Acta Geologica Sinica ‐ English Edition最新文献

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.

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.

The karst groundwater of Cambrian limestone may become an important water source for industry and agriculture in the Pingdingshan area, and is also a potential threat to mining safety. Therefore, to find out the origin, flow paths, and hydrogeochemical processes of karst groundwater beneath the Pingdingshan coalfield, a total of 16 water samples were collected. Our findings confirmed that the karst groundwater is mainly recharged by precipitation. The precipitation can directly supply the deep aquifer of the karst water system through the southwest limestone outcrops, and this area mostly includes the southern part of mines No. 11, No. 9, and the hidden outcrops in the southern part of mine No. 2. What is more, the areas adjacent to the synclinal axis, including mines No. 10, No. 12, and No. 8, may be the main discharge areas. A mixing model of ⁸⁷Sr/⁸⁶Sr and Sr showed that in the southwest Pingdingshan coalfield, the proportion of precipitation decreased gradually from the recharge area to the discharge area, ranging from 89.1% to 17.1%. Besides, the northeast Pingdingshan coalfield is another recharge area for the whole karst system, thus, the infiltrating groundwater will indirectly supply the deep aquifer through Quaternary deposition near the mine No. 13. Our research results can provide theoretical support for the prevention and control of groundwater disasters and the development and utilization of regional groundwater resources in the coalfield in Northern China.

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.

Fracture effectiveness plays a key role in gas productivity of ultra-deep tight sandstone reservoirs, Kuqa depression, Tarim Basin. Based on cores, thin sections, well logging, well testing and production data, the study evaluated fracture effectiveness and illustrated its impacts on gas productivity. High-angle and vertical shear fractures are the most important types. Distribution of effective fractures shows great heterogeneous. Fracture effectiveness is influenced by tectonism, diagenesis and in-situ stress. Earlier fractures or fractures in close to gypsum rock are easier to be filled. Completely filled fractures can be reopened under late tectonism. Dissolution improves local fracture effectiveness. Minerals spanning fracture surfaces protect fracture effectiveness from late compression. Fractures filled with calcite can be activated by acidification. Effective fractures parallel to maximum horizontal principal compressive stress direction show larger aperture. Overpressure can decrease the effective normal stress to maintain fracture effectiveness. With exploitation, decline in pore pressure reduces fracture effectiveness. Linear density, aperture, and strike of effective fractures influence gas productivity. Effective fractures greatly enhance matrix permeability. Therefore, more abundant and larger aperture fractures are always corresponded to higher productivity. However, effective fractures also facilitate late water invasion, especially, both mutually parallel. Intense water invasion leads to rapidly declines in productivity.

Permian sedimentary successions, widely distributed in the eastern Junggar Basin, may record key details on the closure of the Paleo-Asian Ocean in this region. Results of boreholes show that Permian strata is mainly composed of mudstone, sandy mudstone, siltstone, sandstone, sandy conglomerate, dolomite, and limestone. Detrital zircon U-Pb dating results of three sandstone samples yielded the calculated maximum ages of 296 Ma, 278 Ma and 293 Ma, indicating they possibly deposited during the Early Permian. Permian strata in the eastern Junggar Basin show similar rock associations and detrital zircon age data distribution patterns. Our new and compiled age data for sandstone samples in the eastern Junggar Basin and Bogda region yield predominant Paleozoic ages, close to their sedimentary ages. Such detrital zircon age data distribution patterns support the hypothesis that these Early Permian meta-clastic rocks in the eastern Junggar Basin and Bogda region were deposited in a subduction-related basin, and indicate an Early Permian Ocean prevented Precambrian detritus from the Central Tianshan block from depositing in the eastern Junggar Basin and Bogda region.