Journal of Asian Earth Sciences最新文献

Pore structure and fracture determine reservoir quality in deep and ultra-deep tight sandstones, however, the limited availability of core data makes it challenging to describe the complexity of pore-throat structure and characterize the wide range of fractures. To fill this gap, a comprehensive analysis was conducted to evaluate pore structure and fractures from multiple perspectives. Integration of core, thin sections, scanning electron microscopy (SEM), high pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) tests are used to evaluate pore structure and characterize fracture of deep and ultra-deep tight sandstones in the Cretaceous Bashijiqike Formation in the Kuqa Depression. Pore structure and reservoir quality types can be divided into dominantly three types according to characteristics of NMR transverse relaxation time (T₂) distribution and HPMI curves. Correlation analysis among HPMI and NMR parameters with petrophysical parameters indicate that maximum pore throat size (r_max), logarithmic mean of T₂ (T_2gm), irreducible water saturation (S_wi), and reservoir quality index (RQI) are the most sensitive parameters for pore structure characterization. The Type Ⅰ pore structure is composed of intergranular and intragranular pores, and NMR T₂ spectrum shows the highest magnitude, with the calculated mobile fluid saturation content also being the highest. The Type II and III pore structures are composed of intergranular pores, intragranular dissolved pores and intercrystalline micropores in clay minerals. T_2gm of Type II and III are smaller, with the calculated mobile fluid saturation content being the lower.

The fracture features and parameters including fracture density, porosity, aperture, and length are analyzed using image logs and core. The pore structure characteristics are quantificationally and qualitatively analyzed by NMR logs. Integration of pore structure and fractures by image logs and NMR logs are used for prediction of high quality reservoir and hydrocarbon productivity. High hydrocarbon productivity is obtained in well intervals with abundant natural fractures, and favorable pore structure will also contribute to high matrix porosity. Therefore, NMR logs and image logs are crucial for evaluating reservoir quality and predicting productivity when calibrated with core and analysis data. The comprehensive analysis integrating core data with geophysical well logs provide important insights for reservoir quality prediction of deep and ultra-deep tight sandstones.

Mercury (Hg) anomalies in sedimentary records are a newly developed proxy of large volcanism in the geological past. Tuff layers host abundant volcanic ash and record key information on the type of volcanic emission (e.g., arc volcanism and large igneous province eruptions). Here, we measured the Hg isotopic compositions of several tuff layers in South China. Tuff samples in the Late Ordovician Wufeng Formation and the Middle Triassic Guanling Formation mostly show positive Δ¹⁹⁹Hg values of – 0.01 to 0.10‰ and – 0.06 to 0.16‰, respectively, suggesting arc volcanism occurred during these two periods. Tuff samples in the Middle Permian Dachang Layer mostly show near-zero Δ¹⁹⁹Hg values (– 0.08 to 0.00‰), suggesting volcanism was driven by the Emeishan large igneous province eruption. Results of this study verify Hg isotopes as a useful proxy in revealing the type of volcanism in geological history.

We propose a novel fractal analysis-assisted approach for stratigraphic characterization to understand the in-situ subsurface geology of hydrocarbon reservoirs. By employing correlation dimension and multifractal detrended fluctuation analysis, we quantify the self-similarity and long-range dependencies in the geophysical logs, reflecting the unique sedimentation processes inherent in the stratigraphic formations of the Bhogpara oil field in the Upper Assam basin. The fractal properties of the logs associated with various stratigraphic units are used to establish a fractal-based stratigraphic signature. The hierarchy of the studied formations, based on the correlation dimension ranging from 1.98 to 1.13, is follows as: Girujan > Tipam > Barail > Kopili > Prang > Narpuh > Lakadong-Therria. Furthermore, the spectrum width values, was found to range from 0.70 to 2.04, reflecting the spatial correlation, geological complexity, and heterogeneity inherent in each formation as: Kopili > Prang > Narpuh > Barail > Girujan > Tipam > Lakadong-Therria. This method identifies common fractal patterns between wellbores and aids in determining the spatial continuity of stratigraphic units. Our approach effectively detects lateral variations in subsurface formations, linking well log fractality with depositional patterns through various transitional environments, such as lacustrine to shallow marine to fluvial, in presence of different fluids. The petroliferous formation exhibits lowest fractal dimension and weak multifractality. The research utilizes fractal analysis to improve stratigraphic correlation, enhancing subsurface characterization and reservoir delineation, particularly in complex geological environments, and enhancing reliability.

The Jinchuan Ni-Cu sulfide deposit in the northeastern margin of the Tibetan Plateau is one of the largest magmatic sulfide deposits on the earth. However, the post-mineralization exhumation history of this deposit is poorly understood. We report new apatite (U-Th)/He data for bedrocks from the Jinchuan deposit and this new dataset reveals two periods of rapid exhumation at intervals of ∼ 115–95 and ∼ 65–55 Ma. Considering the tecton-thermal events in the adjacent terranes, the earlier stage of rapid exhumation during ∼ 115–95 Ma likely resulted from the Lhasa-Qiangtang collision, whereas the late stage of rapid exhumation during ∼ 65–55 Ma should be a geomorphological response to the initial collision of the Indian and Eurasian plates during the Cenozoic. The rapid exhumation during ∼ 65–55 Ma raises the idea that the northeastern Tibetan Plateau uplifted to a certain extent at the early stage of the India-Asia collision during the early Cenozoic.

The western Yangtze Block is an ideal place for investigating the Precambrian tectonic evolution of the South China Craton and the geodynamic mechanism responsible for the breakup of Rodinia. However, the Neoproterozoic tectonic evolution of the western Yangtze Block has not been fully elucidated; in particular, the driving mechanism has changed from an arc setting to a rift setting. Here, we present new zircon U–Pb geochronology and O–Hf isotopic compositions, whole-rock geochemistry, and Sr–Nd isotope data for Yanbian andesites and Shimian gabbros from the western Yangtze Block, South China. The ca. 850–840 Ma Yanbian andesites are high-Mg andesites with moderate SiO₂ (56.75–60.21 wt%) and high MgO (4.24–6.44 wt%) and Mg^# (52–66). These rocks show enrichments in large-ion lithophile elements (LILEs; e.g., Rb, Ba, Sr, and K) and light rare earth elements (LREEs) and depletions in high-field strength elements (HFSEs; e.g., Nb, Ta and Ti) and display decoupled Nd–Hf isotopes (ε_Nd(t) = +3.9 to + 5.0, ε_Hf(t) = +9.7 to + 13.1) and relatively high zircon δ¹⁸O values (5.82–7.72 ‰); these findings indicate that these rocks were derived from a mantle wedge enriched by slab fluids and sediment melts. The ca. 820–810 Ma Shimian calc-alkaline gabbros are characterized by enriched LREEs and LILEs (e.g., Rb, Sr, and K) and depleted HFSEs (e.g., Nb, Ta and Ti). These rocks show decoupled Nd–Hf isotopes (ε_Nd(t) = +3.2 to + 4.2, ε_Hf(t) = +3.1 to + 7.5) and relatively low zircon δ¹⁸O values (4.28–5.31 ‰), indicating that they were formed by partial melting of mantle wedge metasomatized by sediment and oceanic slab melts. Compiled geochronological data suggest that Neoproterozoic subduction-related magmatism along the western Yangtze Block formed a long-term active arc system. Slab retreat in this subduction zone led to a shift in the tectonic setting from a compressional setting to an extensional setting in the Yangtze Block during the middle to late Neoproterozoic. Moreover, the temporal link between retreating subduction zone and extensional tectonics in the Yangtze Block suggests that protracted subduction retreating resulted in external rifting in Rodinia.

Copper and lead–zinc mineralization in basinal rocks worldwide commonly shows distinct spatiotemporal zonation. The Baiyangping deposit in the Lanping Basin, southwest China, is a large sediment-hosted polymetallic deposit that shows clear and pronounced zoning of Cu versus Pb-Zn mineralization. Here, we analyzed trace elements in pyrite and sphalerite and fluid inclusions in sphalerite, celestine, quartz, and dolomite to seek insights into the origins of the ore-forming fluids and the processes that govern the separation of Cu- from Zn-rich ores. We show that while the later Zn-rich mineralization is characterized by fluid inclusions showing typical hallmarks of basinal brine composition (low homogenization temperature < 230 °C, high salinity > 20 wt% NaCl eq., dominated by Na-K-Ca-Mg chlorides), the earlier Cu-rich mineralization shows markedly distinct composition that more closely resembles deeply-circulated water that has equilibrated with crystalline basement rocks (higher homogenization temperature of 210–280 °C, variable but lower salinity of ∼ 5–15 wt% NaCl, dominated by Na-K-Li chlorides). Pyrites formed during the earlier Cu-rich stage show elevated Cu, Co, Sb and Zn, while pyrites formed during the later Pb-Zn stage are more enriched in As, Pb and Mn. We interpret that the earlier-formed Cu-rich ores were deposited by deeply sourced fluids that ascended along basement-penetrating reverse faults, whereas the later Zn-rich ores were formed by subsequent circulation of fluid from within the basin, whose flux was promoted by shallower strike-slip faults. Hence, the two ore types reflect discrete pulses of chemically distinct hydrothermal fluids. We suggest that the evolving structural controls and fluid pathways during prolonged ore formation allowed sequential pulses of fluids that had acquired different metal budgets by equilibration with different sources. We suggest that this type of two-step process may be more common in sediment-hosted base metal deposits than single-fluid cooling sequential order of precipitation.

The Motuo Fault zone (MTF), the southeast boundary of the eastern Himalaya syntaxis, is an important information carrier to understand the present-day kinematics and geodynamics of the Tibetan Plateau. However, little knowledge has been obtained about its Holocene activity and structural features of the MTF due to the ubiquitous vegetation covering. Interpretations of satellite imageries, outcrop and trench observations, together with the radiocarbon and Optically Stimulated Luminescence (OSL) dating results reveal that (1) the MTF consists of multiple NE–SW-trending branch faults, forming a wide fault system that could be generally divided into the western segment, central segment and eastern segment; (2) the drainage systems, mountain ridges and alluvial fans have been systematically left-laterally deflected or offset along the MTF; (3) two surface-rupturing earthquakes occurred close to or slightly earlier than 9.41 ± 0.94 ky BP and after ∼1540 yr BP, respectively, indicating that the MTF is an Holocene active fault. Due to the very limited exposure, it is quite possible that many paleoseismic events have not been revealed by this study and previous publications. For a better understanding and assessment of the seismogenic behavior of the MTF, a complete paleoseismic sequence and reasonable earthquake recurrence model need to be set up, especially considering the complex geometry of the fault system that probably suggests intricate fault rupturing.

The dynamic rifting model of the Caroline Ridge, an oceanic plateau in the West Pacific, remains unclear. Previous studies have revealed that the crustal width of the Caroline Ridge clearly varies from the northwest to the southeast. Here, we investigate Caroline Ridge rifting using numerical simulation and reveal the relationships between different plateau sizes and rifting. For small-scale oceanic plateaus, the initial rift migration forms a symmetric structure. The strain transfers toward one side of the margin. The rifting manifests as a process from landward migration to seaward migration, accompanied by a transition from a symmetric to an asymmetric structure. With larger initial lithospheric thinning, larger ranges of crustal deformation appear with less basement tectonic subsidence. For large-scale oceanic plateaus, basement tectonic subsidence occurs within narrower ranges. The proximal half-graben structures rapidly abandon with relatively horizontal basement and small fault offsets on the rift flank, leading to a symmetric structure during breakup. With larger initial lithospheric thinning, crustal deformation occurs within a narrower region during the early stage. Landward rift migration dominates the deformation, resulting in an asymmetric structure. On the basis of these results, for the large-scale section of the Caroline Ridge, NW-trending normal faults originated from the landward rift migration. At present, seaward rift migration dominates the Caroline Ridge deformation, forming a relatively symmetric structure. For the small-scale section of the Caroline Ridge, the initial landward rift migration occurred. Larger initial lithospheric thinning may exist beneath the Caroline Ridge during the initiation of Miocene rifting.

Leucogranitic dikes emplaced at the lower-middle crust are particularly widespread in the Himalayas. How the magmatic activities evolved in relation to faulting along the Southern Tibetan Detachment system (STDS) remains debated. In this study, we investigate the multi-stage leucogranitic emplacement in relation to the structural evolution of the detachment fault in the Yalashangbo dome through detailed macro- and microscopic structural observations, LA-ICP-MS monazite U-Th-Pb dating, and whole-rock geochemical analysis of the leucogranitic dikes. We identified late syn- and post-kinematic granitic dikes from the dome. Timing of dominant STDS faulting is constrained from 27 to 17 Ma. Based on synthesis of geochronological and geochemical results of the present and previous studies, we suggest that two dominant stages of granitic emplacement occurred from Eocene to Miocene along the High Himalayas. An early stage of leucogranitic activity was probably ascribed to partial melting of mafic juvenile lower crust due to heating related to asthenosphere upwelling as the result of slab tearing or break-off during the Indian subduction. Emplacement of the leucogranitic melts subsequently softened the middle and lower crust and triggered the initiation of detachment faulting along the STDS as early as ca. 35 Ma. The large-scale detachment faulting likely led to intensive localized decompression melting of the lower crust. Contemporaneous detachment faulting in the lithospheric mantle allowed heat transfer from the asthenosphere to the lower crust, enhancing lower crustal melting and leading to generally higher crystallization temperatures in the late syn-kinematic granites than the post-kinematic ones.

Western Yunnan in SW China preserves abundant rock records related to the eastern Proto- and Paleo-Tethys oceans, yet the transition between the two oceans remains poorly understood. This paper presents new zircon U–Pb ages and Lu–Hf isotopic data, together with whole-rock elemental and Nd isotopic data for the magmatic rocks from Mengding and Menglong in western Yunnan. The Mengding basaltic rocks show oceanic island basalt geochemical features and have similar whole-rock Nd isotopic compositions with ε_Nd(t) values of +3.5–+4.4 and uniform zircon Lu–Hf compositions with ε_Hf(t) values of +6.1–+8.0, similar to the modern Hainan oceanic island basalts. These rocks yielded a zircon U–Pb age of 341 ± 2 Ma, indicating their eruption in a passive continental rift basin setting during the early Carboniferous. The Menglong mafic rocks yielded a similar zircon U–Pb age of 343 ± 3 Ma. The Menglong mafic–ultramafic rocks display E-MORB geochemical signatures, suggesting a within-plate extensional setting. They exhibit uniform Nd isotopic compositions with ε_Nd(t) values of +4.1–+4.4, whereas their zircon Ɛ_Hf(t) values vary from −19.5 to +11.1, indicating a mixed magma source with contributions from asthenosphere mantle and decompression melting of the metasomatized mantle wedge. Our results, together with the regional geological and stratigraphical evidence, suggest that the Menglong mafic–ultramafic rocks are comparable with the Late Devonian–early Carboniferous magmatic rocks in northern Tibet, eastern Myanmar and northern Thailand, likely formed in a post-collision extensional setting in the aftermath of Proto-Tethys closure. Taking account of the zonal distribution of the Late Devonian–early Carboniferous magmatic rocks, we propose that slab detachment of the Proto-Tethys led to an overall extensional tectonic setting, resulting in a series of rift basins in western Yunnan during the Late Devonian to early Carboniferous. The Paleo-Tethys probably opened on one of the rift basins of the Proto-Tethys.