地质力学学报

Geological and evolutionary characteristics of the Gagarin Region on the far side of the Moon

SHAO Tianrui, HAN Kunying, JIN Ming, SHI Chenglong, PANG Jianfeng, DING Xiaozhong

Objective This study focuses on the Gagarin region on the far side of the Moon, aiming to reveal the geological characteristics, distribution features, and genesis of typical areas on the lunar far side. Additionally, it seeks to explore the regional geological evolution history of the Gagarin region. Methods The study primarily employs methods such as multi-source remote sensing data interpretation, regional geological mapping, and quantitative analysis of geological elements' quantity and distribution characteristics. Results (1) 656 impact craters were discovered in the study area, of which 552 have diameters greater than 20 kilometers. Approximately 71.5% of the Gagarin region is covered by ancient basins and their ejecta from the Aitken period. Based on comprehensive area and diameter data, the Aitken period is identified as the geological era with the largest proportion of large impact craters (diameter greater than 70 kilometers) and the largest average diameter. From the Aitken period to the Copernican period, the total area of impact craters in each geological era shows a decreasing trend from old to new. (2) In the study area, six parallel lunar grabens, 62 lobate scarps, one sinuous rille , 50 crater floor fractures, and 70 shallow faults were discovered. It also includes parts of the two longest inferred deep faults on the Moon, originating from the South Pole–Aitken tectonic zone and almost spanning the entire highland tectonic zone. According to Bouguer gravity anomalies and crustal thickness data, linear crustal thickness anomalies extending outward from the South Pole–Aitken basin reach the major basins on the near side of the Moon. (3) The Gagarin region is primarily located in the anorthositic highlands on the far side of the Moon. The rocks mainly consist of ferroan anorthosite (fa) suites, with some crater floors showing magnesium anorthosite (ma) suites. In the central and southern parts of the Gagarin region, low-titanium (TiO₂ > 1.5 and < 4.5) and very low-titanium (TiO₂< 1.5) basalts are sparsely distributed on the floors of certain impact craters and basins. (4) For this study, we selected impact craters such as Aitken and Van der Graaf, with diameters ranging from 350 to 1400 m, for dating analysis. The results of crater size-frequency distribution dating indicate ages of 3.47 GA and 3.32 GA, respectively. (5) The quantitative statistics of impact craters and the dating results of basalt units indicate that the Aitkenian to Imbrian periods were active periods of external dynamic geological processes in the Gagarin region, while the Imbrian period was an active period of internal dynamic geological processes. Conclusion (1) The region’s longest and deepest faults are the result of the combined effects of the South Pole–Aitken impact event and internal and external stresses, including lunar thermal expansion. (2) The variations in the number and size of impact craters in the Gagarin region on the far side of the Moon are related to the evolution of the Earth–Moon system and the solar system. (3) Based on the quantitative statistical results of impact craters and the dating results of basalt units, this study elucidates the regional geological evolution history, and different stages of the geological processes in the Gagarin region were divided according to the active periods and stage characteristics of internal and external dynamic geological processes. Significance The study revealed the geological features of key areas on the far side of the moon, delving into the geological history of the Gagarin region and tentatively establishing a correlation between its geological traits and the lunar evolutionary history.

Analysis of the development characteristics of co-seismic geological hazards and their controlling factors in the Maerkang M_S 6.0 earthquake swarm, Sichuan, on June 10, 2022

SUN Dong, QIN Liang, MENG Minghui, YANG Tao, ZHANG Xu, HU Xiao

Objective This study aims to reveal the distribution patterns and characteristics of co-seismic geological hazards in earthquake swarms, clarify the differences in induced geological hazards by different types of earthquakes, further understand the seismic risks around and within the Bayan Har Block, and provide efficient guidance for the prediction and prevention of secondary geological hazards induced by earthquakes. Methods We take the co-seismic geological hazards of the Maerkang M_S 6.0 earthquake swarm in 2022 as the research focus. Through systematic data and results analysis on earthquake swarm sequence, regional tectonic environment, regional crustal deformation, and post-earthquake short-term geological hazard, the regional and deep structural environment of the Maerkang M_S 6.0 earthquake swarm and the main controlling factors of co-seismic geological hazards are revealed. Results The results show that the Maerkang M_S 6.0 earthquake swarm is a deep-seated sticky-slip earthquake that occurred on a secondary fault in the active strong earthquake zone within the Bayan Har block, a region with extremely strong peripheral boundary activity. The earthquakes with similar magnitudes may be the result of ruptures of secondary faults of the Songgang fault and the successive ruptures of the barriers between them. The earthquake has resulted in 83 newly discovered geological hazard risks, exacerbating deformation in 106 existing hazard spots and triggering multiple high-altitude landslides and a series of fractured mountain slopes. The areas of extreme, high, and medium geological hazard risk in Caodeng Town, the epicenter area after the earthquake, account for 1.62%, 4.80%, and 12.37%, respectively. The occurrence of secondary geological hazards following earthquakes exhibits a positive correlation with the earthquake magnitude, with the number increasing linearly as the magnitude rises. Conclusion The significant differences in the GPS horizontal velocity field and vertical velocity field on both sides of the Darlag–Songgang–Fubianhe fault zone are key factors contributing to the activity of this fault and triggering the recent earthquake.The main controlling factors of co-seismic geological hazards induced by earthquakes are, from primary to secondary, the ruptured fault and its associated faults, seismic magnitude and energy attenuation, terrain slope and altitude difference, and rock mass structure and density of structural surfaces. Significance This study predicts a high risk of future strong earthquakes in the intersection area of the Songgang fault, which triggered this earthquake, and the Longriba active fault. The surrounding area of the seismogenic fault and its associated faults are at a high risk of geological hazards during earthquakes. The findings provides a reference for predicting and controlling the risk of co-seismic geological hazards in this area.

Discussion on the ore-controlling factors in the Longlin–Xilin Sb–Au mining district of western Guangxi, South China

LIU Xiaohu, WANG Xinyu, XIAO Changhao, ZHANG Wengao, LIU Xiangchong, YU Pingping, MAO Cheng’an, FU Wei

Objective Sb deposits are characterized by simple mineral assemblage. The ore-forming ages, sources of ore-forming materials, and genesis of Sb deposits are controversial owing to the absence of suitable minerals for analysis. Sb resources in the South China Sb metallogenic region account for over 83% of the national total, with the Dian–Qian–Gui Sb belt in the southwest being an significant component of this region. Methods Taking the Longlin–Xilin Sb–Au mining district of western Guixi in the central part of the Dian–Qian–Gui Sb belt as an example, this paper systematically summarizes the ore-bearing strata, lithology of ore-bearing wall rocks, ore-bearing structures, and the coexistence relationship of Au and Sb deposits in 86 ore deposits (points) in the area. Combined with the geological characteristics of three typical deposits (Maxiong, Longtan, and Mahao) and the spatiotemporal distribution of Jurassic felsic intrusions, the inherent connection between Sb mineralization and clastic rocks and felsic intrusions was explored. Results (1) Statistics and field works show that the most favorable ore-bearing stratum in the Longlin–Xilin mining district is the Lower Devonian Yujian Formation (D₁y) , followed by the Lower Triassic Luolou Formation (T₁Ll) and the Middle Triassic Banna Formation (T₂b). The lithologies most conducive to mineralization are carbonaceous shale, pyrite-rich sandstone, and siltstone. The Sb content in these strata or lithologies is tens or even hundreds of times higher than the crustal abundance, which has the potential for Sb mineralization. (2) Within the NWW–SEE trending Nongsang–Shijiazhai fault zone in the Longlin area, the middle and late Jurassic felsic intrusions, which have consistent spatiotemporal occurrences with Sb and Au deposits, can directly contribute to antimony mineralization (as sources of Sb and S) and indirectly influence it (as a heat source), both favoring the formation of antimony deposits. (3) Statistical results show that Sb, Au, and Sb–Au deposits account for 48%, 46%, and 6% in the Longlin–Xilin district, respectively. This suggests that the ore-forming fluids for Sb and Au in the study area may originate from different sources. We also can not rule out the possibility that Sb and Au deposits derive from the same fluid. In the latter case, the precipitation of stibnite consumes H₂S in the ore-forming fluid, destabilizing the Au complex in the solution and resulting in localized Au precipitation. This competition between Sb and Au in the fluid for H₂S leads to a negative correlation in the grades of Sb and Au in coexisting deposits. (4) The study area experienced NS-striking compression in the Indosinian period, followed by the NW–SE shortening in the middle–late Jurassic. The intersection of NWW–SEE and NE–SW faults is the favorable ore-bearing space. The NWW–SEE faults displayed strike-slip movement in response to the NW–SE shortening, whereas the NE–SW faults exhibited transpression. Consequently, the NE–SW faults are less conducive to Sb mineralization compared to the NWW–SEE faults. The distribution direction of the NWW–SEE Douhuang–Xilin fault aligns with the axial direction of the main folds in the area, with most fault planes trending northward, displaying horizontal scratches, silicification, and extensional characteristics. The intersection of the Dohuang–Xilin fault and the NE–SW fracture exhibits significant Sb anomalies. Conclusion Based on the above studies, the promising areas we propose for Sb prospecting in Longlin–Xilin mining district are (1) Black shale and pyrite-rich siltstones of the Yujiang Formation in the core of the Xinzhou anticline as the key strata; (2) The periphery of the concealed intrusions within the NWW–SEE Nongsang–Shijaizhai fault (Longlin County) and the intersection area of the NWW–SEE Douhuang–Xilin fault and the NE–SW fault as the favorable areas. [ Significance ] The findings provide new insights into the genesis and metallogenic regularities of Sb–Au deposits in the study area, enriching the theoretical understanding of Au mineralization processes.

Quantitative evaluation of maximum operating pressure and storage capacity for gas-top sandstone reservoir-type gas storage

HU Caiyun, LI Cong, YANG Zhibin, JIA Qian, SUN Yanchun, LI Chaofeng, SUN Junchang, YANG Yuehui, SUN Dongsheng

Objective The maximum operating pressure for underground gas storage facilities designed for oil and gas reservoirs, both constructed and under construction in China, is currently set at the original formation pressure. There have yet to be successful cases of overpressure operation, which significantly impacts the economic benefits of converting depleted oil and gas reservoirs into underground gas storage facilities. This article aims to evaluate the maximum operating pressure and storage capacity of the Nanpu 1-29 gas storage facility from the perspective of the ultimate bearing capacity of cap layers and faults, with the goal of effectively enhancing the construction benefits of the facility. Methods The evaluation of the maximum operating pressure for the Nanpu 1-29 gas storage facility in eastern Hebei is based on the minimum principal stress measured in situ in the mining wells. Different effective porosity calculation methods are employed to quantitatively evaluate the effective storage capacity of gas and oil reservoirs, as well as the incremental capacity after pressure boosting operation, based on their development differences. Results The evaluation of the maximum operating pressure for the Nanpu 1-29 gas storage facility indicates that the minimum principal stress of the cap layers determined by the in-situ measurements in the mining wells is 34.00 MPa. Based on the tensile failure criteria determined by the minimum principal stress, the maximum operating pressure for the tensile failure of the cap layer is 27.20 MPa. Combined with the maximum safe injection pressure corresponding to shear failure of the cap layer (30.60 MPa) and the maximum safe injection pressure corresponding to unstable slip of the fault (27.60 MPa), the final maximum operating pressure for the Nanpu 1-29 gas storage facility is determined to be 27.20 MPa. Based on the effective storage capacity calculation model, considering factors such as the water content of the gas reservoir, residual water and edge porosity as well as the coefficient of influence, the efficiency of gas-driven fluid, and the utilization rate of oil-containing space, the maximum operating pressure increased from the original formation pressure of 22.50 MPa to 27.20 MPa. The practical storage capacity of the gas storage facility increased from 15.46×10⁸ m³ to 18.14×10⁸ m³, an increase of approximately 17.3%. Conclusion (1) The construction of gas storage facilities can be re-evaluated for the maximum operating pressure based on the minimum principal stress measured in situ in the mining wells, and overpressure design can be conducted under appropriate conditions. (2) Overpressure design can effectively increase storage capacity and improve the economic benefits of reservoir construction. Significance The research results have a certain reference value for the quantitative evaluation of the maximum operating pressure and storage capacity of other underground gas storage facilities, and are expected to significantly improve the economic benefits of overpressure-designed reservoir-type gas storage facilities in China.

Estimation of the rock mechanics and in-situ stress parameters of carbonate reservoirs using array sonic logging: A case study of Shunbei No.4 block

LIU Jun, HUANG Chao, ZHOU Lei, CHEN Qun, ZHANG Shenglong

Objective Geomechanical analysis plays a crucial role in exploring and developing oil and gas reservoirs. However, the study of in-situ stress in the Yijianfang–Yingshan formations of the Middle to Lower Ordovician in the Shunbei oilfield has lagged behind. This paper compares and analyzes the rock mechanics and in-situ stress characteristics of different types of carbonate reservoirs in the Yijianfang-Yingshan formations of the Lower Paleozoic Ordovician in the Shunbei No. 4 block, and explores the control effect of micro-grain structures on the in-situ stress and rock mechanics parameters of carbonate reservoirs. It aims to provide fundamental geological data for the evaluating of ultra-deep carbonate reservoirs in the Shunbei area. Methods This study, based on the spring combination model, determined the rock mechanics and in-situ stress characteristics through rock mechanics experiments and array acoustic logging, and characterized the micro-pore structure of the limestone using casting thin sections and X-ray μ-computed tomography analysis. Results The results indicate that the Young's modulus of the Yijianfang–Yingshan formations ranges from 50 to 89 GPa, the compressive strength from 99 to 136 MPa, and the Poisson's ratio from 0.25 to 0.32. The maximum horizontal principal stress of the formations ranges from 200 to 225 MPa, while the minimum horizontal principal stress ranges from 125 to 160 MPa. Conclusion Significant differences in rock mechanics parameters and in-situ stress exist among different types of carbonate reservoirs in the Yijianfang–Yingshan formations. From Type I to Type III and non-reservoir carbonate rocks, the Young's modulus, compressive strength, and maximum horizontal principal stress increase dramatically, while the Poisson's ratio and minimum horizontal principal stress show little change. Compared to micritic limestone, sandy–bioclastic limestone has larger calcite particles and reduced particle cohesion, resulting in decreased compressive strength and Young's modulus. Some calcite particles appear sub-rounded or rounded with higher three-dimensional particle sphericity, also leading to weaker intergranular engagement and further reduction in compressive strength and Young's modulus. The presence of numerous pores and fractures in sandy-bioclastic limestone facilitates frictional sliding and potential failure along microfractures or particle interfaces, contributing to lower compressive strength and Young's modulus as well. In addition, micritic limestones in the Yijianfang–Yingshan formations are affected by hydrothermal activity, with some dolomite being replaced by quartz, which increases the compressive strength and Young's modulus. The macroscopic mechanical characteristics and rock mechanics parameters of the rock are constrained by the micro-grain structures. The low-value zones of the Young's modulus, compressive strength, and maximum horizontal principal stress in the Yijianfang–Yingshan formations are identified as the advantageous reservoir development zones in the Shunbei No. 4 block.