2017年, 第20卷, 第3期　
刊出日期：2017-09-25

  

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  A preliminary investigation of fossil wood from Lower Mekong Basin of Southeast Asia

  Nareerat Boonchai, Marc Philippe, Paul A. Carling, Lyubov Meshkova

  Global Geology. 2017, 20(3): 131-143. https://doi.org/10.3969/j.issn.1673-9736.2017.03.01

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  Nine fossil wood samples from the Mesozoic bedrock and the Quaternary terrace deposits of the Lower Mekong Basin in Southeast Asia including Thailand, Laos and Cambodia were investigated in order to assist in developing a hypothesis about the Mekong River palaeocourses. Six of the samples were conifers assigned to Aga-thoxylon sp., and three were dicots of cf. Pahudioxylon sp. and two unknown taxa (Dicotyloxylon sp.). The preservation of two dicotyledonous wood samples is insufficient for precise identification to family level, such that the samples are not suitable with respect to investigation of the river palaeocourse. However, these findings increase the systematic data of the fossil wood from the Mekong Basin. Further investigations of fossil wood from the Mekong Basin are in progress to gain a regional perspective on the plant communities and to form better reconstruction of the palaeoenvironment.
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  Biostratigraphy of Asmari Formation in Ghare Agha seyed of Farsan region, Chaharmahale Bakhtiari Province, Iran

  Seyed Ahmad Babazadeh, Seyedeh Malihe Hamidzadeh

  Global Geology. 2017, 20(3): 144-152. https://doi.org/10.3969/j.issn.1673-9736.2017.03.02

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  The Asmari Formation consists of shallow marine sedimentary rocks deposited on ramp setting. Larger benthic foraminifera collected from Asmri Formation are dominated by hyaline and porcelanouse forms, including Amphistegina, Nummulites, Archaias, Astrotrillina, Miogypsinella, Miogypsinoides, Lepidocyclina, Operculina, Spiroclypeous and Miliolids. The presence of Nummulites cf. vascus in the lower part of the formation allows the age to be determined as Rupelian. The occurrence of Borelis pygmae is an index taxon of the Rupelian-Chattian and indicates Early Chattian of SBZ 21-22 in the study section. The first appearance of Miogypsinella akadagensis shows Late Chattian (SBZ 23) and defines the upper boundary of the SBZ 21-22. The new data are the first evidences showing that the shallow marine Asmari Formation is attributed to Oligocene (Rupelian-Chattian) age for this region.
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  Geochemistry, U-Pb, Hf isotopic characteristics and geological significance of Zhalaxiageyong trachydacite in Tuotuohe area, Qinghai

  ZHANG Junyi, LI Bile, ZHAO Guoquan, NING Chuanqi, SUN Jing, WANG Guozhi

  Global Geology. 2017, 20(3): 153-163. https://doi.org/10.3969/j.issn.1673-9736.2017.03.03

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  The Zhalaxiageyong lead-zinc-copper polymetallic deposit is a typical porphyry deposit of the Tuotuohe area. Whole-rock geochemical analyses, Zircon U-Pb dating and Hf isotope analysis are undertaken for the ore host trachydacite with the aim of constraining its petrogenesis, magma source and regional tectonic setting. LA-ICP-MS zircon U-Pb dating indicates that the trachydacite was formed in 32.68±0.50 Ma (MSWD=1.6), i.e., Oligocene. The trachydacite is rich in potassium and poor in Mg^#(5.10-9.70), belonging to the peraluminous shoshonite series. The rocks are enriched in LILE (large ion lithophile elements) Rb, Ba, K and LREE, depleted in HFSE (high field strength elements) Nb, Ta, P, Ti, with high Sr and low Y and Yb, having the characteristics of the C type adakite. It is calculated that the initial εHf(t) of the zircons range from -0.92 to 2.07 and their two-stage Hf model ages T_DM2 range from 978 Ma to 1 169 Ma. The magma source should be mainly the partially melt mafic rocks of the thickened Middle Neoproterozoic lower crust of the Northern Qiangtang massif with the addition of ancient aluminosilica material in the melting process. The rocks formed in the tectonic setting of delamination of lithosphere and extension of the thickened crust. During the period of 40-32 Ma, large-scale potassium rich alkaline magmatism occurred in this area. The porphyry metallogenesis is related to the magmatic activities in this period.
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  Parallel numerical simulation on CO₂ geologic storage in Ordos Basin, China

  LEI Honglei, ZHANG Yanjun, WU Fan, HU Zhongjun, YU Ziwang, ZHU Chengcheng, LÜ Tianqi

  Global Geology. 2017, 20(3): 164-169. https://doi.org/10.3969/j.issn.1673-9736.2017.03.04

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  The authors developed a 3-D numerical injection model on parallel TOUGH2-FLAC3D based on the site-specific stratigraphic information in Ordos Basin to simulate the Shenhua CO₂ geological storage (CGS) demonstration project injection process for three years and forecast CO₂ migration and layers displacement after injection for seven years. The results suggested that CO₂ migration did not stop after three years of consecutive injection, but migration rate was slowing down with time. As a result, displacement near injection well increased with injection and decreased slowly after injection. The maximum displacement of surface center occurred near the end of injection, which was only 1.24 cm. According to the displacement result, it is safe to continue injecting for this project.
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  Gas accumulation mechanism in Denglouku Formation of Changling fault depression, southern Songliao Basin, China

  YANG Guang, FAN Yeyu, LIU Changli

  Global Geology. 2017, 20(3): 170-175. https://doi.org/10.3969/j.issn.1673-9736.2017.03.05

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  The Changling gas field is occurs in tight sandstone reservoirs of the Lower Cretaceous Denglouku Formation in the Changling fault depression, southern Songliao Basin, China, which constitutes a new gas-producing area in the depression. Using information on the source-reservoir-cap rock assemblage of the Denglouku Formation, fault activity, and single well burial history of well CS1, together with data on reservoir fluid inclusion and laser Raman spectroscopy, we described the formation of the Changling gas field and determine that this fault depression did not possess suitable conditions for hydrocarbon generation. Coal-derived methane generated from underlying hydrocarbon source rock accumulated in the Lower Cretaceous Yingcheng Formation. At the end of the Late Cretaceous Qingshankou Stage, underwater volcanic eruptions occurred in the northern part of the Changling gas field near Qian'an, resulting in the reactivation of deep faults. Mantle-sourced inorganic CO₂ migrated along faults to hydrocarbon gas reservoirs in volcanic rocks of the Yingcheng Formation;Meanwhile, displaced methane (hydrocarbon gas) migrated upward to sand reservoirs of the Denglouku Formation. The methane accumulated and formed secondary gas reservoirs,Therefore fault activity was the main factor controlling the generation of gas reservoirs in the Denglouku Formation. The main accumulation period of the Yingcheng hydrocarbon gas reservoirs was 82 Ma. Whereas gas reservoir formation in the overlying Denglongku Formation was 79 Ma, slightly later than the time of formation of the Yingcheng gas reservoir in CS1 well area. At 79 Ma, the burial depth of the Denglouku Formation was 1 800-2 000 m, the diagenesis is relatively weak and the physical properties of the reservoir are relatively favorable for accumulation. This period is not only at gas generation peak time of three sets of source rock but also at the reactivation of deep faults during the formation of fault-bound depressions, thereby providing favorable conditions for the migration and accumulation of methane.
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  Multi-component joint inversion of gravity gradient based on fast forward calculation

  YUAN Zhiyi, ZENG Zhaofa, JIANG Dandan, HUAI Nan, ZHOU Fei

  Global Geology. 2017, 20(3): 176-183. https://doi.org/10.3969/j.issn.1673-9736.2017.03.06

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  With the development of gravity gradient full tensor measurement technique, three-dimensional (3D) inversion based on gravity gradient tensor can provide more accurate information. But the forward calculation of 3D full tensor sensitivity matrix is very time-consuming, which restricts its development and application. According to the symmetry of the kernel function, the authors reconstruct the underground source of geological body to avoid repeat computation of the same value, and work out the corresponding relationship between the response of geological body to the observation point and the response of reconstructed geological body to the observation point. According to the relationship, rapid calculation of full tensor gravity sensitivity matrix can be achieved. The model calculation shows that this method can increase the speed of 30-45 times compared with the traditional calculation method. The sensitivity matrix is applied to the multi-component inversion of gravity gradient. The application of this method on the measured data provides the basis for the promotion of the method.
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  Elastic reverse time migration based on vector wavefield decomposition

  GONG Qiming, HAN Liguo, ZHOU Jinju

  Global Geology. 2017, 20(3): 184-190. https://doi.org/10.3969/j.issn.1673-9736.2017.03.07

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  Prestack elastic reverse time migration (RTM) requires multicomponent seismic data. But for multicomponent elastic Kirchhoff migration, there is a limitation that ray theory no longer applies if thegeology becomes complicated.In this paper, the authors have created a new 2D migration context for isotropic, elastic RTM, which included decomposition of the elastic source and receiver wavefields into P and S wave vectors by decoupled elastodynamic extrapolation, which retained the same stress and particle velocity components as the input data. Then we appliedsource-normalized crosscorrelation imaging condition in elastic reverse time migration to compensate the energy of deep strata. We found that the resulting images were nearly identical to the velocity model, and the resolution has been improved. Our method is a wavefielddecomposition based on vector, and we can alsoavoid the problem of polarity reversal of converted shear wave imaging. It proved the applicability of the method proposed in our paper.