Journal of Asian Earth Sciences: X最新文献

The Meishan-Chiayi area of western Taiwan has a large probability of producing a major earthquake in the near future. Historically, one of the largest and most damaging of Taiwan’s earthquakes occurred there. It is, therefore, important to have a well-constrained upper crustal 3-D shear-wave velocity model that can be used to accurately determine ground motion predictions and fault geometry models used in seismic hazard and risk modelling. In this study, we carried out an ambient noise tomography experiment using 100 seismometers deployed with a ∼2 km spacing on a 20 by 20 km grid. The reliable periods of phase velocity from Rayleigh waves are 0.6 to 6.8 s, providing a well-resolved Vs structure from the surface to a depth of around 4 km. The velocity model displays a prominent, roughly northeast-striking change in Vs that follows the projected surface trace of the blind Chiayi thrust. The uplift of relatively higher Vs rocks in its hanging wall, together with a negative to positive change in dVs suggests that it dips gently eastward across the study area. A northward thickening of the lower Vs crust, together with a high negative dVs in the north of the study area is related to an increased thickness of foreland basin rocks across the Meishan fault. The Vs and dVs models provide reasonable evidence that the Meishan fault can be traced at a high angle from its surface rupture to the base of the model at 4 km depth. It cuts the Chiayi thrust.

Permian deposits in the Indian Peninsula have long been a significant source of coal and have great potential for hydrocarbon exploration. Here we present results of megafloral, palynological, and geochemical analysis of the Late Artinskian-Kungurian sediments in the South Karanpura coalfield to assess hydrocarbon generation potential, kerogen types, depositional settings, and thermal maturation. The results suggest anoxic to oxic depositional environments with fluctuating water levels, influenced by terrestrial inputs. The Sirka colliery is considered to be most favorable for hydrocarbon generation due to the palaeodepositional setting dominated by flooded palaeomires. The dominance of degraded organic matter and the rarity of opaque phytoclasts suggest type II/III to type III kerogen material in the palaeomire of the Srika succession, characterized by low-energy dysoxic to anoxic conditions. The thermal maturation values (Tmax 429℃) and the production index (0.01–0.02) indicate that the Sirka area has immature kerogen, but the Giddi colliery has a relatively higher Tmax (average 435℃) placing the studied sample within the mature zone. However, due to deposition in the oxidized swamp, Giddi C has poor potential for hydrocarbon generation, showing that type III/IV material has charcoal input into the sediments. Our findings contribute to global knowledge of coal formations’ oil and gas storage capacity, which has implications for energy resource assessment and exploration strategies.

The northeastern Sunda Strait is a narrow strait separating Java and Sumatra islands. Currently, it forms a seaway between the Java Sea and the Indian Ocean. The geological setting of the region is extremely dynamic, but how the Plio-Pleistocene interplay between sea level oscillations, magmatism, and tectonics, which lead to the current setting, has not been completely understood. We analysed an important set of legacy shallow seismic data from this area to decipher these intricate relationships. Our results indicate that the tectonic extension partly dismantled the Indonesian arc since the Middle Miocene. However, volcanic products formed a barrier between the Sunda Shelf and the Indian Ocean during the Late Pliocene to the Middle Pleistocene. Marine flooding started during the Middle Pleistocene but bypassed the barrier by flooding the NW edge of Java Island. During the Late Pleistocene, high amplitudes and longer periods of the glacial-interglacial cycles ultimately connected the Java Sea with the Indian Ocean. Still, it was only during the Holocene that important erosion made this seaway efficient in transporting seawater between the two reservoirs.

The Late Cretaceous ophiolite mélange in the Salmas area of NW Iran is a part of the Neotethys ophiolites. The mélange includes serpentinized harzburgite, serpentinites, mafic rocks, radiolarite, layered red pelagic limestones and grey and white marbles. Harzburgite main primary mineral phases are olivine, orthopyroxene, clinopyroxene and Cr-spinel. Cr-spinel has Cr₂O₃ contents of 21.13 to 30.18 wt% and high Al₂O₃ (38.67–48.52 wt%), FeO (15.18–18.13 wt%) and MgO (15.18–17.51 wt%) contents. The 100 × Cr/(Cr + Al) ratios of 23 to 34 indicate 9 to 13 % partial melting in the Mid Ocean Ridge (MOR) environment for the origin of the peridotites. An alteration zone is developed around the altered Cr-spinel. Fine-grained minerals assemblage at the spinel crystals’ margin includes Cr-rich chlorite, Cr-rich garnet and spinel-silicate mixture. A 2–5 μm wide transitional zone is developed between the chromite-silicate assemblage and the Cr-rich garnet zone. The chemical variations of major oxides across the alteration zone are mainly diffusion controlled. Al, Cr and Mg have diffused out from the primary spinel and Fe and Mn have diffused into the spinel. Cr-spinel is altered in two stages due to serpentinization. During the first stage and following hydration, spinel reacted with olivine and orthopyroxene to form Cr-rich chlorite and ferrian chromite. Silica formed at this stage. At the second stage, the reaction between the chromite-silicate assemblage and Cr-rich chlorite plus silica form the first stage and Ca²⁺ in the fluid (released from clinopyroxene alteration) produced Cr-rich garnet and H₂O-rich fluid, at temperature between 400 and 600 °C.

The presence of vuggy pore types poses challenges in accurately assessing effective porosity. This study focuses on the significant scientific issue of improving petrophysical evaluation in vuggy carbonate gas reservoirs. The Kangan Formation is one of the main gas reservoir formations in the southern Zagros region, Iran. The main objective of the current research is to distinguish and exclude the influence of vuggy pore types from effective porosity in the reservoir pay zones of the Kangan reservoir.

In the current research, a combination of full suite logs, image logs, core analysis, and thin section studies was employed. The image logs illustrate that vuggy porosity is abundant in the Kangan Formation and these results are confirmed by the available core thin sections, specifically in Zone Kangan_B; Additionally, the cross plots of compressional velocity versus bulk density and total porosity, as a part of rock physics study, indicate the characteristics of the vuggy reservoir. Two methods are utilized to quantify vuggy porosity. The first method, the Velocity Deviation Log (VDL) approach, identifies various available pore types, especially vugs. The second method is a newly proposed approach that can exclude vuggy porosity from the computed effective porosity. In this novel approach, a variable porosity exponent (m) is derived by adopting the Lucia equation to exclude vuggy porosity from the effective porosity computations. Thus, petrophysical evaluation can be implemented based on the constant and variable “m”. Comparing both petrophysical results, it is evident that the amounts of effective porosity and water saturation are modified in the vuggy-bearing intervals. Applying the proposed approach will improve the accuracy of petrophysical properties and lead to the proper calculation of the hydrocarbon volume in the carbonate reservoir rocks containing isolated vugs, particularly in gas-bearing reservoirs where conventional logs are affected by gas contents.

The area offshore of Minamitorishima Island, Northwestern Pacific Ocean, contains large amounts of seafloor mineral resources such as ferromanganese (Fe–Mn) nodules, Fe–Mn crusts, and rare-earth element and yttrium (REY)-rich muds. In this study, we applied stratigraphic Os isotopic dating to a Fe–Mn nodule for the first time to date its formation/depositional age, and mineralogical and texturally characterized a complementary nodule. Based on macroscopic and microscopic observations, the studied Fe–Mn nodules can be divided into three layers: Layers L2, L1, and L0 from core to rim. Under the microscope, the Fe–Mn nodules are dominated by vernadite and Fe-oxyhydroxide. In particular, Layer L1 is dominated by banded-columnar vernadite and contains lower amounts of clay minerals derived from the detrital component than Layers L2 and L0. The bulk major and trace element geochemical compositions of sampled layers in the Fe–Mn nodules all plotted in the hydrogenous field in several discrimination diagrams. The Os isotopic ages determined by fitting to the paleo-seawater ¹⁸⁷Os/¹⁸⁸Os curve can be divided into three clusters (35.7–31.0, 19.0–9.0, and 4.0–1.0 Ma, corresponding to Layers L2, L1, and L0, respectively). These Os isotopic ages indicate that two periods of very slow growth or growth hiatuses occurred during the formation of the Fe–Mn nodule; these age gaps are related to the intermittent (discontinuous) timings of the beginning of Fe–Mn nodule formation offshore Minamitorishima Island.

The staurolite-bearing Avajiq metabasites (AMB) crop out near the NW border of Iran with Turkey and contain the peak assemblage garnet + Ca-amphibole + staurolite + biotite + chlorite1 + ilmenite. The AMB record peak pressure–temperature (P-T) conditions of >1 GPa at ∼650 °C; the moderately high pressure is consistent with the presence of staurolite in metabasalt. Garnets exhibit a weak prograde zoning, with relatively homogenous cores and a slight increase in Mg and decrease in Ca, Fe, and Mn toward the rim, consistent with garnet growth during a clockwise P-T path. Texturally late margarite + biotite + muscovite correspond to retrograde P-T conditions of ∼0.5 GPa at ∼500 °C. This stage was followed by further retrogression under greenschist facies conditions recorded by late pumpellyite + paragonite ± calcite. Based on the MORB-like composition of the AMB, it has been unclear whether the metabasites are associated with Neo-Tethyan ophiolites in the area or formed within a continental setting. However, the petrologic association of metabasites with micaschist, gneiss and marble and upper amphibolite facies conditions of metamorphism indicate that the basaltic rocks likely metamorphosed during Arabia-Eurasia continental collision, following consumption of the intervening Neo-Tethys Ocean and emplacement of the ophiolitic rocks in the region.

This study emphasizes the role of active faults as seismogenic sources in probabilistic seismic hazard assessments (PSHA) in the United Arab Emirates (UAE). An extensive PSHA analysis has been carried out in this study, which incorporates faults alongside area seismic sources within a logic tree-oriented framework. Seismicity is evaluated in terms of spectral acceleration (SA) and Peak ground acceleration (PGA) for return periods of 2,475, 975, and 475 years at 5 % damping. The results are compared with seismic hazard projections derived from previous PSHA studies that utilized only area sources. A key observation is the pronounced impact of the Dibba and Oman range frontal faults on the UAE's northeastern region due to the proximity of the faults. This contributes to higher seismic hazard in this region, with some return periods showing greater hazard levels than those documented in earlier studies. In order to aid seismic hazard design in the UAE, contour maps depicting the SAs at 0.2 and 1.0 s, along with PGA, are developed. The analysis reveals that the highest seismic hazard is predicted along the northeastern borders of the UAE, particularly the coastal area of Ras-Al Khaimah, with a PGA of 0.22 g for a return period of 2,475 years.

The Ordovician Ra’an Shale Member of the Qassim Formation in Saudi Arabia has potential as an unconventional hydrocarbon play. This shale unit, however, has not been studied in detail for its geochemical characteristics, thermal maturity, and reservoir quality. This study aims to investigate this shale member from a well-exposed section at Khashm Ra’an in the Qassim region, central Saudi Arabia. Detailed fieldwork followed by comprehensive geochemical and petrographic analyses were conducted to evaluate the hydrocarbon potential and interpret the depositional setting of this shale member. Integration of field sedimentological description, sample description and thin section petrography facilitated the identification of partially burrowed laminated fish-bearing shale, bioturbated siltstone, graptolite-bearing shale, shale/siltstone and sandstone/shale interbeds in the investigated outcrop. The intervals were sandwiched between the bioturbated sandstone of the underlying Kahfah Sandstone and the overlying Quwarah Formation. These lithofacies are interpreted to have been deposited in a shallow marine depositional setting. The preservation of graptolite in some intervals indicates low bottom dissolved oxygen conditions. The mineralogical analysis indicated that clay minerals, quartz, and feldspars are the major mineralogical components in the study samples. Based on mineralogical compositions, the studied samples were classified into three main lithofacies types; silica-rich argillaceous mudstone, clay-rich siliceous mudstone, and mixed siliceous mudstone lithofacies. Elemental analysis proxies indicated that most of the tested samples were deposited in anoxic environment. The calculated values of the chemical index of weathering (CIW) indicated that samples were exposed to high rates of weathering, which negatively impacted organic matter preservation.

Microscopic investigation identified the pore systems of the Ra’an Member as interparticle, interparticle and fracture pore systems, and observed the diagenetic features in the form of cementation, compaction, and dissolution. Organic geochemical analysis indicates that the samples are mainly of kerogen type III and II/III with poor to fair potential for hydrocarbon generation. This study provides an improved understanding of the depositional setting, organic geochemical, and thermal evolution of the Ra'an Member as a potential unconventional hydrocarbon reservoir.

This paper documents the provenance and palaeoclimatic conditions of the Cretaceous Lumshiwal Formation near the western margin of the Indian plate in the Surghar Range, northwest Pakistan. The combined techniques of petrography, X-ray diffraction (XRD), and bulk rock geochemistry were utilized to reveal the source and mineral-geochemical composition and sandstone type. The petrographic analysis of the sandstones confirms the dominance of quartz, feldspar, and rock fragments, with subordinate occurrences of muscovite, magnetite, and hematite. Heavy minerals consist of tourmaline, titanite (sphene), rutile, cassiterite, monazite, and zircon. The cementing material includes ferruginous clays, jarosite, glauconite, calcite, minor dolomite, gypsum, and silica. The modal composition plot of the sandstones falls into arkose to sub-arkose, with a few lithic arkose varieties. Lithic fragments mainly include granite, with a minor occurrence of granitic gneiss, chert, phyllite, and quartz mica schist. The discriminatory provenance diagram of the sandstones suggests a transitional continental provenance. The bulk rock geochemistry of the sandstones reveals the presence of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, FeO, MnO, CaO, MgO, Na₂O, K₂O and P₂O₅. The petrographical mineral findings were corroborated with XRD, SEM, and bulk rock geochemistry analysis (major element concentrations and their ratios), which collectively all confirm a felsic igneous source. The tectonic discrimination diagram (SiO₂-log K₂O/Na₂O wt %) implies a dominant influx of sediment sourced from the passive continental margin of the uplifted Gondwana Indian plate. Terrigenous sediments found in the Lumshiwal Formation are interpreted to have been derived from granites and granitic gneisses of the Indian Shield. The palaeo-weathering index, including the chemical index of alteration (CIA) and the chemical index of weathering (CIW) of the Lumshiwal Formation confirms a low to moderately weathered source area. The climate discrimination plot (SiO₂ versus Al₂O₃ + Na₂O + K₂O) shows that humid to semi-humid climatic conditions during the deposition of the Lumshiwal Formation.