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Objective Well C902, located in the Ganchaigou area of the western Qaidam Basin, achieved a breakthrough in shale oil exploration as indicated by the high-production oil flow in the Ⅳ-Ⅵ layers of the Upper Member of the Palaeogene Xiaganchaigou Formation. However, ambiguities in evaluation criteria and resource potential limit the efficiency of shale oil development in the Upper Member of the Xiaganchaigou Formation in the Ganchaigou area. Methods Thus, the exploration potential of shale oil in the Ⅳ-Ⅵ layers of the Upper Member of the Xiaganchaigou Formation in the Ganchaigou area was comprehensively evaluated by comparing the lithology, physical properties, pore structure, and organic geochemistry between Well C902 and other wells. Results The complicated lithofacies of laminar/lamellar dolostone, laminar/lamellar limestone, laminar clay shale, and lamellar mudstone suggest the mixed carbonate and siliciclastic deposition of the Ⅳ-Ⅵ layers. The high brittleness index implies the good fracability of the Ⅳ-Ⅵ layers. The intergranular pores and laminar fractures, with a total proportion of 85% in the pore system, were dominant in the Ⅳ-Ⅵ layers that can be roughly classified as low-porosity and ultralow-permeability reservoirs. The organic matter, dominated by type Ⅰ-Ⅱ1, shows an average TOC content of more than 0.9% in the Ⅳ-Ⅵ layers. Both chloroform asphalt "A" and S1+S2 indicate a high hydrocarbon generation capacity. Conclusion Summarily, the widely deposited the Ⅳ-Ⅵ layers of the Upper Member of the Xiaganchaigou Formation have the characteristic of source-reservoir integration and show not only good quality of source rock but also strong fracability of the shale oil reservoir. The excellent source-reservoir-caprock assemblage is also widely developed, which ensures good sealing ability and favors improving oil and gas recovery after fracturing. This study can theoretically improve the development efficiency of shale oil in the Ganchaigou area.

Abstract This paper presents a linear-nonlinear switching control strategy, called Switching Active Disturbance Rejection Control (SADRC), to enhance the disturbance rejection capability of the speed controller in a servo system. SADRC combines the advantages of Linear Active Disturbance Rejection Control (LADRC) and Nonlinear Active Disturbance Rejection Control (NLADRC), and introduces a parameter to switch between nonlinear and linear control, thereby improving the robustness of the servo system. Firstly, the mathematical model of the motor is analyzed as the starting point of the paper. Then, the basic principles of Active Disturbance Rejection Control (ADRC) are analyzed, and improvements are made to address its limitations, resulting in the design of SADRC. The parameters introduced in SADRC are analyzed to determine their appropriate ranges. Finally, the performance of SADRC is validated by comparing the rotational effects of Permanent Magnet Synchronous Motor (PMSM).

CO2 is being considered as an effective alternative working fluid for geothermal applications due to its superior fluid dynamics and heat transfer properties compared to water. Utilizing sedimentary rocks for geothermal energy recovery through a CO2-plume geothermal system, especially in carbonate reservoirs, has been shown to be a practical approach that eliminates the need for hydraulic fracturing. However, uncertainties remain regarding the thermal and hydraulic behavior, particularly the chemical interactions between CO2 and carbonate rocks. This study develops a comprehensive wellbore–reservoir coupling reactive transport model based on specific information obtained from the Ordovician limestone geothermal reservoir in Shandong, China. The model aims to assess the feasibility of heat extraction in carbonate reservoirs by evaluating the heat extraction performance and fluid–rock interaction. The results indicate a rapid temperature drop after CO2 breakthrough due to the Joule–Thomson effect. Simultaneously, the fluid transitions into and maintains a two-phase state throughout the operation. Chemical reactions within the reservoir are not aggressive since complete mixing between unsaturated water and CO2 only occurs in the vicinity of the production well, highlighting the potential of utilizing carbonate reservoirs for efficient heat extraction in geothermal systems. Further research is needed to optimize the performance of CO2-based geothermal systems in carbonate reservoirs.

This study selected ten condition factors from the perspectives of topography, geological structure, hydrology and human for mining subsidence mapping by two types of models (knowledge-driven model and data-driven model) by taking Jining City, China as an example. The knowledge-driven model is achieved by analytic hierarchy process (AHP), and the data-driven model is achieved by frequency ratio (FR), combined with fuzzy comprehensive evaluation (FCE) and entropy evaluation method (EWM). Based on these models, the areas under receiver operating characteristic curve are 0.777, 0.949, and 0.938, which means FR-FCE obtained the highest accuracy. The differences reflected by the three methods are concentrated in Liangshan County and central to northern areas of Weishan County. The best model was proposed for the protection measures and land use management. Finally, the factors system and methods used in this study can be widely applied to the mining subsidence susceptibility evaluation in plain areas.

Yutai Sag is a Mesozoic synsedimentary fault basin. It is characterized by much fault intersection and massive Ordovician carbonate rock in the deep, which indicates a good prospect of geothermal development. To study the characteristics of carbonate thermal reservoirs in this fault basin and evaluate its geothermal resource potential, a hole with a depth of 2 309.31 m was drilled in Yutai Sag. Through comprehensive logging, productivity testing, hydrochemical analysis, gas composition analysis, and geothermal water 14C age determination, the sources and supply of thermal fluid, heat source storage, and ion migration in the interior of the basin are analyzed.The result shows that the temperature anomaly presented in the geothermal well temperature curve and the fault position characterized by many fractures are consistent, indicating the source direction of geothermal water.The geothermal water with high content of Cl- and Na- is related to halite dissolution. The apparent age of geothermal water is 43.5 ka BP near the confluence of faults in the southeastern Yutai Sag, and the corrected age is 10.752 ka BP which is from the late Quaternary Pleistocene to the early Holocene.The study shows that there are relatively active or young geological structures in Yutai Sag, which are one of the thermal sources of thermal reservoirs. The recoverable geothermal resources of the Ordovician thermal reservoir in the area are approximately 2.12×109 GJ, equivalent to 7.27×107 t of standard coal, implying great resource potential and good prospects for development and utilization.

In recent years, oil and gas exploration of the Baikouquan Formation in the northwestern margin of the Junggar Basin has seen major breakthroughs have been made in oil and gas exploration. Sandstone and conglomerate reservoirs are developed in the Baikouquan Formation in this area, but the factors affecting the differences in petro physical properties of these two types of reservoirs are still unclear. In this paper, taking the Baikouquan Formation in the Hongcheguai area on the northwestern margin of the Junggar Basin as an example, a large number of core observations, thin sections, scanning electron microscopy, and petro physical property test data were used to evaluate the petro logical characteristics, reservoir space types, digenesis and pore evolution of the sandstone and conglomerate reservoirs. The results show that the sandstone and conglomerate rocks of the Baikouquan Formation in the study area have a large content of tuffaceous components. The sandstone reservoirs are dominated by primary intergranular pores, and the conglomerate reservoirs are dominated by secondary dissolved pores. In addition, the porosity is negatively correlated with rock permeability for the conglomerate rocks of the Baikouquan Formation. However, the porosity is positively correlated with rock permeability of the sandstone reservoirs. According to the analysis of the digenetic process, the compaction and cementation can lead to a significant decrease in the petro physical properties of the reservoir. While dissolution can improve the petro physical properties of the sandstone reservoirs to a small extent, and can improve that of the conglomerate reservoirs to a greater extent. Moreover, local fractures are important factors affecting the petro physical properties of the conglomerate reservoirs. The existence of abnormal high pressure has a certain effect on the preservation of primary pores. On the whole, the controlling factors of the petro physical properties of the conglomerate reservoirs in the Baikouquan Formation include sedimentation, diagenesis, and tectonic actives, while that of the sandstone reservoirs mainly include sedimentation and diagenesis.

AbstractThe reservoir property of tight sandstones is closely related to the provenance and diagenesis, and multiprovenance system and complex diagenesis are developed in Hangjinqi area. However, the relationship between provenance, diagenesis, and physical characteristics of tight reservoirs in Hangjinqi area has not yet been reported. The Middle Permian Lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin. This research compared the diagenesis-porosity quantitative evolution mechanisms of Lower Shihezi Formation sandstones from various provenances in the Hangjinqi area using thin-section descriptions, cathodoluminescence imaging, X-ray diffraction (XRD), scanning electron microscopy (SEM), and homogenization temperature of fluid inclusions, along with general physical data and high-pressure mercury intrusion (HPMI) data. The sandstones mainly comprise quartzarenite, sublitharenite, and litharenite with low porosity and low permeability and display obvious zonation in the content of detrital components as a result of multiprovenance. Pore space of those sandstone mainly consists of primary pores, secondary pores, and microfractures, but their proportion varies in different provenances. According to HPMI, the order of the pore-throat radius from largest to smallest is central provenance, eastern provenance, and western provenance, which is consistent with the change tend of porosity (middle part>northern part>western part) in Hangjinqi region. The diagenetic evolution path of those sandstones is comparable, with compaction, cementation, dissolution, and fracture development. The central provenance has the best reservoir quality, followed by the eastern provenance and the western provenance, and this variation due to the diverse diagenesis (diagenetic stage and intensity) of different provenances. These findings reveal that the variations in detrital composition and structure caused by different provenances are the material basis of reservoir differentiation, and the main rationale for reservoir differentiation is varying degrees of diagenesis during burial process.

Massive dolostones replacing build-ups and containing H2S of thermochemical sulfate reduction (TSR) origin occur in the Upper Permian reef and bank deposits, northeastern Sichuan, China, despite the absence of gypsum or anhydrite deposits, which are usually present in TSR cases. Fluid chemistry from fluid incluions and δ13C, δ18O and 87Sr/86Sr values from different diagenetic phases were measured to determine the dolomitization regime, and to assess the relationships between the occurrence of dolomitization and H2S accumulation. Dolomitization was initiated by seawater with slightly increased salinities (penesaline) at shallow depths prior to chemical compaction. The micro- to fine-crystalline cloudy dolomite formed with relatively high Na and Sr contents, and with δ13C, δ18O and 87Sr/86Sr values (2.2‰∼4.8‰, −3.9‰∼-5.0‰ and 0.70724∼0.70746, respectively) inherited from seawater. The evaporation of Permian seawater in back-reef and inter-reef lagoons during sea-level fall and the subsequent seepage reflux into reef-beach bodies led to greater Mg 2+ and SO 4 2− concentrations in higher-salinity pore waters. Further massive dolomitization was promoted by compactional flow of hotter residual seawater at shallow to intermediate depths and resulted in the formation of fine- to medium-crystalline clean dolomite with lower Na and Sr contents, more depleted δ 18 O values (-5.1‰∼-6.0 ‰), andδ 13 C (2.5‰∼4.8 ‰) and 87 Sr/ 86 Sr values (0.70726∼0.70741) similar to those of the coeval seawater. The relatively closed hydrodynamic system during burial facilitated SO 4 2− preservation. The whole dolomitization process enriched porewater SO 4 2− , which have been almost exhausted by subsequent TSR, accounting for the high present-day concentrations of H 2 S. The output of this study shows that similar scenarios involving dolomitization driven by condensed MgSO 4 seawater and H 2 S accumulations can occur in evaporite-free settings across a broad range and deserves special attention during deep oil/gas exploitation.

Exploration of hot dry rock in China has just started. In order to accurately evaluate the amount of hot dry rock geothermal resources in China, high-quality hot dry rock drilling cores are needed. However, there are few researches on coring drilling of hot dry rock under the conditions of high temperature, high hardness and high grinding, which seriously restricts the accurate evaluation of hot dry rock geothermal resources. Therefore, independent research and development of the turbodrill Φ127 mm are carried out. The HDR-1 well in Zhangzhou of Fujian and the GR1 well in Gonghe of Qinghai were drilled and the research and application of hot dry rock drilling coring were performed to verify the research and development of turbodrill and the suitability of the KT-140 coring drilling tools and the matching of turbodrill and diamond core bit. The high temperature and hard rock characteristics of turbodrill working under the well conditions are revealed, the test of high temperature of 236 ℃ at the bottom of the bore is withstood, and the application test of turbodrill parameters is obtained. Compared with the conventional coring drilling technology, the turbodrill composite coring drilling technology not only plays an important role in the performance of high rotating speed of the turbodrill, but also makes full use of the characteristics of high temperature resistance, grinding resistance, high ground stress resistance, long service life and low on-site labor intensity, which greatly improves the drilling speed of hot dry rock. The method is of high temperature deep hole hot dry rock underground rotary drill driving core bit coring drilling and is the first successful attempt for hot dry rock of turbodrill coring drilling technology. The field test and application may provide valuable construction experience. The results will also provide a new technical support for hot dry rock scientific drilling and deep geothermal resource exploration in China.

In this paper, GNSS interferometric reflectometry (GNSS-IR) is firstly proposed to estimate ground surface subsidence caused by underground coal mining. Ground subsidence on the main direction of a coal seam is described by using the probability integral model (PIM) with unknown parameters. Based on the laws of reflection in geometric optics, model of GNSS signal-to-noise (SNR) observation for the tilt surface, which results from differential subsidence of ground points, is derived. Semi-cycle SNR observations fitting method is used to determine the phase of the SNR series. Phase variation of the SNR series is used to calculate reflector height of ground specular reflection point. Based on the reflector height and ground tilt angle, an iterative algorithm is proposed to determine coefficients of PIM, and thus subsidence of the ground reflection point. By using the low-cost navigational GNSS receiver and antenna, an experimental campaign was conducted to validate the proposed method. The results show that, when the maximum subsidence is 3076 mm, the maximum relative error of the proposed method-based subsidence estimation is 5.5%. This study also suggests that, based on the proposed method, the navigational GNSS instrument can be treated as a new type of sensor for continuously measuring ground subsidence deformation in a cost-effective way.

AbstractThe Xiagou Formation is the main tight oil reservoir in Qingxi Sag of Jiuquan Basin. Given the poor physical properties and other factors restricting tight oil exploitation and production in this area, studies should focus on microscopic pore structure characteristics. In this study, a nano-CT scanner, a SEM, and an NMR were used to study the pore structure characteristics of a tight carbonate reservoir in Qingxi Sag, Jiuquan Basin. The Xiagou Formation reservoir mainly consists of gray argillaceous dolomite and dolomitic mudstone. The pore categories are mainly elliptic, irregular, intergranular, and intragranular and mostly filled with clay and carbonate cement. Pore space is small, the intergranular or organic pores are mostly separated, and pore-throat is weakly connected. The throats mostly develop with lamellar and tube bundle-like characteristics and with poor seepage ability. The pore-throats mostly span from nanometer to micrometer sizes, and pore diameters are mainly concentrated in the range of 0.01–0.1 and 1–10 μm. It is a unimodal pattern mainly composed of micropores, or a bimodal regular allocation dominated by micropores supplemented by macropores. The relationship between micropore (1 μm) content allocation and mean pore diameter strongly controls the permeability of reservoir rocks. When macropore content reaches more than 85%, or when pore content totals less than 3%, the permeability of a reservoir remarkably increases. At a higher ratio of the average finest throat sectional area and throat-pore of reservoir rock, the throat radius lies closer to the connecting pore radius, pore and throat connectivity improves, and reservoir seepage ability becomes stronger. Based on reservoir capacity and seepage ability, pore structures of the tight carbonate reservoirs in study area are classified into type I (small-pore–thin-throat), type II (thin-pore–thin-throat), and type III (microporous-microthroat) with rock permeability>0.1 mD, 0.05–0.1 mD, and

AbstractThe physical property heterogeneity of tight sandstones was mainly caused by complex alteration of various diagenesis combinations during burial process. However, diagenetic evolution of different diagenesis combinations which generally result in the strong difference and heterogeneity of physical property and pore structure is rarely well understood. The Middle Permian lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin, China. The reservoir heterogeneity of lower Shihezi Formation, which was caused by the differential diagenesis combination, is crucial to efficient exploration and development. Evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight lower Shihezi Formation sandstone in the Hangjinqi area of Ordos Basin was investigated by means of thin-section description, cathodoluminescence (CL) imaging, X-ray diffraction (XRD), scanning electron microscopy (SEM), and homogenization temperature of fluid inclusions. The lower Shihezi Formation sandstones can be divided into four diagenesis combination types according to the reservoir characteristics and diagenetic relationship. The main diagenetic sequence was mechanical compaction-chlorite rim-early pore-filling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation. Differential diagenesis combination was mainly controlled by the petrological characteristics, microfacies, and fault. Low content of rock fragment and high content of detrital quartz were beneficial to the compaction resistance and cementation. The moderate content of pore-filling calcite was conducive to pore space protection and feldspar dissolution. The faults control dissolution and differential diagenesis combination by influencing the migration of acid fluids. Moderate compaction-moderate cementation-moderate dissolution type (BBB type) and weak compaction-moderate cementation-strong dissolution type (CBA type) were in favour of high-quality reservoir development.

Compared with the overpressure conditions of shale reservoirs in the Jiaoshiba area, exploring the controlling factors of changes in shale reservoir physical properties under normal-pressure accumulation is of great significance to shale gas exploration. To achieve this, X-ray powder diffraction, nitrogen adsorption, and scanning electron microscopy were carried out to determine mineral content, permeability, porosity, and pore structure for well core and outcrop samples of the Wufeng–Longmaxi Formation in southeastern Chongqing, China. Field observations and drilling cores revealed abundant structural fractures in southeastern Chongqing, offering potential main storage space for shale gas. Findings also determined that high-quality source rocks are located in the lower part of the Wufeng–Longmaxi Formation, which features low porosity and low permeability, yet existing structural fractures could potentially improve permeability to a certain extent. The main factors controlling the shale reservoir with the above characteristics include the depositional environment, mineral components, total organic carbon content, and tectonic movement. The deep-water anoxic depositional environment was conducive to the complete preservation of organic matter and sedimentation of the biogenic siliceous minerals, which had a strong effect on improving the porosity of organic matter and brittleness of the rocks. The high content of quartz was found to improve rock compression resistance and brittleness, and increasing clay proportion was found to enhance the interlayer pores of clay minerals. Total organic carbon content, specific surface area, pore volume, and fracture development were all found to have direct contributions to shale gas entrapment. Overall, the most important factor resulting in normal-pressure accumulation in southeastern Chongqing was the intense tectonic movements since the Late Jurassic period which destroyed the original overpressure accumulation conditions.

Diagenetic evolution is an important controlling factor of shale gas reservoirs. In this study, based on field outcrop and drilling core data, analytical techniques including X-ray diffraction (XRD), field emission scanning electron microscope combined with a focused ion beam (FIB-FESEM), and energy-dispersive spectroscopy (EDS) analyses were performed to determine the diagenetic evolution of the Longmaxi Formation shale and reveal the effect of diagenetic evolution on the shale gas exploration and development in the Sichuan Basin, Southwest China. The eodiagenesis phase was subdivided into two evolution stages, and the mesodiagenesis phase was subdivided into three evolution stages in the basin margin and center. Absorbed capacity and artificial fracturing effect of the Longmaxi Formation shale gas were related to mineral composition, which was influenced by sedimentary characteristics and diagenetic evolution. The diagenetic system in the basin margin was more open than that in the basin center due to a different burial history. The more open diagenetic system, with more micro-fractures and soluble constitute (e.g., feldspar), was in favor for the formation and preservation of secondary dissolved pores and organic pores in the basin margin. The relatively closed diagenetic system with stronger compaction resulted in deformation of pore space in the central basin.

The thermal evolution process of organic matter is associated with the complete hydrocarbon generation and expulsion process in shale, however, the thermal evolution of organic matter is a long process and cannot be realized without experimental simulations. Although several scholars have substantially studied the thermal evolution of organic matter, it remains a challenging and much debated issue in the studies of organic geochemistry. Volcanic events are crucial in the enrichment of organic matter, and appropriate heating accelerates the thermal evolution of organic matter. However, how strong-rock baking restricts the evolution of organic matter in shale has not been specifically studied. The South Qilian Basin in China is a typical superimposed basin where complex tectonic movements have induced multiple volcanic events, which makes it a favorable location to perform the aforementioned research. This study used the Galedesi Formation shale in the Hala Lake Depression of the South Qilian Basin as an example for investigating the constraints of the volcanic events related to the thermal evolution of organic matter by integrating the results obtained using the geochemical and petrological methods. Our results demonstrate that the lacustrine Galedesi Formation shale of the Hala Lake Depression in the Late Triassic is a typical deep-lake facies deposit with good hydrocarbon generation potential. However, because of the influence of regional tectonic evolution, the burial depth of shale is not deep and the thermal evolution of organic matter is insufficient. Due to the influence of multiple volcanic thermal events in the later stages, the thermal maturity of organic matter in the Galedesi Formation shale generally exceeds 3.0%, which is abnormally high. The apparent carbonization of organic matter can be observed via scanning electron microscopy. Rapid magma baking typically cannot effectively promote the hydrocarbon generation of shale organic matter. Finally, the burial depth of lacustrine shale of the Galedesi Formation in the Hala Lake Depression of South Qilian Basin is too shallow. Organic matter hydrocarbon generation and later shale preservation conditions are not conducive to the enrichment, accumulation, exploration, and development of shale gas.

The Longmaxi and Niutitang Formations are typical shale reservoirs in southeastern Chongqing. Since the Paleozoic, southeastern Chongqing has experienced multistage tectonic movement and diagenesis, resulting in the formation of a large number of natural fractures. In shale reservoirs, fractures not only provide seepage channels for oil and gas migration but also play an important role in the oil and gas reservoir space. Natural fractures provide a flow path from source rock to reservoir during oil filling and connect hydraulic fractures, matrix pores, and boreholes during production. Therefore, identifying the main factors that control the development of natural fractures in the Longmaxi and Niutitang Formations in southeastern Chongqing has a guiding significance for the efficient development of shale reservoirs in this area. By considering the Longmaxi and Niutitang Formation shales as case studies, using field outcrops, drilling cores, and other data, and conducting X-ray diffraction (XRD) analysis as well as total organic carbon (TOC) measurements, the quantitative parameter characteristics and main factors that control the development of natural fractures in reservoir shales were examined in detail. The results obtained showed that there are three types of fractures in the lower Paleozoic shale in southeastern Chongqing, which are structural, diagenetic, and abnormal high-pressure fractures. Among them, the fractures in the Longmaxi Formation shale, which are relatively abundant, predominantly consist of low-angle and high-angle inclined fractures, while the Niutitang Formation predominantly consists of high-angle fractures. Additionally, the investigation of fracture size and fracture density, as well as correlation analyses, showed that the fractures of the Lower Paleozoic shale are predominantly micro fractures that play a key role in improving reservoir seepage. It was also noted that the development of fractures is affected by several factors, including tectonic stress, mineral composition, organic carbon content, and rock thickness. The degree of fracture development was found to be positively correlated with brittle mineral and quartz contents, and high organic matter contents also significantly favored the formation micro fractures.

The dolostone reservoir of the Middle Permian Maokou Formation in Eastern Sichuan has good prospects for oil and gas exploration. Study of dolomitizing genesis of the Maokou Formation is essential for predicting the distribution of the dolostone reservoir. Petrography, in situ geochemistry, Sr-Mg isotopes, and fluid inclusions were carried out on samples from the Maokou Formation in Eastern Sichuan in order to discuss the dolomitizing process. Based on mineral and textural characteristics, dolomites were divided into four components: partially clouded dolomite (PCD), mosaic-like dolomite (MLD), cloudy-centered and clear-rimmed dolomite (CACD), and saddle dolomite (SDD). Results indicate that the Maokou Formation in Eastern Sichuan mainly experienced two stages of dolomitization. PCD, MLD, and cloudy-centered dolomite (CCD) were formed during the early dolomitization. They all show turbid crystal planes and bright orange-red CL and have similar trace element contents, 87Sr/86Sr ratios, and rare-earth patterns, indicating that they might be formed in the same fluid. This is a period when dolomitizing fluids mainly migrated along pores or microcracks and replaced protogenetic calcites, which occurred in the shallow burial stage of the Maokou Formation before the Late Permian. Clear-rimmed dolomite (CRD) and SDD were formed in the late stage of dolomitization. They all have clean crystal planes and darkly red CL. CRD of the ERY profile has trace element contents, 87Sr/86Sr ratios, and rare-earth patterns similar to SDD of the HLCH profile and Well TL6, inferring that both may be formed in the same fluid. Combined with high SrO contents and homogenous temperatures of fluid inclusions of CRD and SDD and Mg-isotopic compositions, they were generated by hydrothermal dolomitization. The hydrothermal fluid stage is related to the movement of the Emeishan Large Igneous Province, which was made up of basaltic magmatic fluids mixing with the surface water. The hydrothermal fluid mainly migrated upwards along structural fractures or faults and filled in structural fractures, occurring in the Late Permian to Middle-Late Triassic.

Microfractures are key for migrating and aggregating hydrocarbon source rocks and fracturing oil-gas exploitation in tight reservoirs. In this study, rock samples from the Lucaogou Formation tight reservoirs in Xinjiang, China, were studied using multidisciplinary techniques to investigate the genetic types and main control factors of microfractures. Results indicated that the Lucaogou Formation mainly developed diagenetic microfractures followed by tectonic microfractures, with slight formations of granular microfractures. These observations were used to clarify the relationship between the development of microfractures and the pore fluid content, lithology, mineral composition, and stratum thickness. A higher pore fluid content corresponded to a lower compressive strength of the rocks and a larger ring count, resulting in a higher probability of failure and microfracture formation. Tight reservoirs containing more quartz and carbonate minerals were found to develop more microfractures. Quartz grains showed fractures at the margins under stress, which increased the pore permeability of rocks. Carbonate minerals tended to form microfractures owing to corrosion. Microfracture formation mechanisms differed depending on lithology, and microfractures were found to develop most in dolomite and dolomitic siltstones and least in mudstone. Muddy rocks developed fewer tectonic fractures because they can easily absorb stress and undergo plastic deformation. Within a certain stratum thickness range, the average single-well fracture space and stratum thickness showed positive correlations. Moreover, the fracture space increased and the fracture density decreased as the stratum thickness increased. When the stratum thickness was less than 2.5 m, the fracture space increased linearly with the stratum thickness, and when the stratum thickness was greater than 2.5 m, the fracture space remained constant. This study will provide an essential scientific basis for enhancing tight oil recovery.

The shale oil reservoir of the Lucaogou Formation in the Jimsar Sag has undergone tectonic movement, regional deposition and complex diagenesis processes. Therefore, various reservoir space types and complex combination patterns of pores have developed, resulting in an intricate pore throat structure. The complex pore throat structure brings great challenges to the classification and evaluation of reservoirs and the efficient development of shale oil. The methods of scanning electron microscopy, high-pressure mercury injection, low-temperature adsorption experiments and thin-slice analysis were used in this study. Mineral, petrology, pore throat structure and evolution process characteristics of the shale oil reservoir were analyzed and discussed qualitatively and quantitatively. Based on these studies, the evolution characteristics and formation mechanisms of different pore throat structures were revealed, and four progressions were made. The reservoir space of the Lucaogou Formation is mainly composed of residual intergranular pores, dissolved pores, intercrystalline pores and fractures. Four types of pore throat structures in the shale oil reservoir of the Lucaogou Formation were quantitatively characterized. Furthermore, the primary pore throat structure was controlled by a sedimentary environment. The pores and throats were reduced and blocked by compaction and cementation, which deteriorates the physical properties of the reservoirs. However, the dissolution of early carbonate, feldspar and tuffaceous minerals and a small amount of carbonate cements by organic acids are the key factors to improve the pore throat structure of the reservoirs. The genetic evolution model of pore throat structures in the shale oil reservoir of the Lucaogou Formation are divided into two types. The large-pore medium-fine throat and medium-pore medium-throat reservoirs are mainly located in the delta front-shallow lake facies and are characterized by the diagenetic assemblage types of weak compaction–weak carbonate cementation–strong dissolution, early medium compaction–medium calcite and dolomite cementation–weak dissolution. The medium-pore fine throats and fine-pore fine throats are mainly developed in shallow lakes and semi-deep lakes. They are characterized by the diagenetic assemblage type of strong compaction–strong calcite cementation–weak dissolution diagenesis. This study provides a comprehensive understanding of the pore throat structure and the genetic mechanism of a complex shale oil reservoir and benefits the exploration and development of shale oil.

In view of the stick-slip phenomenon in deep and hard rock drilling, a new type of torsional impactor that can provide torsional impact vibration was designed. According to the working principle and structural characteristics of the designed torsional impactor, this paper theoretically analyzes the influences of different structural parameters and motion parameters on the impact frequency, impact force, and impact torque of the torsional impactor. The results show that the impact frequency f is directly proportional to the rotational speed VZ of the transmission shaft and the installed number n of torsional impact generating devices. Additionally, the impact force F is directly proportional to sine value of the impact angle α (i.e., sinα), impact hammer mass m, impact hammer rotation speed VZ (i.e., transmission shaft rotation speed), and impact hammer rotation radius r and is inversely proportional to action time Δt of the impact hammer and impact anvil. Furthermore, the impact torque M is directly proportional to the impact force F and rotary radius r of the impact hammer. This article lays a foundation for further theoretical and experimental research of torsional impactors and provides a reference for the design and testing of torsional impactors.

Shale gas accumulates in reservoirs that have favorable characteristics and associated organic geochemistry. The Wufeng-Longmaxi formation of Well Yucan-6 in Southeast Chongqing, SW China was used as a representative example to analyze the organic geochemical and reservoir characteristics of various shale intervals. Total organic carbon (TOC), vitrinite reflectance (Ro), rock pyrolysis, scanning electron microscopy (SEM), and nitrogen adsorption analyses were conducted, and a vertical coupling variation law was established. Results showed the following: the Wufeng-Longmaxi formation shale contains kerogen types I and II2; the average TOC value at the bottom of the formation is 3.04% (and the average value overall is 0.78%); the average Ro value is 1.94%; the organic matter is in a post mature thermal evolutionary stage; the shale minerals are mainly quartz and clay; and the pores are mainly intergranular, intragranular dissolved pores, organic matter pores and micro fractures. In addition, the average specific surface area (BET) of the shale is 5.171 m2/g; micropores account for 4.46% of the total volume; the specific surface area reaches 14.6%; and mesopores and macropores are the main pore spaces. There is a positive correlation between TOC and the quartz content of Wufeng-Longmaxi shale, and porosity is positively correlated with the clay mineral content. It is known that organic pores and the specific area develop more favorably when the clay mineral content is higher because the adsorption capacity is enhanced. In addition, as shale with a high clay mineral content and high TOC content promotes the formation of a large number of nanopores, it has a strong adsorption capacity. Therefore, the most favorable interval for shale gas exploration and development in this well is the shale that has a high TOC content, high clay mineral content, and a suitable quartz content. The findings of this study can help to better identify shale reservoirs and predict the sweet point in shale gas exploration and development.

Underground coal mining-induced ground subsidence (or major ground vertical settlement) is a major concern to the mining industry, government and people affected. Based on the probability integral method, this paper presents a new ground subsidence prediction method for predicting irregularly shaped coal mining area extraction-induced ground subsidence. Firstly, the Delaunay triangulation method is used to divide the irregularly shaped mining area into a series of triangular extraction elements. Then, the extraction elements within the calculation area are selected. Finally, the Monte Carlo method is used to calculate extraction element-induced ground subsidence. The proposed method was tested by two experimental data sets: the simulation data set and direct leveling-based subsidence observations. The simulation results show that the prediction error of the proposed method is proportional to mesh size and inversely proportional to the amount of generated random points within the auxiliary domain. In addition, when the mesh size is smaller than 0.5 times the minimum deviation of the inflection point of the mining area, and the amount of random points within an auxiliary domain is greater than 800 times the area of the extraction element, the difference between the proposed method-based subsidence predictions and the traditional probability integral method-based subsidence predictions is marginal. The measurement results show that the root-mean-square error of the proposed method-based subsidence predictions is smaller than 3 cm, the average of absolute deviations of the proposed method-based subsidence predictions is 2.49 cm, and the maximum absolute deviation is 4.05 cm, which is equal to 0.75% of the maximum direct leveling-based subsidence observation.

Abstract A slow-wave structure improvement for enhancing the 2π-mode electronic efficiency is embodied in the validation of an extended interaction oscillator (EIO), which has an electronic efficiency of 6.52% at 0.22 THz from particle-in-cell (PIC) calculations. A 2π-mode bi-periodic slow-wave structure (BPSWS) with staggered long and short slots is utilized for optimizing the circuit performance. The proposed BPSWS has the capability of combining the respective advantages for both π and 2π-mode in terms of coupling performance and output performance, thus supporting a strongly-coupled 2π-mode with higher coupling capability. Compared with the typical mono-periodic SWS (MPSWS), the adopted strongly-coupled 2π-mode effectively improves the characteristic impedance M2R/Q by 103% to 66.79 Ω, the coupling coefficient by 66% to 0.497, and the normalized wave-amplitude by 22%. Accordingly, 503 W of average output power can be derived for the BPSWS-EIO with a 25.7 kV and 0.3 A sheet beam injected. Cold-test experiments were conducted, confirming that the 0.22 THz structure exhibits commendable fabrication precision and consistency and thus demonstrates the expected frequency response.