Liujiagou Formation Research Articles

Paradigm for Determining the Optimal Ultradeep and Super‐thick Saline Aquifer for High‐TDS Mine Water Geological Storage

AbstractSaline aquifers are the most popular waste and CO2 injection and storage reservoirs worldwide. This project proposes that several optimal injection positions should be investigated as hydraulic pressure‐focused positions, in order to relieve the high demands of pump performance. The comprehensive indices (Fi) representing the injectivity of different burial depths were obtained by using information entropy, based on the mercury injection experimental data of 13 rock samples. The results demonstrated that the burial depths of No. 4, No. 1 and No. 2 in the Liujiagou Formation were the most suitable positions for hydraulic focused injection, which means the upper 30 m thickness could be regarded as the hydraulic focused range in the saline aquifer with an average thickness of 400 m. In addition, some laboratory experiments and in situ tests were carried out for the purpose of certifying and analyzing results, including SEM, XRD, brittleness index and logging. The results suggested that the rock samples at the No. 4, No. 1 and No. 2 burial depth ranges have loose microstructure, weak cementation, as well as dual pores and fractures. The lithology is mainly quartz and feldspar, but the clay mineral content is high (10%–25%), which is positive for dissolution. The lithology is suitable for hydraulic fracturing to form extended cracks and micro‐fissures during high‐TDS (total dissolved solids) mine water injection, because of the high brittleness index. Finally, a theoretical and technical framework for high‐TDS mine water injection was established, based on operating pilot engineering. Some theoretical defects and drawbacks learned from the field practices were summarized and solutions proposed. The research in this study could provide guidance and a paradigm for the inexpensive treatment of high‐TDS mine water by injection and storage.

Ecological-Based Mining: A Coal–Water–Thermal Collaborative Paradigm in Ecologically Fragile Areas in Western China

A substantial reduction in groundwater level, exacerbated by coal mining activities, is intensifying water scarcity in western China’s ecologically fragile coal mining areas. China’s national strategic goal of achieving a carbon peak and carbon neutrality has made eco-friendly mining that prioritizes the protection and efficient use of water resources essential. Based on the resource characteristics of mine water and heat hazards, an intensive coal–water–thermal collaborative co-mining paradigm for the duration of the mining process is proposed. An integrated system for the production, supply, and storage of mining companion resources is achieved through technologies such as roof water inrush prevention and control, hydrothermal quality improvement, and deep-injection geological storage. An active preventive and control system achieved by adjusting the mining technology and a passive system centered on multi-objective drainage and grouting treatment are suggested, in accordance with the original geological characteristics and dynamic process of water inrush. By implementing advanced multi-objective drainage, specifically designed to address the “skylight-type” water inrush mode in the Yulin mining area of Shaanxi Province, a substantial reduction of 50% in water drillings and inflow was achieved, leading to stabilized water conditions that effectively ensure subsequent safe coal mining. An integrated-energy complementary model that incorporates the clean production concept of heat utilization is also proposed. The findings indicate a potential saving of 8419 t of standard coal by using water and air heat as an alternative heating source for the Xiaojihan coalmine, resulting in an impressive energy conservation of 50.2% and a notable 24.2% reduction in carbon emissions. The ultra-deep sustained water injection of 100 m3·h−1 in a single well would not rupture the formation or cause water leakage, and 7.87 × 105 t of mine water could be effectively stored in the Liujiagou Formation, presenting a viable method for mine–water management in the Ordos Basin and providing insights for green and low-carbon mining.

Computed X‐ray Tomography Investigation of Porosity and Permeability of the Liujiagou Formation Sandstone Exposed to CO2‐Saturated Brine

AbstractIn order to improve CO2 capture, utilization and storage (CCUS) to solve carbon emission, sandstone from the Triassic Liujiagou Formation (LF) from the Ordos Basin in China was investigated using permeability tests and computed X‐ray tomography (CT) scanning. The presence of reactive minerals within the geological CO2 sequestration target storage formation can allow reaction with injected CO2, which changes the porosity and permeability of the LF beds, affecting storage effectiveness. To investigate the effect of chemical reactions on the pore structure and permeability of sandstone cores representing the LF CO2 storage, tests were conducted to analyze the changes in porosity and permeability of sandstone cores induced by CO2‐saturated brine at different reaction times (28‐day maximum reaction period). Porosity and permeability of the sandstone increased after reaction with CO2‐saturated brine due to mineral dissolution. The sandstone exhibited an increase in porosity and permeability after 15 days of reaction with CO2‐saturated brine. Moreover, there was an increase in the volume of large pores in the sandstone after the 28‐day period. The pore network of the sandstone was established through CT results, and the porosity calculated based on the obtained pore network was close to that measured in the test, demonstrating the feasibility to use CT to study the evolution of the microstructure of sandstone after long‐time exposure to CO2‐saturated brine.

The variations of terrestrial trace fossils and sedimentary substrates after the end‐Permian extinction in the Dengfeng area, North China

Evolution of terrestrial infauna in the Early Triassic enables to understand terrestrial biotic recovery after the end‐Permian mass extinction (EPME). Six ichnospecies in two ichnogenera, including Skolithos linearis, S. verticalis, S. annulatus, S. isp., Planolites montanus, and P. beverleyensis, were recognized from the Lower Triassic Liujiagou Formation (Induan Stage) and Heshanggou Formation (Olenekian Stage) in the Baiping section of the Dengfeng area, North China. The ichno‐ecological analyses of ichno‐taxonomic abundance, ichnodiversity, bioturbation index, burrow size, and tiering level revealed that terrestrial infauna expanded from epibenthic to endobenthic through the Early Triassic. The tiering deepening suggested that tracemakers gradually began to excavate into the sediments instead of moving on the microbial mats, indicating that the burrowing capability of trace‐making animals has recovered in the Olenekian. Four types of sedimentary substrates were summarized: non‐bioturbated matground, matground with bioturbated patches, weakly bioturbated mixed ground, and moderately bioturbated mixed ground. Occurrence of the substrate of mixed ground may be a sign of the initial recovery of the terrestrial ecosystem after the EPME.

Deep Groundwater Flow Patterns Induced by Mine Water Injection Activity

Mine water injection into deep formations is one of the effective approaches for reducing the drainage from coal mines in the arid and semi-arid region of the Ordos basin, China. Many coal mines are attempting to execute the related projects. Under the influence of groundwater protection, the understanding of regional groundwater flow is becoming highly important to the mine water monitoring, whereas quite few academic research teams focus on the deep groundwater flow pattern by mine water injection. This paper reveals the spatial distribution of Liujiagou Formation that is in positive correlation with the terrain, and its local thickness is influenced by the dominant W-E and NE-SW directions of geological structures. Only a part of sandstone rocks consists of aquifers, the rest 61.9% of relatively dry rock provide the enhanced storage space and partial mudstone aquicludes decrease the possibility of the vertical leakage for mine water. The dynamic storage capacity is evaluated at 2.36 Mm3 per 1 km2 and over 25.10 billion m3 in this study area. Two hydrogeologic cross-sections of basin-scale identify the W-E and N-S regional groundwater flow directions, with the lower Yellow River catchment becoming the discharged region. The hierarchically and steadily nested flow systems containing coal mining claims are influenced by coal mining activity. The groundwater depression cone in a shallow coal measure aquifer is caused by mine water drainage whereas the groundwater mound in Liujiagou Formation is generated by mine water injection activity. The numerical simulation revealed that the groundwater head rebound is slightly decreased and will not recover to its initial baseline within 500 years due to its low porosity and permeability. This study elucidates the deep groundwater flow patterns induced by mine water injection and provides a practical methodology for the management and pollution monitoring of mine water injection activity.

Geochemistry of the Permian–Triassic boundary strata in the southwest margin of the North China Craton: Provenance and tectonic implications

The Late Permian Sunjiagou Formation and Early Triassic Liujiagou Formation were studied to understand the palaeoclimatic and palaeoenvironmental changes in the southwest margin of the North China Craton across the Permian–Triassic boundary, as well as the tectonic evolution of their provenance sediment. Major, trace, and rare earth elements in 20 clastic rock samples from the strata above and below the boundary were analysed. Overall, the Sunjiagou Formation samples showed an average index of compositional variability (ICV) and chemical index of alteration (CIA) of 1.53 and 73.59, respectively, indicating high‐maturity sediments in a warm and humid environment with moderate chemical weathering, whereas Liujiagou Formation samples had an average ICV and CIA of 2.40 and 70.1, respectively, indicating low‐maturity sediments in a cold and dry environment with mild chemical weathering. These strata were composed of first‐cycle sediments in a tectonically active area. The provenance of the Sunjiagou Formation was dominated by quartz‐rich sedimentary rocks, whereas that of Liujiagou Formation had both quartz‐rich sedimentary rocks and intermediate volcanic rocks. The provenance tectonic settings were mainly the continental island arc and active continental margin. Thus, the study area was in a terrestrial, freshwater sedimentary environment from the Late Permian to Early Triassic, and the climate gradually changed from warm and humid to dry and hot, with multiple small‐scale periodic evolutions. Taken together, these findings further confirm that the entire study area was in an oxidizing environment and the oxidation degree in the Early Triassic was higher than that in the Late Permian.

Sedimentology of the latest Permian to Early Triassic in the terrestrial settings of the North China Basin: Low-latitude climate change during a warming-driven crisis

Abstract In terrestrial settings, the Permo-Triassic mass extinction is commonly linked to major changes in sedimentological and climatic conditions that include a switch from meandering to braided fluvial systems and increased aridity. We examined the predominantly terrestrial strata of North China to reveal that, on the contrary, there was little substantial sedimentological change during the Permo-Triassic mass extinction. The crisis level occurs in the upper Sunjiagou Formation, where a range of environments, including low-sinuosity rivers, distally terminating alluvial fans, and mud-dominated coastal plains are recorded. A major sedimentological change occurred slightly higher, within the lowest Triassic, and involved a switch to braided fluvial and shallow lacustrine/fluviodeltaic conditions of the Liujiagou Formation. Later in the Early Triassic, fine-grained, playa lake, and alluvial plain facies became widespread (Heshanggou Formation). Paleosols are present throughout the basin fill and record a transition from Aridisols and Vertisols in the Late Permian to Inceptisols in the Early Triassic followed by gleyed Aridisols, Vertisols, and Inceptisols later in the Early Triassic. Previously reported aeolian facies from Lower Triassic strata were not encountered in this study and were unlikely to have occurred given the prevalence of lacustrine and fluvial conditions at this time. Overall, the North China Basin experienced an increase in seasonal rainfall in the latest Permian and a base-level rise that introduced marine conditions into the southwest of the basin. During the Early Triassic, seasonal or interannual rainfall was frequent, with strong evaporation in a hot climate. Overall, substantial sedimentological changes occurred in the terrestrial Permo-Triassic environments of North China, but despite earlier claims from studies in North China and elsewhere, there was no abrupt transition in fluvial styles around the Permo-Triassic mass extinction.

Late Permian–Middle Triassic magnetostratigraphy in North China and its implications for terrestrial-marine correlations

A detailed magnetostratigraphic study, linked to a new latest Permian U–Pb ID-TIMS age, was undertaken on the continental Shichuanhe section (SCH) in North China in order to provide a magnetic polarity scale for the Late Permian–early Middle Triassic interval. Tilt-corrected mean directions of the characteristic remanent magnetization pass the reversal test and correspond to a site paleolatitude of 18.1°N during the Early Triassic, consistent with previous results from the North China Block. The magnetostratigraphy shows close similarity with previous studies, allowing interregional correlations with both marine and non-marine records. Normal magnetozone SCH3n, constrained by an absolute age of 252.21±0.15 Ma from an ash bed 3.5 m below its base, is unambiguously correlated to the earliest Triassic normal magnetochron LT1n. Our newly established magnetostratigraphic framework and published carbon-isotope chemostratigraphy, indicate that the Permian–Triassic Boundary is ca. 8 m above the base of SCH3n (within the middle part of the Sunjiagou Formation) at SCH. The overlying reverse polarity dominated interval (SCH3r–SCH5r) ranges to the middle Liujiagou Formation, and straddles an interval from the mid-Griesbachian to mid-Smithian. The base of the Olenekian is provisionally located in the lower part of the Liujiagou Formation, near the base of magnetozone SCH5n. The succeeding thick normal magnetozone SCH6n persists into the upper Heshanggou Formation, with the inferred Smithian–Spathian boundary located in the upper part of the Liujiagou Formation. The transition from reverse magnetozone SCH6r to the overlying normal magnetozone SCH7n, coincides with a clear erosional contact with the base of the Ermaying Formation. Consequently, magnetozone SCH7n is matched to the Early Anisian magnetochron MT3n, with the Olenekian–Anisian boundary interval missing. Our new timescale provides additional magnetostratigraphic constraints on the timing of the terrestrial ecological crisis in North China, which is found to lie within reverse magnetozone SCH2r (equivalent with reverse magnetochron LP3r), a level some 270±150 kyrs before the main marine extinction, that falls in the overlying normal magnetochron LT1n.

Leakage Plugging Technology and Its Application at the Bottom of Liujiagou Formation in Shaanxi Province

Leakage Plugging Technology and Its Application at the Bottom of Liujiagou Formation in Shaanxi Province

Minewater deep transfer and storage

Subjected to the environmental influence of minewater drainage, a new method of minewater deep transfer and storage (MWDTS) was proposed and trialed in the eastern margin of Ordos basin, China. Based on the scientific issue, why, where, how, what and the consequence of MWDTS were presented in this paper. Two target geological mediums, including Baotashan coarse sandstone (BCS) in Yan'an Formation of Jurassic and Liujiagou Formation (LFm) of Triassic, were compared and analyzed to acquire the ideal aquifer as the groundwater reservoir. LFm was eventually selected as the final minewater reservoir and the scientific well named MC-1 was constructed, and the permeability of LFm reservoir was enhanced by artificial hydraulic fracturing. The laboratory tests including SEM, XRD, mercury injection, mechanical strength, microscopic permeability evolution and hydrochemical composition, as well as the field tests covering groundwater table recovery and different pressure water injection, revealed that the deep LFm was the optimal minewater reservoir. The original LFm was low porosity at 7.5% and lower permeability at 5.31 × 10−6 m/d to 6.19 × 10−6 m/d. The space with natural & artificial hydraulic fracture and pores was the main storage space. The pressure of hydraulic fracturing should be greater than 31.5 MPa, the continuous MWDTS would change the mechanical strength, and the whole permeability of the screening section was shown 1788 to 2858 times greater than natural. As the continued reinjection of minewater, the total amount of 1.00 million cubic meters had conducted. The capacity of MWDTS in the rejection well was predicted at 2.85 to 3.68 million cubic meters. On the basis of technical and economic evaluation, it would reduce the environmental pollution caused by minewater drainage and be one option method for minewater management of coal mine enterprises in the Ordos Basin.

Characteristic and paleoenvironment significance of microbially induced sedimentary structures (MISS) in terrestrial facies across P-T boundary in Western Henan Province, North China

AbstractWe report varied microbially induced sedimentary structures (MISS), and other sedimentary surface textures, from the Induan (Early Triassic) Sunjiagou Formation and Liujiagou Formation in the Xingyang, Dengfeng, Jiyuan and Yiyang areas, western Henan Province, North China. Microanalysis shows that these MISS are characterized by a U-shaped structure, thin clayey laminae, and discontinuous mica sheet that are arranged parallel to the bedding plane, as well as directionally oriented quartz grains floating in lamina, which are indicative of a biogenic origin. The MISS of the studied area were probably affected by four main factors, including the end-Permian mass extinction, the megamonsoon, the adapted sedimentary environment, and the sediment supply, and they possess significant stratigraphic correlation. Abundant microbial-related sedimentary structures from the study area indicate that continental ecosystems were severely devastated in the aftermath of the Permian-Triassic biocrisis. These sedimentary structure assemblages, including MISS, red beds, conglomerate layers, and calcareous concretions in the western Henan Province, show a specific, post-extinction continental ecosystem that was characterized by microflora dominance, monotonous and rare fossils, extreme hot climate, soil ecosystem devastation, and poor vegetation.

Intensifying aeolian activity following the end‐Permian mass extinction: Evidence from the Late Permian–Early Triassic terrestrial sedimentary record of the Ordos Basin, North China

AbstractSedimentary successions provide direct evidence of climate and tectonics, and these give clues about the causes of the mass extinction around the Permian–Triassic boundary. Terrestrial Permian–Triassic boundary strata in the eastern Ordos Basin, North China, include the Late Permian Sunjiagou, Early Triassic Liujiagou and late Early Triassic Heshanggou formations in ascending order. The Sunjiagou Formation comprises cross‐bedded sandstones overlaid by mudstones, indicating meandering rivers with channel, point bar and floodplain deposits. The Liujiagou Formation was formed in braided rivers of arid sand bars interacting with some aeolian dune deposits, distinguished by abundant sandstones where diverse trough and planar cross‐bedding and aeolian structures (for example, inverse climbing‐ripple, translatent‐ripple lamination, grainfall and grainflow laminations) interchange vertically and laterally. The Heshanggou Formation is a rhythmic succession of mudstones interbedded with thin medium‐grained sandstones mainly deposited in a shallow lacustrine environment. Overall, the sharp meandering to braided to shallow lake sedimentary transition documents palaeoenvironmental changes from semi‐arid to arid and then to semi‐humid conditions across the Permian–Triassic boundary. The die‐off of tetrapods and plants, decreased bioturbation levels in the uppermost Sunjiagou Formation, and the bloom of microbially‐induced sedimentary structures in the Liujiagou Formation marks the mass extinction around the Permian–Triassic boundary. The disappearance of microbially‐induced sedimentary structures, increasingly intense bioturbation from bottom to top and the reoccurrence of reptile footprints in the Heshanggou Formation reveal gradual recovery of the ecosystem after the Permian–Triassic boundary extinction. This study is the first to identify the intensification of aeolian activity following the end‐Permian mass extinction in North China. Moreover, while northern North China continued to be uplifted tectonically from the Late Palaeozoic to Late Mesozoic, the switch of sedimentary patterns across the Permian–Triassic boundary in Shanxi is largely linked to the development of an arid and subsequently semi‐humid climate condition, which probably directly affected the collapse and delayed recovery in palaeoecosystems.

Early Triassic Conversion from Source to Sink on the Southern Margin of the North China Craton: Constraints by Detrital Zircon U-Pb Ages

Provenance analysis of sediments provides important constraints on basin formation and orogenic processes. With the aim to define the sedimentary provenance and tectonic evolution of the southern margin of the North China Craton, this paper presents new detrital zircon U-Pb data from Early Triassic sediments in the Yiyang area. The results showed major peaks at 1848, 458, 425, and 268 Ma and subordinate peaks at ca. 2500, 872, and 957 Ma on age spectra from the Liujiagou Formation. The Heshanggou Formation exhibited a major age peak at 445 Ma and subordinate peaks at 755 and 947 Ma. Integrated with the analysis of sandstone detrital compositions, we suggest that the sources of the Liujiagou Formation were mainly a mixture of the southern margin of the North China Craton and the North Qinling Orogenic Belt, whereas the Heshanggou Formation was derived primarily from the North Qinling Orogenic Belt. Age comparisons of detrital zircon geochronology collected from different basins in the North China Craton indicated that the paleogeography of the North China Craton during the Early Triassic was strongly asymmetric, wherein the uplifted highland along the southern margin of the North China Craton was relatively lower than the northern margin. Meanwhile, the marked shift in source region from the Liujiagou to the Heshanggou formations provides a constraint regarding the conversion from denuded zone to deposited zone along the southern margin of the North China Craton in the Early Triassic, which controlled the evolution of the provenance and sedimentary system.

Provenance analyses of early Mesozoic sediments in the Ningwu basin: Implications for the tectonic–palaeogeographic evolution of the northcentral North China Craton

ABSTRACTThis paper reports results from detrital zircon U–Pb geochronology, Hf isotopic geochemistry, sandstone modal analysis, and palaeocurrent analysis of the early Mesozoic strata within the Ningwu basin, China, with the aims of constraining the depositional ages and sedimentary provenances and shedding new light on the Mesozoic tectonic evolution of the northcentral North China Craton (NCC). The zircons from early Mesozoic sandstones are characterized by three major populations: Phanerozoic (late Palaeozoic and early Mesozoic), late Palaeoproterozoic (with a peak at approximately 1.8 Ga), and Neoarchaean (with a peak at approximately 2.5 Ga). Notably, three Phanerozoic zircons in the Early Triassic Liujiagou Formation were found to have positive εHf(t) values and characteristics typical of zircons from the Central Asian Orogenic Belt (CAOB). Therefore, the CAOB began to represent the provenance of sediment in the sedimentary basins in the northern NCC no later than the Early Triassic (261 Ma), implying that the final amalgamation of the NCC and CAOB occurred before the Early Triassic. The U–Pb geochronologic and Hf isotopic results show that the Lower Middle Triassic sediments were mainly sourced from the Yinshan–Yanshan Orogenic Belt (YYOB), and that a sudden change in provenances occurred, shifting from a mixed YYOB and CAOB source in the Middle Jurassic to a primarily YYOB source in the Late Jurassic. The results of the sandstone modal analysis suggest that the majority of the samples from the Lower Middle Jurassic rocks were derived from either Continental Block or Recycled Orogen sources, whereas all the samples from the Upper Jurassic rocks were derived from Mixed sources. The change in source might be ascribed to the southward subduction and closure of the Okhotsk Ocean and the resulting intense uplift of the YYOB during the Late Jurassic. This uplift likely represents the start of the Yanshan Orogeny.

A Geostatistical Study in Support of CO2 Storage in Deep Saline Aquifers of the Shenhua CCS Project, Ordos Basin, China

The Shenhua Carbon Capture and Storage (CCS) project at the Shenbei Slope injection site in North Yulin is the first 100,000 ton-per-year scale CCS pilot project in China with an injection operation lasting nearly 3 years. In this study, we investigate various geostatistical methods and their impact on the respective geologic models on which simulation is performed to understand the phenomena observed during 3 years of Shenhua CCS operations. Although there was a brief period of wellhead pressure increase at the injection well, it unexpectedly dropped for most of the time. Another interesting observation showed that the majority of CO2 gas injection was received by the topmost sandstone Liujiagou formation instead of the basement limestone Majiagou formation, which was predicted to have much more injectivity and storage capacity. Based on the current geostatistical methods and available data, 3 steps of reservoir modeling and flow simulation are carried out and they go from having homogeneous property models to incorporating standard 2-point geostatistical methods to using object-based models. The layer-cake models generate a rather uniform plume shape and increased pressure response. Meanwhile, two-point statistical models add more complexity to the size and shape of CO2 plume, however are not capable of reproducing the pressure decline behavior. These results demonstrate homogeneous and 2-point geostatistical models are inadequate in interpreting subsurface heterogeneity, both due to their method and data limitations. Further work is being done with object-based models to produce a system of meandering rivers based on the geological concept of Shenhua injection site. This will help offset the data limitation and bring our model closer to geologic reality.

Reactive transport modeling of long-term CO2 sequestration mechanisms at the Shenhua CCS demonstration project, China

Carbon dioxide injection into deep saline aquifers results in a variety of strongly coupled physical and chemical processes. In this study, reactive transport simulations using a 2-D radial model were performed to investigate the fate of the injected CO2, the effect of CO2-water-rock interactions on mineral alteration, and the long-term CO2 sequestration mechanisms of the Liujiagou Formation sandstone at the Shenhua CCS (carbon capture and storage) pilot site of China. Carbon dioxide was injected at a constant rate of 0.1 Mt/year for 30 years, and the fluid flow and geochemical transport simulation was run for a period of 10 000 years by the TOUGHREACT code according to the underground conditions of the Liujiagou Formation. The results show that different trapping phases of CO2 vary with time. Sensitivity analyses indicate that plagioclase composition and chlorite presence are the most significant determinants of stable carbonate minerals and CO2 mineral trapping capacity. For arkosic arenite in the Liujiagou Formation, CO2 can be immobilized by precipitation of ankerite, magnesite, siderite, dawsonite, and calcite for different mineral compositions, with Ca2+, Mg2+, Fe2+ and Na+ provided by dissolution of calcite, albite (or oligoclase) and chlorite. This study can provide useful insights into the geochemistry of CO2 storage in other arkosic arenite (feldspar rich sandstone) formations at other pilots or target sites.

An object-based modeling and sensitivity analysis study in support of CO2 storage in deep saline aquifers at the Shenhua site, Ordos Basin

The Shenhua Carbon Capture and Storage (CCS) project at the Shenbei Slope injection site in North Yulin is the first 100,000 ton/year scale CCS pilot project in China with an injection operation lasting nearly 3 years. While the project turned into an operational success with 300,000 tons of CO2 being sequestered, several aspects of reservoir dynamic behavior and the role of reservoir heterogeneity are not clearly understood. For example, although there was an initial period of wellhead pressure (WHP) increase at the injection well, WHP incrementally declined for most of the time. The majority of CO2 was received by the topmost sandstone of the Liujiagou formation in the injection interval instead of the lowermost limestone of the Majiagou formation, suggesting strong reservoir heterogeneity. Knowledge of the key reservoir processes and properties that resulted in these observations would help for predicting long-term storage safety and aiding the design of a larger-scale CCS operation at the same site. In this study, we investigate various object-based models and important reservoir parameters to determine aspects of heterogeneity that have the most impact on pressure behavior at the injection well and the observed plume dynamics after injection. Simulation results suggest that the extent of CO2 plume is most sensitive to the geometry of fluvial channels, and specifically, the amount of connected sandbodies that facilitate lateral fluid migration. Along with the injection rate, sandbody permeability plays a significant role in determining pressure near the injection well and in the reservoir. Further work will incorporate the dynamics of CO2 flow down the injection wellbore to derive a more accurate representation of injection rate under the reservoir condition. As drilling and analog data also suggest that fractures may exist in the Liujiagou formation, future work will also construct geomechanical models to investigate the effect of fractures on injection and storage.

Simulation and Analysis of Long-Term CO2 Trapping for the Shenhua CCS Demonstration Project in the Ordos Basin

The Shenhua CO2 capture and sequestration (CCS) project has achieved its goal of injecting 100,000 tons/year CO2 into the saline aquifers of the Ordos Basin. This study analyzes the geochemical interactions between CO2, formation fluid, and host rock of the major formations in the Ordos Basin, assesses the CO2 trapping capabilities, and predicts the final mineral forms of injected CO2. Reactive transport simulations are performed using a 2D radial model, which represents a homogeneous formation. The results show that 80% of injected CO2 remains as free supercritical gas in each formation after injection, while most of CO2 is sequestrated in different carbonate mineral assemblages after 10,000 years. The CO2 mineral trapping capacities of the Shiqianfeng and Shihezi formations are smaller than the Liujiagou formation. Calcite, dawsonite, and siderite are stable CO2 trapping minerals, while dolomite, ankerite, and magnesite are not. The increase in porosity and permeability of the three formations in the first 100 years agrees with the observation from the Shenhua CCS Project. Also the decrease in porosity and permeability after 100 years shows agreement with other modelling studies using the similar methods. These results are useful for the evaluation of the geochemical process in long-term CO2 geological storage.

Experimental Study on the Effects of Stress Variations on the Permeability of Feldspar-Quartz Sandstone

The multistage and discontinuous nature of the injection process used in the geological storage of CO2 causes reservoirs to experience repeated loading and unloading. The reservoir permeability changes caused by this phenomenon directly impact the CO2 injection process and the process of CO2 migration in the reservoirs. Through laboratory experiments, variations in the permeability of sandstone in the Liujiagou formation of the Ordos CO2 capture and storage (CCS) demonstration project were analyzed using cyclic variations in injection pressure and confining pressure and multistage loading and unloading. The variation in the micropore structure and its influence on the permeability were analyzed based on micropore structure tests. In addition, the effects of multiple stress changes on the permeability of the same type of rock with different clay minerals content were also analyzed. More attention should be devoted to the influence of pressure variations on permeability in evaluations of storage potential and studies of CO2 migration in reservoirs in CCS engineering.

Microbial mats in the terrestrial Lower Triassic of North China and implications for the Permian–Triassic mass extinction

Evidence for microbial mats has been reported repeatedly from marine Lower Triassic rocks, but scarcely mentioned in post-mass extinction terrestrial facies. Here, we report from the terrestrial Lower Triassic Liujiagou Formation in North China the presence of five kinds of microbially induced sedimentary structures (MISS) or sedimentary surface textures, including “old elephant skin” textures, wrinkle structures, palimpsest ripples, “Manchuriophycus” structures and sand cracks. Terrestrial microbial communities that produced these MISS adapted not only to periodically desiccated conditions, but also to the storm-dominated palaeoenvironments in the Liujiagou Formation. The Permian–Triassic mass extinction (PTME) in North China is marked by the die-off of plants, disappearance of coal beds, extinction of pareiasaurs among tetrapods, decreased bioturbation levels and a dramatic change of sedimentary systems through the Sunjiagou Formation. The Sunjiagou Formation recorded the turnover from an ever-wet to a progressively drier and hotter climate and it spans the PTME in North China. Following this mass extinction, MISS became much more common and widespread, suggesting that the mass extinction provided favourable biological and environmental conditions for the development of the MISS in terrestrial ecosystems, especially the decreased bioturbation intensity and grazing pressure associated with increased temperature and climatic drying. In the upper part of the Liujiagou Formation and overlying Heshanggou Formation, the disappearance of MISS coupled with increased bioturbation might indicate an improvement of terrestrial ecosystems and the beginning of the Triassic biotic recovery. However, as the investigation of MISS in space and time through the geological record is in its early stages, further geobiologic and geochemical studies, as well as high-precision isotopic dating from Permian–Triassic terrestrial successions, are needed to fully reveal the timing and pattern of the Early Triassic terrestrial ecosystem reconstruction.