China Geology最新文献

Palaeoclimatic and palaeoenvironmental reconstructions of the Cryogenian Period have attracted attention in relation to the debated “Snowball Earth ” hypothesis and the early evolution of metazoan life. The carbon cycle and redox conditions of the Sturtian-Marinoan non-glacial interval have been subjected to much controversy in the past decades because of the lack of a high-resolution stratigraphic correlation scheme. As one of the typical Sturtian-Marinoan interglacial deposits, the Datangpo Formation was widely distributed in South China with shales continuously deposited. The previous zircon dating data of the Datangpo Formation provide important ages for global constrain of the Sturtian-Marinoan non-glacial interval. Here we present a high-resolution straitigraphic study of the organic carbon isotopes of the Datangpo Formation from a drill core section in northern Guizhou Province. Based on measured episodic δ¹³C_org perturbations, three positive shifts and three negative excursions are identified. A δ¹³C_org-based chemostratigraphic correlation scheme is proposed herein that works well for the Datangpo Formation regionally. Meanwhile, the δ¹³C_org vertical gradients changed dynamically throughout the formation. This discovery implies that a significant ocean circulation overturn might have occurred in the upper Datangpo Formation, coinciding with the potential oxygenation.

©2023 China Geology Editorial Office.

Landslide is a serious natural disaster next only to earthquake and flood, which will cause a great threat to people’s lives and property safety. The traditional research of landslide disaster based on experience-driven or statistical model and its assessment results are subjective, difficult to quantify, and no pertinence. As a new research method for landslide susceptibility assessment, machine learning can greatly improve the landslide susceptibility model’s accuracy by constructing statistical models. Taking Western Henan for example, the study selected 16 landslide influencing factors such as topography, geological environment, hydrological conditions, and human activities, and 11 landslide factors with the most significant influence on the landslide were selected by the recursive feature elimination (RFE) method. Five machine learning methods [Support Vector Machines (SVM), Logistic Regression (LR), Random Forest (RF), Extreme Gradient Boosting (XGBoost), and Linear Discriminant Analysis (LDA)] were used to construct the spatial distribution model of landslide susceptibility. The models were evaluated by the receiver operating characteristic curve and statistical index. After analysis and comparison, the XGBoost model (AUC 0.8759) performed the best and was suitable for dealing with regression problems. The model had a high adaptability to landslide data. According to the landslide susceptibility map of the five models, the overall distribution can be observed. The extremely high and high susceptibility areas are distributed in the Funiu Mountain range in the southwest, the Xiaoshan Mountain range in the west, and the Yellow River Basin in the north. These areas have large terrain fluctuations, complicated geological structural environments and frequent human engineering activities. The extremely high and highly prone areas were 12043.3 km² and 3087.45 km², accounting for 47.61% and 12.20% of the total area of the study area, respectively. Our study reflects the distribution of landslide susceptibility in western Henan Province, which provides a scientific basis for regional disaster warning, prediction, and resource protection. The study has important practical significance for subsequent landslide disaster management.

©2023 China Geology Editorial Office.

As a prerequisite and a guarantee for safe and efficient natural gas hydrates (NGHs) exploitation, it is imperative to effectively determine the mechanical properties of NGHs reservoirs and clarify the law of the change in the mechanical properties with the dissociation of NGHs during NGHs production tests by depressurization. Based on the development of Japan’s two offshore NGHs production tests in vertical wells, this study innovatively proposed a new subsea communication technology—accurate directional connection using a wet-mate connector. This helps to overcome the technical barrier to the communication between the upper and lower completion of offshore wells. Using this new communication technology, this study explored and designed a mechanical monitoring scheme for lower completion (sand screens). This scheme can be used to monitor the tensile stress and radial compressive stress of sand screens caused by NGHs reservoirs in real time, thus promoting the technical development for the rapid assessment and real-time feedback of the in-situ mechanical response of NGHs reservoirs during offshore NGHs production tests by depressurization.

©2023 China Geology Editorial Office.

Conductor and suction anchor are the key equipment providing bearing capacity in the field of deep-water drilling or offshore engineering, which have the advantages of high operation efficiency and short construction period. In order to drill a horizontal well in the shallow hydrate reservoir in the deep water, the suction anchor wellhead assembly is employed to undertake the main vertical bearing capacity in the second round of hydrate trial production project, so as to reduce the conductor running depth and heighten the kick-off point position. However, the deformation law of the deep-water suction anchor wellhead assembly under the moving load of the riser is not clear, and it is necessary to understand the lateral bearing characteristics to guide the design of its structural scheme. Based on 3D solid finite element method, the solid finite element model of the suction anchor wellhead assembly is established. In the model, the seabed soil is divided into seven layers, the contact between the wellhead assembly and the soil is simulated, and the vertical load and bending moment are applied to the wellhead node to simulate the riser movement when working in the deep water. The lateral bearing stability of conventional wellhead assembly and suction anchor wellhead assembly under the influence of wellhead load is discussed. The analysis results show that the bending moment is the main factor affecting the lateral deformation of the wellhead string; the anti-bending performance from increasing the outer conductor diameter is better than that from increasing the conductor wall thickness; for the subsea wellhead, the suction anchor obviously improves the lateral bearing capacity and reduces the lateral deformation. The conduct of the suction anchor wellhead assembly still needs to be lowered to a certain depth that below the maximum disturbed depth to ensure the lateral bearing stability, Thus, a method for the minimum conductor running depth for the suction anchor wellhead assembly is developed. The field implementations show that compared with the first round of hydrate trial production project, the conductor running depth is increased by 9.42 m, and there is no risk of wellhead overturning during the trial production. The method for determining the minimum conductor running depth in this paper is feasible and will still play an important role in the subsequent hydrate exploration and development.

©2023 China Geology Editorial Office.

The Ordos Basin is the largest continental multi-energy mineral basin in China, which is rich in coal, oil and gas, and uranium resources. The exploitation of mineral resources is closely related to reservoir water. The chemical properties of reservoir water are very important for reservoir evaluation and are significant indicators of the sealing of reservoir oil and gas resources. Therefore, the caprock of the Chang 6 reservoir in the Yanchang Formation was evaluated. The authors tested and analyzed the chemical characteristics of water samples selected from 30 wells in the Chang 6 reservoir of Ansai Oilfield in the Ordos Basin. The results show that the Chang 6 reservoir water in Ansai Oilfield is dominated by calcium-chloride water type with a sodium chloride coefficient of generally less than 0.5. The chloride magnesium coefficients are between 33.7 and 925.5, most of which are greater than 200. The desulfurization coefficients range from 0.21 to 13.4, with an average of 2.227. The carbonate balance coefficients are mainly concentrated below 0.01, with an average of 0.008. The calcium and magnesium coefficients are between 0.08 and 0.003, with an average of 0.01. Combined with the characteristics of the four-corner layout of the reservoir water, the above results show that the graphics are basically consistent. The study indicates that the Chang 6 reservoir in Ansai Oilfield in the Ordos Basin is a favorable block for oil and gas storage with good sealing properties, great preservation conditions of oil and gas, and high pore connectivity.

©2023 China Geology Editorial Office.

The Daheishan supergiant porphyry molybdenum deposit (also referred to as the Daheishan deposit) is the second largest molybdenum deposit in Asia and ranks fifth among the top seven molybdenum deposits globally with total molybdenum reserves of 1.65 billion tons, an average molybdenum ore grade of 0.081%, and molybdenum resources of 1.09 million tons. The main ore body is housed in the granodiorite porphyry plutons and their surrounding inequigranular granodiorite plutons, with high-grade ores largely located in the ore-bearing granodiorite porphyries in the middle-upper part of the porphyry plutons. Specifically, it appears as an ore pipe with a large upper part and a small lower part, measuring about 1700 m in length and width, extending for about 500 m vertically, and covering an area of 2.3 km². Mineralogically, the main ore body consists of molybdenite, chalcopyrite, and sphalerite horizontally from its center outward and exhibits molybdenite, azurite, and pyrite vertically from top to bottom. The primary ore minerals include pyrite and molybdenite, and the secondary ore minerals include sphalerite, chalcopyrite, tetrahedrite, and scheelite, with average grades of molybdenum, copper, sulfur, gallium, and rhenium being 0.081%, 0.033%, 1.67%, 0.001%, and 0.0012%, respectively. The ore-forming fluids of the Daheishan deposit originated as the CO₂-H₂O-NaCl multiphase magmatic fluid system, rich in CO₂ and bearing minor amounts of CH₄, N₂, and H₂S, and later mixed with meteoric precipitation. In various mineralization stages, the ore-forming fluids had homogenization temperatures of > 420°C–400°C, 360°C–350°C, 340°C–230°C, 220°C–210°C, and 180°C–160°C and salinities of > 41.05%–9.8% NaCleqv, 38.16%–4.48% NaCleqv, 35.78%–4.49% NaCleqv, 7.43% NaCleqv, and 7.8%–9.5% NaCleqv, respectively. The mineralization of the Daheishan deposit occurred at 186–167 Ma. The granites closely related to the mineralization include granodiorites (granodiorite porphyries) and monzogranites (monzogranite porphyries), which were mineralized after magmatic evolution (189–167 Ma). Moreover, these mineralization-related granites exhibit low initial strontium content and high initial neodymium content, indicating that these granites underwent crust-mantle mixing. The Daheishan deposit formed during the Early-Middle Jurassic, during which basaltic magma underplating induced the lower-crust melting, leading to the formation of magma chambers. After the fractional crystallization of magmas, ore-bearing fluids formed. As the temperature and pressure decreased, the ore-bearing fluids boiled drops while ascending, leading to massive unloading of metal elements. Consequently, brecciated and veinlet-disseminated ore bodies formed.

©2023 China Geology Editorial Office.

To reveal the microbial community composition of regional shallow porous brackish groundwater and its response characteristics to groundwater environment, the first and second aquifers in Taocheng District, Hengshui City were selected, and 10 groundwater source samples were collected for hydrochemical analysis and microbial 16S RNA gene V4–V5 regional sequencing. The results showed that the shallow brackish groundwater in the study area is weakly alkaline and has high ion content. The hydrochemical types are SO₄·Cl-Na·Mg type and HCO₃·Cl-Na·Mg type as a whole. The spatial zonation of the abundance and diversity of groundwater microorganisms is obvious. The number of endemic bacteria in groundwater from upstream, midstream to downstream is 11, 135 and 22 respectively, with a total of 22 bacteria. Proteobacteria is the most dominant in groundwater level (38.82%–86.88%), and there are obvious differences in different sections. At the genus level, the main dominant species in each group and sample are Pseudomonas and Hydrogenophaga. In terms of composition difference, Pseudohongiella, Pseudorhodobacter and Limnohabitans are the representatives of UR, MR and LR. On the whole, the composition of flora in groundwater in the study area is sensitive and closely related to hydrochemical processes. Species abundance is affected by alkaline and high salinity environmental indicators, while species diversity is related to depth and dissolved oxygen in weak reduction environment.

©2023 China Geology Editorial Office.