratios indicate that in Stage I the lake developed under warm and humid conditions, while in Stage II climate changed to hot and arid/semiarid conditions before returning to warm and humid climates in Stage III. Correlation analysis among proxies for palaeoclimate, terrestrial influx (Al2O3 and TiO2 contents), sedimentation rate ((La/Yb) N ratios), palaeosalinity (Sr/Ba ratios and B/Ga ratios), palaeoproductivity (P/Ti ratios and Sibio contents), redox conditions (Corg/P ratios), and TOC values revealed that palaeoclimate had little direct impact on OM accumulation, with palaeoenvironmental conditions the main controlling factor for OM accumulation. OM accumulation in Stage I was primarily controlled by redox conditions, followed by palaeoproductivity, terrigenous influx, and sedimentation rate. In Stage II, high palaeoproductivity and anoxic bottom water conditions controlled OM accumulation, resulting in the deposition of oil shales with abundant hydrocarbon potential. In Stage III, poor OM preservation related to oxic redox condition was the primary controlling factor for OM accumulation. We suggest that variations in lake palaeoenvironments induced by palaeoclimate change controlled OM accumulation and petroleum potential.

Geo-storage of hydrogen in organic-rich shales: Multicomponent selectivity of organic nanopores

The gas adsorption capacity of kerogen is commonly obtained through experimental isothermal adsorption procedures. These experiments employ mathematical models called adsorption isotherms, which bridge the gap between empirical data and theoretical frameworks to offer a continuous representation of the underlying processes in the depleted shale formation with some fraction of resident adsorbed natural gas. However, an exploration of the adsorption behavior of H2, in the presence of CH4, C2H6, C3H8, and CO2 in organic-rich porous media of varying pore size remains an unaddressed area of study. Considering this, in this study, we conduct a molecular simulation of the adsorption and diffusion behavior of hydrogen (H2), methane (CH4), ethane (C2H6), propane (C3H8), and carbon dioxide (CO2) within a representative kerogen organic nanopore structure. In particular, we studied hydrogen adsorption behavior in organic-rich shale formations in varying nanopore size and multi-gas components. We also evaluate the suitability of different adsorption models (Langmuir, Langmuir-Freundlich, Toth) for depicting the adsorption characteristics of single species. For the multicomponent mixture (H2/CO2/CH4/C2H6/C3H8), we apply Langmuir ratio correlation model (LRC). The findings indicate C3H8 demonstrates the highest adsorption potential, while CH4 ranks lowest, followed by H2, CO2 and C2H6 exhibit intermediate potential. Furthermore, the adsorption of each species is hindered by organic nanopores of 1 nm, as opposed to 2 nm nanopores. For multicomponent adsorption, hydrogen exhibits minimal presence at equilibrium, which can be attributed to its higher diffusivity compared to other gases. Notably, the Langmuir, Toth, and Langmuir-Freundlich models exhibit ability to accurately depict the overall adsorption trends single-component. However, for the multicomponent mixture (H2/CO2/CH4/C2H6/C3H8), the LRC is found reasonable effective to capture the adsorption.

Astronomically forced cycles in Lower Carboniferous Luzhai Formation shales, Guizhong Depression, South China

Early Carboniferous Luzhai Formation developed continuous and thick shales, which is a critical succession for hydrocarbon exploration and production in southern China. However, no high-resolution stratigraphic sequence framework or cyclostratigraphic studies on early Carboniferous (Tournaisian Stage) are available for this organic-rich shale formation. In this research, Milankovitch cycle evidence has been reported in two boreholes of Luzhai Formation in Guizhong Depression. Natural gamma-ray logging data allowed the establishment of a ∼5.9-Myr floating astronomical timescale (FATS) based on stable long eccentricity (405-kyr) cycles in Luzhai Formation. We anchored FATS to Devonian and Carboniferous boundary age (359.3 ± 0.4 Ma), and an absolute timescale for intervals of 359.3 to 353.4 Ma was developed. Correlation coefficient (COCO), evolutionary COCO, and evolutionary TimeOpt method (eTimeOpt) were employed for quantitatively fit observed stratigraphic cycles to astronomical cycles for providing possible range of sedimentation rates. Sedimentary noise model was adopted for detecting orbital forcing-controlled high-resolution sea-level changes which were correlated to ∼1.2-Myr obliquity amplitude modulation (AM) cycles. Sedimentary noise models (autocorrelation coefficient analysis, ρ1, and dynamic noise after orbital tuning analysis, DYNOT) revealed a correlation between obliquity AM cycle (∼1.2-Myr) and sea level changes in early Carboniferous Luzhai Formation. The ∼1.2-Myr obliquity AM was the main driver of third-order sequence development, the 3rd-order sequence boundaries were related to maximum values of DYNOT median curve and minima values of ∼1.2-Myr amplitude modulation (AM) curve.

Bayesian inversion for rapid multiwell interpretation of well logs and core data: Applications to unconventional organic-shale formations

Multimineral analysis is widely used to calculate in situ porosity, fluid saturation, and mineralogy of rocks penetrated by a well. It delivers weight/volumetric concentrations of rock solid/fluid constituents by combining multiple borehole geophysical measurements and often is referred to as petrophysical joint inversion. Recently, a probabilistic method was developed for improved petrophysical estimation of rock constituents and their uncertainty from well logs. This method mitigates borehole and instrument-related environmental effects present in the measurements and efficiently propagates the uncertainty from measurement noise and rock-physics models (RPMs) to compositional/petrophysical estimations. The probabilistic estimation method is extended to the challenging conditions of multiple neighboring wells penetrating similar rock formations where borehole/drilling environmental conditions, borehole instruments, and measurement noise may vary from well to well. A calibration step is performed in a few key wells with core data and/or advanced borehole measurements; it enables the same RPMs and prior models to be implemented in nearby wells but with limited measurements. In addition, a precomputed surrogate model constructed with radial basis function interpolation is implemented for accurate and efficient nuclear-property calculations. The multiwell interpretation method is verified using synthetic and field examples of organic-rich shale formations. Results find that the probabilistic method (1) improves rock petrophysical/compositional estimations by mitigating borehole environmental effects and incorporating a priori knowledge, (2) yields regionally consistent compositions among wells, and (3) quantifies the uncertainty of the estimations. As a result, the probabilistic approach is especially suitable for assessing petrophysical/compositional properties in multiwell settings with complex rock constituents and/or limited borehole measurements.

Effects of Astronomical Cycles on Laminated Shales of the Paleogene Shahejie Formation in the Dongying Sag, Bohai Bay Basin, China

Laminated shales are widely developed in the Dongying Sag and have attracted much attention as an oil reservoir. Macroscopically, these shales generally have multi-scale cyclicity, which is closely related to the development of laminae. Therefore, analyzing the origin of their cyclicity is helpful to understanding the formation mechanism of laminated shales and the vertical heterogeneity of shale reservoirs, which are of great significance for continental shale oil exploration and development. In this study, a gamma ray (GR) logging series, high-resolution elemental geochemical data, high-resolution core scanning photos and grayscale data, and mineralogical data were used to characterize the cyclicity of shale at different scales, and their relationship with different astronomical cycles was discussed. The results show that the Es3L and Es4U shale in the Dongying Sag has cyclicity from the meter-scale to the ten-meter scale and then to the hundred-meter scale, which is mainly manifested by periodic changes in organic matter abundance, mineral composition, element abundance, and grayscale. These cycles of different scales coincide with different astronomical periods. Specifically, the hundred-meter scale cyclicity is mainly controlled by the very long orbital period; the ten-meter scale cyclicity is mainly related to the eccentricity cycle; while the precession period is the main driver of the meter-scale cyclicity. Finally, we propose a simplified model for illustrating the formation of rhythmic organic-rich shale. This study is helpful to understanding the origin of continental organic-rich shale and predicting shale reservoir properties.

Multiple geological events controlling rock types development: A case study of Wufeng-Longmaxi formation in southern Sichuan Basin, China

During Late Ordovician to Early Silurian, global geological events such as volcanic activity, glaciation, and mass extinction affected sedimentary environments and organic matter (OM) accumulation, and controlled the development of organic-rich shales. Taking the Wufeng-Longmaxi Formation (WF-L Fm) shales in southern Sichuan Basin as the research object, based on the core, thin section and scanning electron microscopy (SEM) observations, combined with X-ray fluorescence (XRF) two-dimensional element distribution maps, X-ray diffraction (XRD), trace elements, carbon isotope composition and total organic carbon (TOC) data, the relationship between geological events and sedimentary environments, OM accumulation as well as rock types development are analyzed, and the sedimentary environments response to geological events is elaborated. Eight rock types have been identified, and the minerals occurrence model has been characterized. By using Cu content, Mo content, δ13C and TOC to measure paleoproductivity, Sr content and Sr/Cu to judge paleoclimate, Sr/Ba to measure paleosalinity, and U/Th, V/Cr, V/Ni, and Ni/Co to analyze redox conditions, the sedimentary environments of the WF-L Fm shales can be furtherly divided into 7 intervals. In general, the deep water hypoxic-anoxic environments from Interval Ⅰ to Interval Ⅴ is conducive to the formation of organic-rich shales. The organic-rich shales formed in the anoxic environments from Interval Ⅰ to Interval Ⅴ are related to the fact that volcanic activities not only release a large amount of nutrients, promote biological prosperity and improve productivity, but also make the water in a sulfide anoxic environment, which is conducive to the preservation of OM. The rapid rise and fall of sea level caused by the Hirnantian glaciation harmed the living environments of organisms, led to the mass extinctions and provided material sources for OM accumulation, resulting in high TOC in the rock types of Interval IV. Irregular gravity flow can enhance terrigenous input in deep water for a short time and dilute OM, which is one of the reasons for the low TOC in the rock types of Interval Ⅵ to Interval VII. Upwelling current enhances nutrient flux in surface water and improves primary productivity of water, thus affecting the development of WF-L Fm organic-rich shales. Based on the aforementioned interpretations, the sedimentary model of the southern Sichuan Basin was established.

Pore Structure and Gas Content Characteristics of Lower Jurassic Continental Shale Reservoirs in Northeast Sichuan, China

The Jurassic shale in the northeastern Sichuan Basin is one of the main target intervals for continental shale gas exploitation. Research on the pore structure and gas-bearing properties of shales is the key issue in target interval optimization. Through core observation, geochemistry, bulk minerals, scanning electron microscopy, nitrogen adsorption, and isothermal adsorption experiments, various lithofacies with different pore structure characteristics were clarified. In addition, the factors that control gas-bearing properties were discussed, and a continental shale gas enrichment model was finally established. The results show that the Jurassic continental shale in the northeastern Sichuan Basin can be classified into six lithofacies. Organic pores, intergranular pores, interlayer pores in clay minerals, intercrystalline pores in pyrite framboids, and dissolution pores can be observed in shale samples. Pore structures varied in different shale lithofacies. The contact angle of shales is commonly less than 45°, leading to complex wettability of pores in the shales. Free gas content is mainly controlled by the organic matter (OM) content and the brittleness in the Jurassic shale. The adsorbed gas content is mainly controlled by the OM content, clay mineral type, and water saturation of the shales. The enrichment mode of the Lower Jurassic continental shale gas in the northeastern Sichuan Basin is established. Paleoenvironments control the formation of organic-rich shales in the center part of lakes. The "baffle" layer helps the confinement and high pressure, and the complex syncline controls the preservation, forming the enrichment pattern of the complex syncline-central baffle layer.

Predicting shale mineralogical brittleness index from seismic and elastic property logs using interpretable deep learning

The mineralogical brittleness index (MBI) of organic-rich shale formations is one of the key parameters to identify the optimal production well locations and optimize hydraulic fracturing. Since we as a community don't understand the exact physical relationship between the MBI and seismic properties from well logs, we have used traditional approaches like the log-based brittleness index (LBI) and the elastic brittleness index (EBI) to quantify the rock brittleness from seismic data and well logs. The LBI method is easy to use but is empirically derived from the porosity and sonic logs. On the other hand, the EBI method is dependent on the average values of Young's modulus and Poisson's ratio but is not physically meaningful in practice. Therefore, we develop a deep learning approach to obtain a more reliable MBI model from seismic properties and enhance the interpretability with Shapley values. First, we analyze the statistical relationship between the MBI and eight seismic properties from well logs and distinguish the influential input variables for the MBI prediction, such as bulk density, Young's modulus, and Poisson's ratio. Second, we find a multivariate linear regression (MLR) model with three input properties and quantify the relative statistical contribution of each input based on Shapley values. Third, we use a deep neural network technique to derive the nonlinear estimation model with a better fit to the MBI data than the traditional methods. We test and verify our approach on field log and core data from the Wolfcamp shales in the Permian Basin, Texas. In conclusion, this workflow can provide a more interpretable and accurate MBI estimation from seismic properties to enhance unconventional shale reservoir characterization.

Organic matter enrichment due to high primary productivity in the deep-water shelf: Insights from the lower Cambrian Qiongzhusi shales of the central Sichuan Basin, SW China

• Cambrian shales in the Sichuan Basin were formed under humid anoxic conditions with low-moderate productivity. • High primary productivity was responsible for organic matter enrichment. • Much higher primary productivity occurred in the deep-water shelf than in the shallow-water shelf. • Hydrothermal upwelling can increase primary productivity. The formation and enrichment mechanisms of organic-rich shales under different marine environments remain unclear. Recently, Cambrian Qiongzhusi (QZS) shales with greater thickness and total organic carbon (TOC) content were found in a deep-water shelf (DWS) than in the adjacent shallow-water shelf (SWS) of the central Sichuan Basin, and these locations provide ideal conditions for determining the mechanisms underlying differences in organic matter (OM) enrichment. In this study, we investigated the TOC, pyrolysis, organic carbon isotopes (δ 13 C org ), biomarkers, and elemental geochemistry of the QZS shales. Based on a detailed comparison between shales from the DWS to the SWS, an OM enrichment model of different shelf conditions was established and the reasons for the higher TOC content in the DWS were clarified. The deposition of QZS shales was accompanied by a large-scale regional transgression. Against the background of an overall humid climate, the water mass at the bottom of the DWS was more restricted, resulting in limited external material exchange and supply. The water mass of the DWS represented by the bottom of the QZS was anoxic and showed the development of euxinic conditions. The biogenic Ba (Ba bio ) content of the samples in the DWS under deep basinal conditions was significantly greater than that in the SWS region, which reflects higher primary productivity and is associated with a large accommodating space and upwelling flows caused by the hydrothermal activities in the DWS. Under conditions in which OM enrichment is dominated by productivity, the DWS is a more favorable location for the formation of high-quality shales.

Astronomical forced sequence infill of Early Cambrian Qiongzhusi organic-rich shale of Sichuan Basin, South China

Early Cambrian Qiongzhusi Formation has attracted great attention due to shale gas finding. However, this organic-rich shale formation is yet no robust high-resolution stratigraphic sequence framework. Here the evidence on Milankovitch cycles in three wells drilled through Qiongzhusi (QZS) Formation in Sichuan Basin has been reported. The logging data made it possible to develop a floating timeline calibrated to stable eccentricity cycles of 405-kyr within ~10.92 and ~7.34 Myr intervals in intracratonic rift and low bulge of Sichuan Basin, respectively. In this research, astronomical time scale was anchored to the UPb Zircon age of QZS Formation/Maidiping Formation boundary (526.86 ± 0.16 Ma), developing an absolute timescale for considered intervals extending from 526.86 to 515.94 Ma. Average spectral misfit and correlation coefficient analyses were applied to quantitatively measure the fitting of the witnessed sedimentary cycles to astronomical periods and provide a possible sedimentation rate range. A sedimentary noise model was chosen to detect high-resolution sea-level variations under the control of orbital forcing and variations of sea-level were linked to obliquity modulation cycles of about 1.2 Myr. The orbital obliquity modulation cycles witnessed for early Cambrian QZS Formation (~1.2 Myr) were correlated to eustatic variations of sea level generated based on sedimentary noise model (DYNOT and ρ1). Furthermore, correlations between sequence stratigraphy and cyclostratigraphy were discussed, and the third-order sequence boundaries well corresponded to minima ~1.2 Myr orbital obliquity modulation curve and high values of DYNOT. The previous sequences framework was demonstrated to be isochronous and credible.

Adsorption characteristics and thermodynamic analysis of shale in northern Guizhou, China: Measurement, modeling and prediction

The Lower Cambrian Niutitang Formation shale is the principal organic-rich shale formation in the northern Guizhou Province of China, which has great shale gas development prospects. In this study, the isothermal adsorption data from 26 shale samples from Niutitang Formation in northern Guizhou were obtained through the isothermal adsorption experiment of self-sampled specimens, along with published data. On this basis, we analyzed the control effects of temperature and physical parameters on adsorption capacity. An adsorption model with two new physical meaning parameters was constructed, and the accuracy of the model was validated. In addition, the isosteric enthalpy of adsorption in shale was further calculated, and the factors that affected the isosteric enthalpy were discussed. The results showed that the isosteric enthalpy decreased with an increase in temperature, and increased with an increase of TOC, but was controlled at a higher TOC. TOC-normalized heat revealed a more obvious segmental change rule for thermal maturity. Finally, we provided a new method to predict adsorption isotherms at other temperature conditions through the isosteric enthalpy of adsorption, and obtained better prediction results. This study provided a new insight for further understanding of shale adsorption characteristics from the perspective of thermodynamics.

Effect of oxidative breakers on organic matter degradation, contaminant mobility and critical mineral release during shale-fracturing fluid interactions in the Marcellus Shale

Oil and gas production from organic-rich shale formations has become viable through advancements in multistage hydraulic fracturing. However, fluctuations in oil and gas prices, coupled with the sharp decline in gas production after the initial days of fracturing operations, drive operators to devise strategies for enhancing hydrocarbon production. The use of highly reactive fracturing fluids that include strong oxidizing agents, also known as breakers, can potentially increase well productivity. The oxidative breakers are used to reduce the viscosity of gel-based fluids after the proppant is transported into fracture zones of the target formation. These breakers can also degrade the organic matter, enhancing the release of hydrocarbons. However, chemical byproducts generated by the interaction of oxidative breakers with the shale matrix have not been extensively studied. This study investigated the fluid-rock interactions between Marcellus Shale and synthetic hydraulic fracturing fluid (HFF) solutions comprising three different oxidative breakers, i.e., ammonium persulfate, sodium bromate, and sodium hypochlorite, commonly used in the Appalachian Basin, USA. Our results demonstrate that the type of oxidizing breakers used in the HFF controlled the type and amount of volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) produced. In all HFF reacted effluents, we observed the transformation of VOCs and the presence of organic acids in variable amounts. However, effluents from HFF containing sodium bromate and sodium hypochlorite breakers showed the presence of several halogenated organic compounds.Changes in major ions and mineralogy indicate that carbonate dissolution and barite precipitation were ubiquitous in all shale reacted effluents. Our results also demonstrate that the addition of oxidative breakers increased the concentration of several major and trace elements in the effluents. These elements fall under the critical mineral (CM) or critical element category due to their high demand in emerging technologies and susceptibility to supply chain disruption due to variety of factors. However, before oxidative breakers can be used at a larger field scale to enhance the release and recovery of CM and hydrocarbons, a better understanding is required of the potential environmental impacts associated with the generation and transformation of contaminants during breaker-fluid-rock interaction.