Sep 2007, Volume 1 Issue 3
    

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  • Longmu Co–Shuanghu plate suture and evolution records of paleo-Tethyan oceanic in Qiangtang area, Qinghai–Tibet plateau
    LI Cai, DONG Yongsheng, HUANG Xiaopeng, ZHAI Qingguo, ZENG Qinggao
    2007, 1(3): 257-264. https://doi.org/10.1007/s11707-007-0032-3
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    Three Late Triassic plate sutures, namely, the Longmu Co Shuanghu suture, Kunlun Qinling suture and Xijinwulan Jinshajiang suture, have been recognized on the Qinghai Tibet plateau. Data show that the last two sutures have no essential elements of the northern boundary of Gondwana. This paper briefly introduces the facts of the existence of the paleo-Tethys Ocean in the Longmu Co Shuanghu suture, i.e., the ages and natures of Early Paleozoic ophiolites, Permian ophiolites, Devonian Permian radiolarian cherts, accretionary relics of different natures, two types of tectonic m?lange zones and ophiolites. The starting time of paleo-Tethys Ocean in Qiangtang may be traced back to the Early Paleozoic and the records about the oceanic basin evolution lasted from the Late Devonian to Late Triassic. It is thought that the Longmu Co Shuanghu suture was the site for the extinction of the paleo-Tethys Ocean on Qinghai Tibet plateau and an important window for the reconstruction and inversion of the early-stage formation and evolution of the Qinghai Tibet plateau, as well as the northern boundary of Gondwana wich the geoscience community has paid attention to in the past few decades.
  • Post-collisional adakitic biotite plagiogranites from Guangtoushan pluton (Mianxian, central China): Petrogenesis and tectonic implication
    QIN Jiangfeng, LAI Shaocong, LI Yongfei
    2007, 1(3): 299-303. https://doi.org/10.1007/s11707-007-0036-z
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    The post-collisional Guangtoushan pluton intruded into the Mianlue suture, Central China. Geochemically, the Guangtoushan biotite plagiogranites show many close compositional similarities to high-silica adakites from the supra-subduction zone setting, but tend to have a higher concentration of K2O (3.22%&ndashi;3.84%). Chondrite-normalized rare earth element patterns are characterized by high ratios of (La/Yb)N, concave-upward shapes of the heavy rare earth element (HREE), and a lack of significant Eu anomalies. In conjunction with high abundances of Ba and Sr, as well as low abundances of Y and HREE, these patterns suggest a feldspar-poor, garnetv?ohibole-rich fractionation mineral assemblage. Coupled with previous studies, we suggest that the Guangtoushan biotite plagiogranites were likely to be caused by subducting slab break-off during the late orogenic stage in the West Qinling orogenic belt.
  • Abundances of chemical elements of the granitoids in different geotectonic units of China and their characteristics
    SHI Changyi, YAN Mingcai, CHI Qinghua
    2007, 1(3): 309-321. https://doi.org/10.1007/s11707-007-0038-x
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    On the basis of actual analytical data of 767 composited samples collected mainly from about 750 large to middle representative granitoid bodies all over China, the average chemical compositions and element abundances of about 70 chemical elements of SiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O, K2O, H2O+, CO2, TFe2O3, Ag, As, Au, B, Ba, Be, Bi, Cd, Cl, Co, Cr, Cs, Cu, F, Ga, Ge, Hf, Hg, Li, Mn, Mo, Nb, Ni, P, Pb, Rb, S, Sb, Sc, Se, Sn, Sr, Ta, Th, Ti, Tl, U, V, W, Zn, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y in alkalifeldspar granite, syenogranite and adamellite in 7 geotectonic units in China such as Tianshan Xing’an orogenic series, Sino–Korean metaplatform, Kunlun–Qilian–Qinling orogenic series, Yunnan–Tibet orogenic series, Yangtze metaplatform, South China–Youjiang orogenic zone and Himalayan orogenic belt, are calculated and presented in this paper. In addition, the characteristics of petrochemical parameters, trace element contents and rare earth element distributions of different rock types of the granitoids in different geotectonic units are also sufficiently discussed.
  • Characteristics of rare earth elements of lacustrine exhalative rock in the Xiagou Formation of Lower Cretaceous in Qingxi sag, Jiuxi basin
    WEN Huaguo, ZHENG Rongcai, GENG Wei, FAN Mingtao, WANG Manfu
    2007, 1(3): 333-340. https://doi.org/10.1007/s11707-007-0040-3
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    The exhalative rock occurring in the Xiagou Formation of Lower Cretaceous in Qingxi sag, Jiuxi basin is a sort of a rare lacustrine white smoke type, rich in ferrodolomites and albites. This paper introduces the geological background, mineral association, and lithology of the exhalative rock, and discusses its REE geochemical characteristics and connection with hydrothermal environment. It is shown that the exhalative rock has basal characteristics of hydrothermal depositional formation of LREE⟩HREE, with positive &deltaCe and negative &deltaEu, which is different from the characteristics of marine exhalative rock. Since the REE pattern and exceptional distribution of &deltaEu and &deltaCe are highly similar to the characteristics of alkalescent tholeiitic basalt in the same layer, the exhalative rock of Xiagou Formation is considered to be closely related to the origin of basalt in the same horizon. The fact that the amount of REE of exhalative rock decreases outwards indicates that exhalative rock in the Xiagou Formation may be connected with lacustrine hydrothermal convection circulation.
  • Physical and chemical environments of abnormal vitrinite reflectance evolution in the sedimentary basins
    QIU Nansheng, XIE Mingju, WANG Weixiao
    2007, 1(3): 341-350. https://doi.org/10.1007/s11707-007-0042-1
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    Based on the tested data of pressure and vitrinite reflectance of some wells in sedimentary basins, abnormal high pressure is regarded as not the only factor to retard the increase of vitrinite reflectance (Ro). Apart from the types of the organic matter, the physical environment (temperature and pressure) and chemical environment (fluid composition and inorganic elements) will result in the abnormal vitrinite reflectance values in the sedimentary basins. This paper tested trace elements and vitrinite reflectance data from the the abnormal high pressure and normal pressure strata profiles, respectively, and found that the acidic and lower salinity starta are favorable for the increase of Ro. By discussing the corresponding relationship between the contents of some trace elements in the mudstone and the vitrinite reflectance values, the typical trace elements were found to suppress and/or catalyze the vitrinite reflectance of organic matter, while the elements of Ca, Mn, Sr, B, Ba and P may result in the retardation of Ro. However, elements of Fe, Co, Zn, Ni and Rb may catalyze the organic matter maturation. This study is conductive to the organic maturation correction, oil and gas assessment and thermal history reconstruction by the paleothermometry.
  • Deep-seated gas generation and preservation condition in China
    LIU Wenhui, ZHANG Dianwei
    2007, 1(3): 351-357. https://doi.org/10.1007/s11707-007-0043-0
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    Deep-seated gas is defined in this paper as natural gas generated under the combined action of high temperature, high pressure, and environment media. As to organic matter vertical evolution, deep-seated gas is natural gas, which is generated and deposited under the position of an oil generation window. Deep-seated gas exploration is an important potential field for oil-gas exploration. Also, it is an inexorable trend to the further development of oil and gas provinces. In this paper, the authors will examine and distinguish the concept of deep-seated gas, and discuss the distribution and gas source of deep-seated gas. It is pointed out that kerogen, assemble dissoluble organic matter and disperse dissoluble organic matter all have contributed to deep-seated gas generation, especially disperse dissoluble organic matter in composite and superimposed sedimentary basin. In the end, according to the structural evolution and hydrocarbon source rock depositional distribution, the distribution of deep-seated gas in China is predicted.
  • The seismic property of reef flat in the Changxing–Feixianguan Formation, Northwest Sichuan Province, China
    LIU Shu, TANG Jianming, GUO Xusheng, ZHAO Shuang
    2007, 1(3): 358-365. https://doi.org/10.1007/s11707-007-0044-z
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    The development of a carbonate reservoir is dominated by its primary sedimentary environment. Generally, a good reservoir must be the oolite of a bank or bioclastics of an organic reef in a marginal mesa, which must deposit on the slope of the mesa. Therefore, the analyses of sequence stratigraphy are the key for predicting the presence of carbonate reservoirs. A typical sequence pattern can be seen in the Puguang gas field. As an independent lithological body, the mechanical property of a reef flat is very different from the surrounding rock, causing stress to concentrate. Stress centralization will, in turn, cause a lot of fractures and faults to develop, and ultimately, result in a gas chimney, which is a seismic smear zone beneath or above the reef flat. Known gas fields, such as the Puguang gas field, also possess gas chimneys. Sequence stratigraphy patterns in the marginal mesa and gas chimney near the reservoir are very obvious in all known fields in southeast and east Sichuan Province. By analyzing the seismic features of all known gas fields, a model to predict carbonate reservoirs has been set up. Using this prediction model, we have found that there is a typical reef flat of the marginal mesa in the Malubei structure of Tongnanba structural belts whose sequence stratigraphy pattern is very much like that of the Puguang gas field; and in the Cangxi Jiulongshan structure, there is another reef flat which is symmetrical to the Malubei reef. The Cangxi reef flat possesses the largest platform edge shallow facies domestically found, and has a typical prograding sequence. Its gas chimney is very obvious too. Because the Cangxi reef flat is in the very vicinity of the hydrocarbon kitchen and the depression is much bigger than that of the Puguang gas fields, we predict that there may be a gigascopic gas field in Cangxi County. There may also be a large gas field in the Malubei structure.
  • Methods and applications of 3-D wave equation common-azimuth prestack migration
    CHENG Jiubing, WANG Huazhong, GENG Jianhua, MA Zaitian
    2007, 1(3): 366-372. https://doi.org/10.1007/s11707-007-0045-y
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    To tackle the difficulties of a 3-D full volume prestack migration based on the double-square-root (DSR) one-way wave equation in practical applications, the common-azimuth migration approach is first discussed using dual-domain wave propagators under the theoretical frame of crossline common-offset migration. Through coordinate transforming, a common-azimuth prestack tau migration technology that recursively continues the source and receiver wavefields and picks up the migrated results in the two-way vertical traveltime (tau) direction is developed. The migrations of synthetic data sets of SEG/EAGE salt model prove that our common-azimuth migration approaches are effective in both depth and tau domains. Two real data examples show the advantages of wave-theory based prestack migration methods in accuracy and imaging resolution over the conventional Kirchhoff integral methods.
  • Interpretation of aeromagnetic anomalies of the Sulu region, eastern China and implications for deep geology
    ZHAO Sutao, LIU Qingsheng, QIAO Qingqing, FU Yuanyuan
    2007, 1(3): 373-379. https://doi.org/10.1007/s11707-007-0046-x
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    By using data on the 1:100 000 aeromagnetic anomalies of the Sulu orogenic belt, we designed three simulated geotraverses, in which deep seismic reflection and other geophysical investigations have been completed. Based on the features of magnetism of the three profiles, and under the constraints of deep seismic reflection data, together with the magnetism of the core petrology at the Chinese Continental Scientific Drilling (CCSD) pilot-hole and areal geology, the three inversions of magnetic anomalies are carried out. The characteristics of terrane structure are presented: the rocks are mostly composed of eclogite, marble, and gneiss at the depth of 5 km. At the depth between 5 and 7 km under the surface, inverse magnetic bodies are mainly the ultra high pressure metamorphic (UHPM) rock slices containing a lot of coesite-bearing eclogite. At the depth between 7 km and the bottom of upper crust are the rocks of the gneiss, granite and granite diorite that underwent ultra high pressure metamorphic process. Middle crust (10–19 km) is mostly composed of UHPM gneiss and granite that intruded later. The rocks of acid and basic granulite dominate the lower crust. Based on the inversed results of the three simulated geotraverses, we know that the UHPM rock slices of the three profiles are dipping north, stacking each other and being uplifted to the earth s surface, which may be the result of the North China craton’s subduction and exhumation in the Triassic.
  • Improving resolution of gravity data with wavelet analysis and spectral method
    QIU Ning, CHANG Yanjun, HE Zhanxiang
    2007, 1(3): 380-387. https://doi.org/10.1007/s11707-007-0047-9
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    Gravity data are the results of gravity force field interaction from all the underground sources. The objects of detection are always submerged in the background field, and thus one of the crucial problems for gravity data interpre tation is how to improve the resolution of observed information. The wavelet transform operator has recently been introduced into the domain fields both as a filter and as a powerful source analysis tool. This paper studied the effects of improving resolution of gravity data with wavelet analysis and spectral method, and revealed the geometric characteristics of density heterogeneities described by simple shaped sources. First, the basic theory of the multiscale wavelet analysis and its lifting scheme and spectral method were introduced. With the experimental study on forward simulation of anomalies given by the superposition of six objects and measured data in Songliao plain, Northeast China, the shape, size and depth of the buried objects were estimated in the study. Also, the results were compared with those obtained by conventional techniques, which demonstrated that this method greatly improves the resolution of gravity anomalies.