Acta Geologica Sinica ‐ English Edition最新文献

This study focuses on tuffaceous clastic rocks of the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin. It aims to explore the influence of sedimentation and parent rock on this kind of reservoir development. The results show that the tuffaceous components formed by the denudation of ultramafic and mafic rocks can transform into chlorite coating or hematite, while those from intermediate rock denudation can be dissolved or transformed into illite. Sedimentary facies and lithofacies are essential in controlling the evolutionary result of tuffaceous components. Matrix-supported medium conglomerate and grain-supported medium-fine conglomerate that developed in the fan delta plain, with a closed original geochemical systems, have been in the oxidizing environment for a long time. The tuffaceous matrices mainly transforms into hematite or illite. These minerals occupy the primary pores and are difficult to dissolve by felsic fluids, which inhibits the development of high-quality reservoirs. The grain-supported sandy fine conglomerate developed in the fan delta front was in the underwater reductive environment with an open original geochemical system. The tuffaceous matrices not only can transform into chlorite coating to strengthen the particle's compaction resistance, but also can be fully dissolved, which promotes the formation of high-quality reservoirs.

The formation of Mesozoic natural gas in the Pengyang area of southwestern Ordos Basin is discussed, from the perspective of microbial community characteristics, in order to clarify the relationship between the origin of natural gas and its associated indigenous microbial community. The types and diversity of indigenous microbial communities associated with the oil reservoir were studied by means of collecting reservoir formation water samples from exploration wells. The indigenous microbial communities in the Chang 8 member of the Yanchang Formation were primarily distributed within Proteobacteria and Firmicutes, including the specific species and genera of Methylobacter, Pseudomonas, Haibacter, Toxobacillus, Acinetobacter and Adura actinomyces. The results of diversity analysis shows that the number of common genes was 5448, while the number of unique genes and information was less. This reflects the fact that the strata in the study area are relatively closed and not invaded by external water sources, which leads to the development of biological community diversity. In conjunction with the analysis of geochemical characteristics of oil and gas reservoirs in this area, this indicates that the study area possesses the necessary geological conditions for microbial degradation. It is the first time that the species and diversity of the indigenous microbial community in the Ordos Basin have been analyzed, showing that microbial degradation is the main cause of natural gas formation here, changes the characteristics of crude oil in this area and provides first-hand information on the impact of indigenous microorganisms on the reservoir.

The Southwest Tianshan is the suture zone between the Central Tianshan and the Tarim Craton. To better illustrate the subduction polarity of the Southwest Tianshan Ocean, a systematic detrital zircon LA-ICP-MS U-Pb geochronology of sedimentary rocks of the Devonian Apadaerkang Formation and the Carboniferous Akeqiayi Group of the Biedieli area in the Wushi region is the focus. Detrital zircon ages indicate that the youngest grains of the targeted quartz sandstones are Late Devonian and Early Carboniferous in age, respectively. These ages also have relatively similar age spectra, indicating a similar tectonic setting and source in the Biedieli area during the Devonian to Carboniferous. The main spectrum peaks occur at 2360, 1960, 810, 640 and 440 Ma, with a principal age peak in the early Paleozoic (474–430 Ma), and the Central Tianshan Terrane is indicated as the main source. The detrital zircon geochronology and sedimentary characteristics of the Biedieli rocks indicate that the Paleozoic sedimentary rocks in the northern Wushi region were formed at an active continental margin, associated mainly with the early Paleozoic subduction of the South Tianshan Ocean.

A new species of Bothriolepis (Placodermi, Antiarcha), B. zhujiangyuanensis sp. nov., is described from the Eifelian (Middle Devonian) Shangshuanghe Formation at the Pearl River Source Scenic Area, Qujing, Yunnan Province, southwestern China and is characterized by a broad postpineal plate bearing a straight anterior margin, a premedian plate wider than long, and a weakly developed dorsal median ridge of the trunk shield. As the eighth Bothriolepis taxon in China, B. zhujiangyuanensis sp. nov. provides a wealth of morphological data to clarify the intrageneric interrelationships. Maximum parsimony analysis, using a revised data set of the genus with 76 ingroup taxa and 72 characters, yields a well-resolved consensus tree for Bothriolepis. The phylogenetic result shows a sister-group relationship between B. zhujiangyuanensis and B. babichevi from Kazakhstan, corroborating the biogeographic proximity between the South China and Kazakhstania blocks during the Middle Devonian.

Black soil is essential for maintaining regional food security and promoting global agricultural production. Understanding the weathering process of parent material and the accumulation of organic carbon is crucial to comprehending the developmental history and future trends of black soil, especially against the background of large-scale global cultivation and climate change. Although the importance of black soil formation and evolution cannot be ignored, the relevant research is still very scarce. In this study, a typical eight-meter-deep soil core was collected from the Keshan area of the Songnen Plain, Northeast China, where surface black soil developed on paleo-sediments. Using ¹⁴C dating, the formation age of the black soil was determined. Based on the characteristics of the geochemical composition, grain size and the magnetic susceptibility of the sediments, it was demonstrated that the black soil and its parent material originated from reworked loess. Furthermore, the mass transfer coefficient (τ) of some elements was determined, in order to explore the soil weathering process. By calculating the transported amount of alkaline and alkaline-earth elements, the weathering rate of parent material to black soil was found to be weak, at 0.16 kEq·ha^–1·year^–1. Combining the results of dating and carbon density in the different layers of black soil, the accumulation rate of organic carbon was determined as follows: rapidly increasing in the initial period of 13.2–2.2 ka, reaching its maximum average value of 34.0 g·cm^–2·a^–1 at 2.2–0.8 ka, then showing a decreasing trend with an average value of -77.5 g·cm^–2·a^–1. Compared with regional climate change, Keshan black soil has developed under a colder and wetter climate during the Holocene. Predictably, ongoing global warming may lead to the degradation of black soils in the Songnen Plain, as well as in other regions. Our results will enrich geological knowledge of black soil formation and future evolutionary trends.

The Curvicubitidae is a Triassic cicada family with some taxa systematically debatable. Here a new curvicubitid species, Beaconiella tongchuanensis sp. nov., is established from the Middle Triassic Tongchuan Entomofauna in NW China. Restudy of the two Australian species of Beaconiella Evans, 1963 (B. fennahi and B. multivenata) confirms that they are different species and first reveals the hindwing venation of B. multivenata. A detailed restudy of Curvicubitus triassicus Hong, 1984 confirms the treatment of Curvicubitus as a junior synonym of Beaconiella. A key to all the six species of Beaconiella in Curvicubitidae is first given. Although reported from only three sites, the family Curvicubitidae potentially was globally distributed during the Middle–Late Triassic, implying that this family probably originated as early as the Early Triassic or even the Permian.

Overpressure is widely developed in deep petroliferous strata in the hinterland of the Junggar Basin. However, a comprehensive understanding of its origin and effect on hydrocarbon distribution and enrichment remains lacking. In this study, we employ five empirical methods and comprehensive geological analysis to identify the origin of overpressure, and uncover the effect of overpressure on hydrocarbon enrichment. The results indicate that disequilibrium compaction is not a significant factor in overpressure generation. Instead, hydrocarbon generation, pressure transfer and diagenesis are the primary causes. The empirical methods support this conclusion. There is a positive correlation between overpressure intensity and source rock thickness and maturity. Notably, widespread cross-formational migration of hydrocarbon bearing fluid occurred, with sandstone overpressure exceeding that of adjacent shale in non-source strata. Furthermore, there is a distinct transformation from smectite to illite near the top of overpressure. Hydrocarbon generation pressurization and pressure transfer significantly effect hydrocarbon enrichment. The overpressure caused by hydrocarbon generation drives hydrocarbon migration and accumulation, establishing an optimal energy configuration between reservoir and cap rock. Faults play a crucial role in hydrocarbon vertical migration and pressure relief. The overpressure in reservoirs can reduce the porosity and permeability thresholds and enhance the charging capacity of oil and gas.

Western Yunnan is an important area for Mesozoic marine strata, development of which helps to reconstruct paleoenvironments in the eastern Tethys. To bring knowledge of eastern Tethys up to that of western Tethys, this study focuses on its sedimentary characteristics and paleoenvironmental significance concentrating on the Xiushan section of the Simao Basin with analyses of the petrology, element geochemistry, and C-O-Sr isotopes. Samples are micrite, bioclastic limestone, marl, and mixed calcareous clastic rocks with the ⁸⁷Sr/⁸⁶Sr values of limestone ranging from 0.708255 to 0.708933; the δ¹³C values of the limestone range from –5‰ to +1.5‰ and the δ¹⁸O values range from –13.7‰ to –9.1‰. Based on the results, it is concluded that the Middle Jurassic limestones in the Simao Basin were deposited in a tidal flat environment with a strong influence of terrigenous input. At least one transgression event occurred during the depositional period. The paleoclimate as a whole changed from dry hot to humid hot and back to dry hot again. These enriched and improved results provide further data support for a comparison of the paleoenvironments between the eastern and western Tethys.

Water inrush is a common disaster in submarine mining. The key to preventing this disaster is to restrict the water-conducting fissure propagation induced by mining from forming a flow channel to communicate with the overlying aquifer. The mechanical behavior of hydraulic fractures under stress disturbance lies at the heart of the problem. Hence, the multiple parallel bond—grain-based model (multi Pb-GBM) is introduced to explore the hydraulic fracture evolution law of crystalline granite under the influence of stress disturbance. The results show that: hydraulic fracturing under stress disturbance is clearly affected by the stress wave frequency; the higher the frequency is, the more difficult it is to crack, but the crack propagation speed is faster after crack initiation; the propagation direction of a crack is deflected towards the propagation direction of the stress wave and the crack dip angle is controlled by the maximum principal stress; the internal crystal boundary of the same mineral is the most stable one among the three contacts, the contact boundary between different minerals being the most fragile one. This research not only has a practical application value to seabed mining engineering, but also has important theoretical significance in enriching deep rock mechanics theory.