To better understand how cyclic effective stress variations affect the mechanical and transport properties of reservoir rocks during underground hydrogen storage (UHS), this study conducted proportional loading experiments on three porous sandstones varying in porosity and permeability. Proportional loading experiments were conducted under different stress paths, simulating variations in effective stress caused by cyclic UHS. Mechanical properties, porosity, and permeability were evaluated based on the mean effective stress and the number of loading cycles. Scanning electron microscopy (SEM) analysis provided insights into the microscopic deformation processes responsible for observed macroscopic behaviours. Results indicate distinct deformation mechanisms influenced by stress paths. Under cyclic triaxial loading conditions (K = 0), high-porosity sandstones initially exhibit compaction but transition to dilatancy-dominated deformation, characterised by microcrack development and grain rearrangement. This dilatancy behaviour paradoxically results in a negative correlation between porosity and permeability. This phenomenon arises because fragmented grains obstruct pore throats, modifying the pore structure and causing localised variations in porosity distribution, which in turn adversely impacts permeability. Conversely, the low-porosity sandstone consistently exhibits compaction-driven deformation, with porosity loss closely correlating with permeability reduction. Under cyclic proportional loading conditions (K > 0), all sandstones exhibit predominant compaction, particularly under repeated cyclic loading. The mechanical and transport properties initially evolve mainly with increasing mean effective stress irrespective of stress paths. However, during cyclic loading, both bulk and pore compressibilities significantly depend on the applied stress paths, becoming notably larger at higher stress path values. Consequently, greater accumulations of inelastic strain and subsequent porosity and permeability loss occur under elevated stress path conditions. SEM observations revealed that these inelastic strains predominantly originate from grain fracturing, contact wear, and compaction or consolidation of clay-rich grain boundaries under cyclic loading. Furthermore, permeability evolution across all samples follows an exponential decay trend, emphasising the cumulative impact of cyclic loading-induced microstructural changes. These findings elucidate critical process-driven mechanisms governing mechanical and transport property evolution in reservoir rocks under cyclic stress conditions, thereby informing the design and operational safety assessments of underground hydrogen storage facilities.

Pore connectivity and ultimate imbibed porosity are two important parameters used to assess the shale oil reservoir property, the proper appraising of which could facilitate the efficient flow of oil from the matrix and an improvement in recovery efficiency. In previous studies, the uncertainty in sample dimensions and the extra-long stable time during imbibition experiments exploring pore connectivity and ultimate imbibed porosity showed a lack of discussion, which influenced the accuracy and efficiency of the SI experiments. In this study, SI experiments with shale samples of different thicknesses are carried out to acquire the two parameters in a short period of time. As a result, the pore connectivity of sample D86-5 from the Qingshankou Formation (Fm) in the Songliao Basin fluctuates with the increase in thicknesses, with an average of 0.265. The water penetrates sample D86-5 of all thicknesses, so the ultimate imbibed porosity fluctuates around 3.7%, and the stable time increases with thicknesses. The pore connectivity of sample Y172 from the Shahejie Fm in the Bohaiwan Basin fluctuates around an average of 0.026, which is much smaller than that of D86-5. The ultimate imbibed porosity of Y172 decreases with thicknesses because the penetration depth is so small that the pores cannot be fully accessed, and the stable time increases before becoming stable with fluctuations. The method is examined using the samples from the Liushagang Fm in the Beibuwan Basin measuring around 400 μm: the ultimate imbibed porosity of BW1-1 and BW1-3 is 5.8% and 18.1%, respectively, the pore connectivity of BW1-1, BW1-2, and BW1-3 is 0.086, 0.117, and 0.142, respectively, and the results can be obtained within a day. In comparison, the average pore connectivity of the 400 μm samples from Qingshankou, Shahejie, and Liushagang Fms is 0.324, 0.033, and 0.097, respectively, and the average ultimate imbibed porosity of these Fms is 3.7%, 3.1%, and 12.0%, respectively. Based on the above results, a quick method for measuring the two parameters with thin samples by spontaneous imbibition is established, providing a fast solution for the evaluation of the sweet spot.

The Aptian–Albian interval represents a significant cooling phase within the Cretaceous “hothouse” climate, marked by dynamic climatic fluctuations. High-resolution continental records are essential for reconstructing terrestrial climate and ecosystem evolution during this period. This study examines a lacustrine-dominated succession of the Shahezi Formation (Lishu Rift Depression, Songliao Basin, NE Asia) to access paleo-weathering intensity and paleoclimate variability between the Middle Aptian and Early Albian (c. 118.2–112.3 Ma). Multiple geochemical proxies, including the Chemical Index of Alteration (CIA), were applied within a sequence stratigraphic framework covering four stages of lake evolution. Our results indicate that a hot and humid subtropical climate predominated in the Lishu paleo-lake, punctuated by transient cooling and drying events. Periods of lake expansion corresponded to episodes of intense chemical weathering, while two distinct intervals of aridity and cooling coincided with phases of a reduced lake level and fan delta progradation. To address the impact of potassium enrichment on CIA values, we introduced a rectangular coordinate system on A(Al2O3)-CN(CaO* + Na2O)-K(K2O) ternary diagrams, enabling more accurate weathering trends and CIA corrections (CIAcorr). Uncertainties in CIA correction were evaluated by integrating geochemical and petrographic evidence from deposits affected by hydrothermal fluids and external potassium addition. Importantly, our results show that metasomatic potassium addition cannot be reliably inferred solely from deviations in A-CN-K diagrams or the presence of authigenic illite and altered plagioclase. Calculations of “excess K2O” and CIAcorr values should only be made when supported by robust geochemical and petrographic evidence for external potassium enrichment. This work advances lacustrine paleoclimate reconstruction methodology and highlights the need for careful interpretation of weathering proxies in complex sedimentary systems.

During underground hydrogen storage (UHS) operations, reservoir rocks often experience time-dependent deformation under long-term stress, which can alter the microstructure and subsequently affect the stability and hydrogen storage efficiency. Therefore, understanding and predicting these time-dependent deformation of reservoir rocks under in situ conditions and its impact on rock properties are crucial for ensuring the long-term safe operations of UHS. This study investigates the time-dependent mechanical and transport behaviour of three representative porous sandstones-St Bees, Castlegate, and Zigong-through constant stress (creep) and multilevel stress creep experiments. These tests were designed to simulate the in situ conditions (1.3-2.6 km depth) of the underground hydrogen storage process at a laboratory scale. In the constant stress experiments, permeability and porosity were measured concurrently to reveal the impact of time-dependent deformation on the transport properties of porous sandstones. In the multi-level stress creep tests, long-term pore pressure cycling was applied to simulate hydrogen injection and withdrawal, and the results were compared with those from experiments under constant pore pressure. This allowed for a systematic assessment of the influence of pore pressure fluctuations on the mechanical response and transport characteristics of the sandstones. The research results indicate that all three sandstones exhibit stable creep behaviour, with the steady-state creep rate increasing as temperature and stress increased. The high-porosity St Bees and Castlegate Sandstones show higher steady-state creep rates under the same conditions compared to the low-porosity Zigong Sandstone. The creep behaviours of the three sandstones under in situ conditions can be well described by Burgers model. The permeability of the three sandstones gradually decreased during the experiments, and this trend become more obvious as the stress and temperature increases. Microstructural analysis reveals that the deformation mechanism of the high-porosity St Bees Sandstone is dominated by dilatancy. Although shear-induced deformation causes the feldspar and quartz clusters to fracture, creating new voids and increasing the overall porosity, the fractured debris from these clusters block the throats, complicating the pore structure and leading to a significant permeability loss. The deformation mechanisms of Castlegate and Zigong Sandstone, on the other hand, are dominated by compaction, with pore compression and microcrack closure being the primary causes of porosity, permeability losses. Pore pressure cycling increases the creep rate of sandstones, accumulating more inelastic strain especially in St Bees Sandstone, but has limited effect on the properties of Castlegate and Zigong Sandstones.

The production of gas and condensate from liquid-rich shale reservoirs, particularly within heterogeneous lacustrine systems, remains a critical challenge in unconventional hydrocarbon exploration due to intricate multiphase hydrocarbon partitioning, including gases (C1–C2), volatile liquids (C3–C7), and heavier liquids (C7+). This study investigates a 120-meter-thick interval dominated by lacustrine deposits from the Lower Cretaceous Shahezi Formation (K1sh) in the Songliao Basin. This interval, characterized by high clay mineral content and silicate–pyrite laminations, was examined to identify the factors controlling hybrid shale gas condensate systems. We proposed the Hybrid Shale Condensate Index (HSCI), defined as the molar ratios of (C1–C7)/C7+, to categorize fluid phases and address shortcomings in traditional GOR/API ratios. Over 1000 samples were treated by geochemical pyrolysis logging, X-ray fluorescence (XRF) spectrum element logging, SEM-based automated mineralogy, and in situ gas desorption, revealing four primary controls: (1) Thermal maturity thresholds. Mature to highly mature shales exhibit peak condensate production and the highest total gas content (TGC), with maximum gaseous and liquid hydrocarbons at Tmax = 490 °C. (2) Lithofacies assemblage. Argillaceous shales rich in mixed carbonate and clay minerals exhibit an intergranular porosity of 4.8 ± 1.2% and store 83 ± 7% of gas in intercrystalline pore spaces. (3) Paleoenvironmental settings. Conditions such as humid climate, saline water geochemistry, anoxic bottom waters, and significant input of volcanic materials promoted organic carbon accumulation (TOC reaching up to 5.2 wt%) and the preservation of organic-rich lamination. (4) Laminae and fracture systems. Silicate laminae account for 78% of total pore space, and pyrite laminations form interconnected pore networks conducive to gas storage. These findings delineate the “sweet spots” for unconventional hydrocarbon reservoirs, thereby enhancing exploration for gas condensate in lacustrine shale systems.

We investigate the evolution of poro-mechanical, transport properties and strength characteristics of different sandstones during the cyclic underground hydrogen storage (UHS). Therefore, we selected three different types of sandstones: fine-grained St Bees (∅ =19~22%), coarse-grained Castlegate (∅=18~20%), and coarse-grained Zigong (∅=7~11%). These sandstones exhibit significant porosity, grain size, and mineralogical differences. The samples were imaged using micro-CT to characterise their initial microstructure and then subjected to cyclic loading experiments under hydrostatic as well as various deviatoric stress paths. The aim is to simulate the in-situ stress during cyclic UHS at depths of ~1.5-3km. The permeability of the samples was measured at different stress points. After completing the cyclic loading tests, we performed repeat micro-CT characterization as well as scanning electron microscopy (SEM) analysis to record the permanent changes in the microstructure caused by the stress cycles. The experimental results show that at shallower depths (low-stress state), the high porosity Castlegate sandstone (∅=18~20%) and the St Bees sandstone (∅=19~22%) exhibit an increase in elastic modulus during the tests, experiencing strain hardening due to compaction. The permeability of both sandstones decreases with an increase in mean stress, independent of the stress path. The fine-grained St Bees sandstone shows more significant accumulative inelastic strain and higher permeability loss than the coarse-grained Castlegate sandstone at the same stress state. In contrast, the low-porosity Zigong sandstone (∅=7~11%) shows no significant changes in mechanical properties, and its permeability loss is related to the closure of the initial microcracks. At greater depths (high-stress conditions), the mechanical and transport properties of the fine-grained St Bees sandstone exhibit an evident dependence on the stress path. During stress cycling under deviatoric stress conditions, the rock experienced a noticeable weakening indicated by a reduction in elastic modulus. The porosity of the sandstone decreased by 0.8~1.4% due to the combined effects of compaction and dilatancy, with a permeability loss exceeding 50%. The application of deviatoric stress led to lower permeability than hydrostatic tests conducted under the same mean stress. In contrast, the coarser-grained Castlegate and Zigong sandstones show an insignificant stress path dependence in their mechanical and transport properties. Due to compaction, these sandstones experience increased intergranular contact, leading to reduced porosity, increased elastic modulus, and strain hardening. The lower-porosity Zigong sandstone shows a higher sensitivity of permeability to stress than the higher-porosity Castlegate sandstone, which is related to its more complex pore structure. Microstructural analysis reveals that factors such as porosity, particle size, microfractures, and the presence and distribution of compliant components like clay minerals are the primary causes for the variations in the poro-mechanical and transport properties of the three sandstones under cyclic stress. Therefore, in addition to the depth of the reservoir, grain size (and their distribution) and mineralogical characteristics play a significant role in the selection of hydrogen storage candidates.

Total organic carbon (TOC) content, a classic indicator of rock organic richness, is widely used in geological archives for paleoenvironmental interpretation and petroleum system modeling. However, organic carbon (OC) undergoes significant alteration and loss upon burial, rendering present-day TOC measurements inadequate for reflecting original OC levels. Many approaches have been developed to restore such OC loss based on mass balance principles and Rock-Eval parameters, yet these methods rely on implicit assumptions that introduce uncertainties and have not been tested. Based on a reevaluation of previous restoration methods, this study proposed a mass balance framework with a refined algebraic scheme to reconstruct buried (pre-catagenesis) TOC. A one-at-a-time sensitivity analysis method was introduced to quantify the propagation uncertainties in the model by examining the responses of TOC restoration outputs (TR, f, and sigma TOC) to variations in key inputs (TOCpd, BIpd, HIpd, HIo, Cc, alpha, and beta). Simulated Rock-Eval data, derived from HI-Tmaxsigmoid models, was utilized in sensitivity analysis to avoid the influence of source rock heterogeneity and organo-facies variations. Both the simulated and experimental results demonstrate that the proposed model improves the TOC restoration accuracy by accounting for the rock mass changes and OC deductions due to hydrocarbon expulsion. Furthermore, the uncertainties arising from S1 "carry-over" and mineral matrix effects are resolved through the new equations. This study, from a sensitivity analysis perspective, summarizes the impacts of input parameters in perspectives of kerogen kinetics and thermal maturation, offering a guideline for more robust TOC restoration and evaluation.

Lakes are sensitive indicators of the balance between accommodation and sediment supply, recording high‐resolution changes in palaeoenvironmental conditions. Long‐lived rift lake basins, however, are predominantly controlled by episodic accommodation changes and pronounced basinward facies shifts, complicating the generalisation of tectonic and climatic controls on rift lake successions. This study proposes a sequence framework and depositional pattern for asymmetric half‐grabens in syn‐rift lake basins by characterising the lacustrine fan‐delta deposits of the Lower Cretaceous Shahezi Formation in the Songliao Basin. Detailed sedimentologic and petrographic analyses identified 24 lithofacies categorised into seven facies associations. A sequence stratigraphic framework was constructed to outline the tectono‐stratigraphic evolution during the syn‐rift phase. The results indicate that the syn‐rift Lishu palaeo‐lake is characterised by its relatively small size, steep slopes, poorly developed and siliciclastic‐dominant shoreline strata and significant input of allochthonous biodetritus. The syn‐rift deposits show a distinct threefold conglomerate–sandstone–mudstone motif, with a complete cycle comprising a prolonged retrogradational phase (LST and TST) and a brief progradational phase (HST). Basin‐bounding faults accelerated hinterland erosion and increased sediment feeder system slopes by rotating hangingwall blocks; consequently, rapid sediment transport and localised gravitational collapse caused the common occurrence of soft‐sediment deformation structures and sublacustrine fan conglomerates. The substantial increase in accommodation space, resulting from fault‐generated subsidence, triggered lake expansion and further contributed to the development of transgressive system tracts and continuous mudstone deposition. These mudstones, rich in terrigenous organic matter and allochthonous fossils, correlate with carbonaceous mudstones, coals and conglomeratic sandstones in proximal overfilled sections, indicating a dynamic interplay between fan delta progradation and Lake Shoreline transgressions. This study proposes a depositional model within a sequence stratigraphic framework for non‐marine sediment accumulation in asymmetric half‐grabens bounded by active faults. The findings offer insights that complement existing models developed for marine rift systems.

Shale gas exploration in the Yangtze plate has been hindered by strong heterogeneity of gas compositions resulting from magmatism. Borehole core and gas samples from Upper Ordovician-Lower Silurian in the Upper Yangtze plate and the Lower Yangtze plate were analyzed to explore the effects of magmatism on gas accumulation. The analyses included nitrogen isotope of gas, pore structure and X-ray photoelectron spectroscopy of shales, and U-Pb ages of fracture-filling calcites. Our results suggest that magmatism caused the development of micro-fractures in shale reservoirs and accelerated the thermal evolution rate of organic matter. Magmatism in the Upper Yangtze plate predates the peak of gas generation of Upper Ordovician-Lower Silurian shales. Generation and expulsion of hydrocarbon at high temperatures removes 12C from carbon reservoir. After the magmatism, the continuous subsidence of the strata led to further hydrocarbon generation of organic matter. Moreover, the self-sealing property of shale reservoirs were enhanced by the closure of micro-fractures, development of graphite structures, and enrichment of 13C in residual methane, which favor the efficient accumulation of shale gas. Conversely, magmatism in the Lower Yangtze plate occurred after the peak of gas generation of Upper Ordovician-Lower Silurian shales. Magmatism compromised shale gas reservoirs by disrupting sealing conditions, facilitating methane diffusion and atmospheric nitrogen influx. Additionally, overburden pressure led to the collapse of graphitized OM-hosted pores, further inhibiting gas accumulation. Our findings underscore the pivotal role of magmatism in various stages of shale gas accumulation and are critical for exploration strategies in sedimentary basins with prevalent magmatic activities.

The utilization of CO2-Enhanced Coal Bed Methane (CO2-ECBM) technology is pivotal in realizing the environmentally responsible and efficient exploitation of Coalbed Methane (CBM) energy resources. The optimization of carbon capture, utilization, and storage (CCUS) for carbon reduction mandates a nuanced understanding of the diverse geological attributes present in CBM reserves globally. Traditional estimations of CO2-ECBMs carbon sequestration potential have predominantly relied on rudimentary empirical models, notably those proposed by the United States Department of Energy (DOE), which overlook the intrinsic geological conditions and the physicochemical properties of subsurface fluids. Addressing these limitations, our study implements the advanced DR/Henry mixed adsorption model in tandem with the Peng-Robinson equation of state (PR-EOS). This approach meticulously identifies the critical parameters governing the mass exchange ratios between CO2 and CH4, pertinent to in-situ geological environments. Subsequently, we have formulated a comprehensive carbon sequestration potential assessment framework. This innovative model adheres to the mass conservation principles for individual CO2 and CH4 components, taking into account the specific surface and stratigraphic conditions prevalent. Employing this refined methodology, we evaluated the CO2-ECBM carbon sequestration potential of the 40 evaluation units of extensional, compressive, and cratonic continental coal bearing basins in China's three major temperature-pressure systems across different depth domains and coal ranks within 2000 m. Our findings reveal that the theoretical carbon sequestration capacity of China's continental coal-bearing basins is approximately 59.893 billion tons. Concurrently, the potential ECBM output stands at an estimated 4.92 trillion cubic meters, underscoring the substantial environmental and energy benefits inherent in harnessing CO2-ECBM technology effectively. The regional analysis revealed that North and Northwest China hold the highest sequestration and recovery potential, followed by the Northeast and Southern regions, respectively. Specific areas, including the eastern edge of the Ordos Basin and southern Junggar Basin, Qinshui, Huoxi, Xishan, and other areas in Shanxi, present promising future prospects for geological carbon storage in unrecoverable coal seams.

Initial water saturation has a significant influence on spontaneous imbibition, and it impacts the imbibition rate and imbibed liquid dynamic distribution among pore networks. At present, the effect of initial water saturation on spontaneous imbibition is still unclear, especially the water film effect caused by initial water saturation varies during imbibition. To address these issues, we prepared the samples with varied initial water saturation through vapor adsorption and then conducted spontaneous imbibitions on these samples, which were monitored by nuclear magnetic resonance (NMR). Samples were selected from tight sandstone reservoirs in the eastern Ordos Basin. From the NMR spectrum corresponding to different imbibition duration, the left wing of the left peak at the initial water saturation is usually higher than the ones after imbibition, mainly caused by the water film effect. When the water film on the surface of large pores is too thin, it is prone to be interpreted as water in small pores because the water film has a quick relaxation time than the water in the center part of pores. The water film effect disappears when the whole pores are filled with water, and the inversed small pores decrease. The relaxation time of the water film is shorter than 0.51 ms in the tested samples. This study is conducive to understanding the effect of initial water saturation on liquid dynamic distribution during imbibition, especially the water film effect variation during imbibition and the T2 value of the water film.

After flowback, the residual fracturing fluid will reduce the gas seepage space and influence natural gas production, which attracts widespread attention. In this study, the irreducible water saturation was investigated, and its controlling factors were clarified. We target the Upper Paleozoic Taiyuan and Shihezi Formations, which belong to a tight gas reservoir in the eastern Ordos Basin. The main experiments include porosity, permeability, mineral composition, nitrogen adsorption, mercury intrusion porosimetry, nuclear magnetic resonance, and high-speed centrifugation. The specific surface area is very low and varies from 0.95 to 4.03 m(2)/g, and the median pore-throat diameter ranges from 28.6 to 698.6 nm. Through the T-2 cutoff value, the water saturation can be divided into movable water saturation (S-mov) and irreducible water saturation (S-irr). Furthermore, the S-irr can be divided into water saturation in large pores controlled by the small throat (S-irrl) and water saturation controlled by the capillary force (S-irrc). In both formations, the S-irr has a negative relationship with porosity, permeability, and average pore diameter and exhibits a positive relationship with the specific surface area. The S-irrl has a positive relationship with the median pore-throat diameter in Taiyuan Formation, but the S-irrl has a weak relationship with the median pore-throat diameter in Shihezi Formation. The Sirrc has a negative relationship with porosity, permeability, and average pore diameter and displays a positive relationship with the specific surface area in Taiyuan Formation, but the S-irrc has a weak relationship with these parameters in Shihezi Formation. The relationship difference between Taiyuan and Shihezi Formations was mainly caused by the pore structure, demonstrated by the amplitude ratio in three peaks. Based on the above analysis, this study is conducive to understanding the mechanism of water occurrence and its controlling factors.

Imbibition under overburden pressure can simulate the imbibition behavior in reservoir conditions during hydraulic fracturing, about which the mechanism is still unclear. This study investigated the imbibition with overburden pressure using a nuclear magnetic resonance (NMR) displacement design. The main contribution of this study is that the initial imbibition rate under confining pressure can reflect the pore connectivity of reservoirs under overburden pressure and a method for appraising the pore connectivity under confining pressure was established. The tight sandstone samples were collected from the Upper Paleozoic Taiyuan and Shihezi Formations in Ordos Basin. The Taiyuan Formation presents the apparent double-peak structure from NMR spectra, and liquid fills into small pore preferentially as a whole. When the imbibition time is on a square root scale, the cumulative imbibition height at the initial imbibition period is not stable, which deviates from the linear principle, and the initial imbibition rate ranges from 0.077 to 0.1145. The Shihezi Formation shows a dominant peak structure from NMR spectra, and the liquid has no obvious filling order as a whole. When the imbibition time is on a square root scale, the cumulative imbibition height at the initial imbibition period also deviates from the linear principle, and the initial imbibition rate ranges from 0.0641 to 0.1619.

In-situ fluid phase behavior is important in determining hydrocarbon contents and the multiphase flow through shale reservoirs. The gas-to-oil ratio (GOR) has been recognized as a critical indicator of fluid types. However, little is known about the impact of fluid phase variation across the thermal maturity on shale oil/gas production (e.g., estimated ultimate recovery, EUR). According to the specific gravity ratio of oil/gas, the producing GOR was converted and normalized into a mass fraction of gas in total hydrocarbons (M-GOR) to compare North American shale oil/gas plays with Chinese shale oil and hybrid gas-condensate plays. A correlation between M-GOR, the fluid phases, and production data was established to identify five phase stages of flow. M-GOR varies systematically with the different production zones, which shows promise in rapidly indicating the well production performance and high production stages of shale oil/gas plays. The hybrid shale gas condensate index, T-max, and total gas contents were integrated to present the fluid types and maturity of shale gas-condensates, which indicates fluid phase and production variation across thermal evolution. The results offer a unique perspective on the shale oil reservoir producibility based on the impact of GOR on fluid phases and EUR from the dominant global oil/gas plays.

Because it is necessary to focus on differences in regional oil reservoirs and determine the priority of the CCUS‐EOR (Carbon capture, utilization, and storage‐enhanced oil recovery) deployment under China's net‐zero CO 2 emission target, systematic and regional evaluations of CO 2 sequestration capacity in major oil basins are needed considering the geofluid properties—carbon sequestration capacity in place (CSCIP)—where the ‘in place’ indicates actual geological formation conditions underground, e.g., formation temperature and pressure. Therefore, physical properties of geofluids at different depths with different geologic temperatures and pressure conditions are considered for the CO 2 sequestration capacity evaluation in place, including shallow (800–2000 m), medium (2000–3500 m), deep (3500–4500 m) and ultra‐deep (4500–8000 m) depth intervals. A modified evaluation model with four grading levels is proposed, combining the P‐V‐T equations of state (EOS) and evaluation equations of the Carbon Sequestration Leadership Forum (CSLF), including theoretical, effective, practical, and CCUS‐EOR CSCIP, which is more consistent with geofluid physical properties underground, to make the grading evaluation and ranking of the CSCIP in China's major oil basins. Then, the grading CSCIP of 29 major oil basins in China was evaluated based on the petroleum resources evaluation results of the Ministry of Natural Resources of China (MNRC) during China's 13 th Five‐Year Plan period. According to the grading evaluation results, suggestions for China's CCUS‐EOR prospective regions are given as follows: shallow oil fields of the Songliao Basin in Northeast China, shallow–medium oil fields of the Bohai Bay Basin in East China, medium oil fields of the Zhungeer Basin in West China, and medium oil fields of the Ordos Basin in Central China; all are potential areas for the CCUS‐EOR geological sequestration in China's onshore oil basins. In addition, in China's offshore oil basins, shallow–medium oil fields of the Bohai Sea and shallow oil fields of the Pearl River Mouth Basin have potential for CCUS‐EOR geological sequestration.

Organic carbon (OC) burial in lakes has been identified as an efficient sink in the global carbon cycle. Abundant input of terrestrial-derived OC leads to high variability in OC origin and type, but its role in determining organic rich sediments has been overlooked in prior studies. Here, we investigated the OC source and concentration of the Lower Cretaceous (Middle Aptian to Lower Albian) Shahezi Formation (Songliao Basin, NE Asia) to reveal the burial of terrestrial OC in relation to syn-rift lake evolution and paleoclimate change. The sequence stratigraphic framework of fan-deltaic and lacustrine successions was established by identifying depositional facies and sequence boundaries. The lacustrine-dominated interval was further subdivided into four facies associations (i.e., lake shore to littoral siltstones, shallow-littoral mudstones, sublittoral, and profundal) and a few cyclic, parasequence-order packages, using 90 m of continuous cores and high sampling frequency of RoqSCAN SEM-EDS. Multiple independent proxies (macerals identified using correlative light and electron microscopy, pyrolysis indices, Ro, TOC/TN, and & delta;13Corg) suggest that the organic fraction of the highly mature mudstones was predominantly contributed by terrestrial-derived OC (gas-prone type III/IV kerogen). A direct correlation between depositional facies, chemical weathering proxies (CIA, CIAcorr, and Ln(Al2O3/Na2O)), and OC burial (TOC and HI) has been established. A steep syn-rift slope, a warm-humid climate, abundant vegetation, and the subaqueous transport of OC-bearing sediments (e.g., massive mud-rich conglomerates and sand-to pebble-bearing mudstones), may have jointly promoted the high input of terrestrial OC. A comparison between two Lower Cretaceous terrestrial records from high and low paleolatitudes suggests that the shift from the syn-rift to post-rift phase was accompanied by an increase in TOC concentration and a change toward Type I kerogen of aquatic origin. The tectonically-controlled evolution of rift basins might be an important forcing function for the change of OC sources and concentrations, which is responsible for long-term OC burial in hinterland environments.& COPY; 2023 Published by Elsevier B.V.

Rock fabric and its influence on residual oil distribution are key issues to the highly efficient development of shale oil. This study targeted the rock fabric and residual oil distribution, and samples were selected from the first and second members of the Upper Cretaceous Qingshankou Formation in the Songliao Basin. Multiple methods were used to analyze rock fabric, including material composition, scanning electron microscopy (SEM), micro-computed tomography (micro-CT), and low-temperature nitrogen adsorption (LT-NA). The residual oil distribution was investigated by methods of oil extraction and LT-NA. The organic matter of most samples belongs to type I, and clay is the main mineral component, which can be as high as 65.4%. There are a lot of inorganic pores at the nanoscale, while the organic pores mainly range from 10 nm to 2 mu m, and at the microscale, the pore connectivity is poor in both formations. The apparent and intrinsic average specific surface areas (SSAs) are 5.35 and 10.23 m2/g, respectively, indicating that the average SSA of post-oil extraction is nearly 2 times that of pre-oil extraction. The residual oil has a wide distribution among different pores, ranging from 1 to 200 nm. In most cases, the residual oil mainly exists in pores between 1 and 5 nm, indicating small pores holding abundant oil. The pore space ratio for residual oil has a negative relationship with clay content, total organic matter (TOC), pyrolysis hydrocarbon (S2), and intrinsic average pore diameter, indicating that higher clay content is detrimental to liquid hydrocarbon generation. Higher TOC and S2 mean less generated liquid hydrocarbon, and a higher intrinsic average pore diameter means fewer nano organic pores for liquid hydrocarbon. This study is conducive to understanding the rock fabric of lacustrine shale and its influence on residual oil distribution.

Low-resistance shale reservoirs have prospected in many high-mature shale gas plays with distinct yields. It is worth investigating the reasons for the difference in the production of low-resistance shale. And this needs to start from the genetic mechanism of shale low resistance. The genesis of shale low resistance is currently considered to be closely related to high-mature organic matter, which is the most important storage space for shale gas. Therefore, it is necessary to study the relationship among organic matter properties, pore structure of shale reservoir space, and shale resistivity. Herein, the typical low-resistivity shale in southern China was examined using X-ray photoelectron spectroscopy, gas adsorption, high-pressure mercury intrusion porosimetry. The results show that compared with conventional resistivity shale, low resistivity shale has smaller pore volume and specific surface area and higher organic matter graphitization degree. The high degree of graphitization significantly reduces the rock resistivity. In addition, graphitization changes the mechanical properties of organic matter. Under the action of compaction and tectonic movement, the macropores decrease sharply, the mesopores increase first and then decrease, and the micropores change little with the degree of graphitization. The change in the size and shape of the organic pores results in the collapse of the organic pores and the contact of the pore walls, which further increases the migration path of the electronic currency on the conductive organic matter and makes the rock resistivity lower. When the degree of graphitization exceeds 15%, poor pore development leads to lower resistivity, and due to poor reservoir space, such shales are extremely risky for exploration.

Remarkable breakthroughs have been achieved in exploration of marine shale gas, acting as important cleaner energy resources in complex tectonic regions, encouraging us to accurately estimate different reservoir capacity of tectonically deformed gas shales. In particular, organic matter (OM) pore system, acting as the most important storage space for high-over-mature marine gas-shale reservoirs in South China has not yet been specifically targeted and comparatively studied in a regional and differential tectonic deformation regime. A set of shale drilling core samples from the Upper Silurian to Lower Ordovician Wufeng-Longmaxi Formations in the Sichuan Basin and its periphery were targeted by a multi-methodical approach utilizing organic geochemistry and mineralogical investigations, FE-SEM observation and digital image processing and extraction, and combined fluid intrusion (N2 and CO2 adsorption analysis). From undeformed shales (UDS) through slightly deformed shales (SDS) to intensely deformed shales (IDS), an obviously and progressively “Triple Jump” reduction in multi-scale pore volume and specific surface areas, and characteristic parameters for OM pores including the plane porosity (Phi) (the average dropped from 23.14% to 10.33%), the equivalent circle diameter (ECD) (the average dropped from 28.38 nm to 10.09 nm), the perimeter over area (PoA) (the average rose from 0.1126 to 0.2718), and the dominant pore diameter (DOM size) (the average dropped from 113.80 nm to 20.78 nm). The reactivation of seepage channels and intrusion of brittle minerals are proposed as two main microscopic forcing mechanisms, updated and innovative conceptual models are proposed to reconstruct an OM pore response processes under a differential tectonic deformation regime in panoramic view for Lower Paleozoic marine gas shale reservoirs of China. Thus, an evolution of spatial resolution spanning a total of three different scales respectively as structural styles, shale reservoir architectures, and microscopic petrological compositions are together portrayed.

The gas content in shale reservoirs is often determined by the micro storage and sealing capacities of the reservoir. Deep shale reservoirs are in the high- or over-thermale maturity stage and have complex pore structure and connectivity, which are highly heterogeneous in vertical distribution. Research on the gas-bearing property of deep shale reservoirs is limited by these complex microscopic conditions. To analyze the gas-bearing characteristics of deep shale reservoirs, this work collected and summarized data on total organic carbon content, mineral composition, porosity, water saturation, and gas content measured on-site for the Longmaxi Formation in the Sichuan Basin in southern Sichuan, China. Then, experimental methods, such as Xray photoelectron spectroscopy, transmission electron microscope, low-pressure N-2 adsorption, spontaneous imbibition, and high-pressure methane adsorption, were used to analyze the micro storage and sealing capacities of the deep shale reservoirs. The results show that, different from shallow shale reservoirs (<3500 m), deep shale reservoirs have a higher graphitization degree and water saturation. An abundance of graphite structures often leads to weak resistance of organic matter to compression, deformation, or even collapse of pores in organic matter and severe damage to the gas storage space. However, a higher degree of graphitization can enhance the ability of the shale reservoirs to adsorb gas and self-sealing. The high water saturation in the reservoirs can interact with clay minerals and negatively affect the gas accumulation, storage, and transmission capacities of the shale reservoirs. However, the upper shale reservoirs with higher water saturation can seal the lower shale reservoirs, helping it preserve shale gas. Based on the vertical distribution of graphite structure, clay minerals contents, lithofacies, and water content in deep shale reservoirs, the essential microscopic conditions for deep shale reservoirs to have high gas content were proposed. This paper provides a detailed explanation and evaluation of deep shale's storage and sealing capacities at the microscopic scale and can serve as a reference for further identifying the patterns for high-yield and rich shale gas reservoirs and improving deep shale gas exploration technologies.