Detection and Evaluation of Gas Hydrates in the Eastern Nankai Trough by Geochemical and Geophysical Methods   总被引：2，自引：0，他引：2  

Ryo Matsumoto    Hitoshi Tomaru  Hailong Lu 《Resource Geology》2004,54(1):53-67

Abstract: Interstitial waters extracted from the sediment cores from the exploration wells, “BH‐1” and “MITI Nankai Trough”, drilled ～60 km off Omaezaki Peninsula in the eastern Nankai Trough, were analyzed for the chloride and sulfate concentrations to examine the depth profiles and occurrence of subsurface gas hydrates. Cored intervals from the seafloor to 310 mbsf were divided into Unit 1 (～70 mbsf, predominated by mud), Unit 2 (70–150 mbsf, mud with thin ash beds), Unit 3 (150–250+ mbsf, mud with thin ash and sand), and Unit 4 (275–310 mbsf, predominated by mud). The baseline level for Cl “concentrations was 540 mM, whereas low chloride anomalies (103 to 223 mM) were identified at around 207 mbsf (zone A), 234–240 mbsf (zone B), and 258–265 mbsf (zone C) in Unit 3. Gas hydrate saturation (Sh %) of sediment pores was calculated to be 60 % (zone A) to 80 % (zones B and C) in sands whereas only a few percent in clay and silt. The total amount of gas hydrates in hydrate‐bearing sands was estimated to be 8 to 10 m³ of solid gas hydrate per m², or 1.48 km³ CH₄ per 1 km². High saturation zones (A, B and C) were consistent with anomaly zones recognized in sonic and resistivity logs. 2D and high‐resolution seismic studies revealed two BSRs in the study area. Strong BSRs (BSR‐1) at ～263 mbsf were correlated to the boundary between gas hydrate‐bearing sands (zone C) and the shallower low velocity zone, while the lower BSRs (BSR‐2) at～289 mbsf corresponded to the top of the deeper low velocity zone of the sonic log. Tectonic uplift of the study area is thought to have caused the upward migration of BGHS. That is, BSR‐1 corresponds to the new BGHS and BSR‐2 to the old BGHS. Relic gas hydrates and free gas may survive in the interval between BSR‐1 and BSR‐2, and below BSR‐2, respectively. Direct measurements of the formation temperature for the top 170 m interval yield a geothermal gradient of ～4.3d?C/ 100 m. Extrapolation of this gradient down to the base of gas hydrate stability yields a theoretical BGHS at～230 mbsf, surprisingly ～35 m shallower than the base of gas hydrate‐bearing sands (zone C) and BSR‐1. As with the double BSRs, another tectonic uplift may explain the BGHS at unreasonably shallow depths. Alternatively, linear extrapolation of the geothermal gradient down to the hydrate‐bearing zones may not be appropriate if the gradient changes below the depths that were measured. Recognition of double BSRs (263 and 289 mbsf) and probable new BGHS (～230 mbsf) in the exploration wells implies that the BGHS has gradually migrated upward. Tectonically induced processes are thought to have enhanced dense and massive accumulation of gas hydrate deposits through effective methane recycling and condensation. To test the hypothetical models for the accumulation of gas hydrates in Nankai accretionary prism, we strongly propose to measure the equilibrium temperatures for the entire depth range down to the free gas zone below predicted BGHS and to reconstruct the water depths and uplift history of hydrate‐bearing area.  相似文献   

Mechanical stratigraphy as a factor controlling the development of a sandbox transfer zone: a three-dimensional analysis   总被引：3，自引：0，他引：3  

Antonio Ravaglia  Claudio Turrini  Silvio Seno 《Journal of Structural Geology》2004,26(12):2269-2283

In thrust belts, fold–fault terminations are common features of the structural architecture and can pose complicated problems to unravel, in particular when two or more terminations are in close proximity. Such terminations usually reflect pre-existing attributes. Amongst the many factors, lateral variations in the mechanical stratigraphy can control along-strike geometry and kinematics of fault-related folds.A displacement transfer zone was produced in a compressional sandbox model by means of two adjacent, mechanically different stratigraphic domains. The experiment allowed two discrete chains to develop in the different domains, so that a complex structural setting occurred in the connecting area. Periclinal folds, oblique thrust fronts and oblique ramps developed in the resulting transfer zone. The interaction between periclines in the transfer zone produced lateral culminations in the folded structures. The analysis of displacement across the structural domains revealed that a significant loss of slip along the faults occurred in the relay zone. In this area, imbricate faulting was partially replaced by layer-parallel shortening. A linear relationship appears to exist between the bed length of the thrust sheet and the related fault slip.  相似文献   

坡度阀值与坡面泥石流--以重庆市北碚区为例   总被引：5，自引：0，他引：5  

曾凡伟  徐刚  李青  桑琰云  夏大庆 《山地学报》2004,22(5):568-571

通过对重庆市北碚区的 2 1个坡面泥石流的实地调查表明 ,坡度对坡面泥石流发生具有重要控制性作用。在对坡面泥石流流域大量原始坡度进行统计分析后发现 ,各坡面泥石流的坡度分布具有明显的正态分布特征 ,且平均坡度分布具有显著的规律。根据这一分布规律 ,将本区坡面泥石流发生的坡度阀值确定为三类 :第一类的坡面泥石流植被覆盖率高 ,远离居民区 ,人类影响活动微弱 ,坡度阀值为 32 4° ;第二类坡面泥石流无一例外均位于林地结合部 ,森林覆盖率很低 ,人类活动影响十分剧烈 ,坡度阀值为 2 7 3° ;第三类的两条坡面泥石流相邻 ,均位于观音峡峡谷地带 ,它们的发生完全受地形条件的控制 ,坡度阀值为 36 8°。本文从定量的角度阐述了坡度阀值与坡面泥石流发生的关系 ,为生态脆弱区的划分、工程建设、防灾减灾和政府决策提供服务  相似文献   

几种土壤的细沟侵蚀过程及其影响因素   总被引：15，自引：0，他引：15       下载免费PDF全文

蔡强国  朱远达  王石英 《水科学进展》2004,15(1):12-18

通过间隔为11h的二次人工模拟降雨和冲蚀槽试验研究了10种土壤的侵蚀过程,发现在第2次降雨中大多数土壤出现了细沟侵蚀形式。采样微形态观察发现,在第1次降雨中形成结皮的土壤在第2次降雨中出现了细沟侵蚀;产流产沙观测发现所有出现细沟侵蚀的土壤在第1次降雨中径流含沙量逐渐减少,而第2次降雨中有一个先剧增后减少的过程。土壤理化分析及多元逐步回归分析结果表明,土壤颗粒组成、有机质含量、铁铝含量等因素对细沟侵蚀的影响表现不明显,而与土壤团聚体有关结构指标、细沟侵蚀、产流产沙速率及总量呈显著相关关系。分析表明,团聚体分散度、崩解速率与渗透系数之比两个指标能较好地预测细沟侵蚀发生的可能性,同时也能很好地预测侵蚀产沙量。  相似文献   

牛顿内摩擦定律和达西定律的改进     

熊道锟 《岩土工程技术》2004,18(6):275-278

根据物体运动的摩擦力定律和物体变形破坏的库仑定律对牛顿内摩擦定律作了改进 ,进而改进了达西定律。用改进的达西定律与实验资料进行拟合 ,阐述了达西定律与实际偏离的原因。  相似文献   

New heat flow data from the immediate vicinity of the Kola super-deep borehole: Vertical variation in heat flow confirmed and attributed to advection   总被引：1，自引：0，他引：1  

D. Mottaghy  R. Schellschmidt  Y.A. Popov  C. Clauser  I.T. Kukkonen  G. Nover  S. Milanovsky  R.A. Romushkevich 《Tectonophysics》2005,401(1-2):119-142

We present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12.4-km deep Kola super-deep borehole, NW Russia. Our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure (1.6 km). Our results confirm earlier results on vertical heat flow trends of in the uppermost part of the Kola super-deep hole, and imply that the thermal regime is not in steady-state conductive conditions. In an area of 3-km × 5-km measurements were performed in 1–2-km deep boreholes surrounding the Kola super-deep hole and on core samples from these holes. Temperature logs are available from 36 holes. Core data exists from 23 boreholes with a total length of 11.5 km at a vertical resolution of 10 m. We carried out a very detailed study on thermal conductivity with regard to anisotropy, inhomogeneity and temperature dependence. Tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements. Additionally, we measured specific heat capacity, heat generation rate, density, porosity, and permeability on selected subsets of core samples. Heat flow from 19 boreholes varies between 31 and 45 mW m⁻² with an average value of 38 mW m⁻². In most boreholes the vertical heat flow profiles show a considerable variation with depth. This is consistent with observations in the upper part of the Kola super-deep borehole. We conclude that this variation is not caused by technical operations but reflects a natural process. It is considered to be due to a combination of advective, structural and paleoclimatic effects. Preliminary 3-D numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors: advective heat transfer turns out to have a major influence on the vertical variation of heat flow, although transient changes in surface temperature may also cause a significant variation. Heterogeneity of the rocks in the study area is less important.  相似文献   

沾化凹陷Ro分布规律及影响有机质成熟度的因素   总被引：6，自引：0，他引：6  

李胜利  于兴河  陈建阳  黎运秀  王永诗 《地质力学学报》2005,11(1):90-96

结合实际资料，综合确定R0=0．4％为该区生烃门限(含低熟油)，相应的深度门限平均为2250m左右，温度门限平均在90℃左右。造成R。值相对较低的相关因素如下：有机质成分、温度、压力、埋深等。通过比较，沾化凹陷中渤南洼陷和孤北洼陷生烃能力强，而富林洼陷生烃能力相对较弱。  相似文献   

Effect of sill spacing and sediment size grading on scouring at grade‐control structures     

Andrea Marion  Mario A. Lenzi  Francesco Comiti 《地球表面变化过程与地形》2004,29(8):983-993

Sequences of arti?cial steps are sometimes used to reproduce the natural step–pool morphology of high‐gradient streams. The depth, length and shape of the scour holes in gravel‐bed rivers can be predicted reasonably using recently developed formulae. However, the properties of the scour holes can sometimes be affected by the distance between structures. This effect is called ‘geometrical interference’ and leads to a reduction of the scour hole compared to its potential size. Geometrical interference may occur in sequences of arti?cial steps in high‐gradient torrents, where structures are sometimes built at distances of a few tens of metres apart, but may also apply to natural step–pool systems. In this paper, a series of tests have been conducted to determine the effect of bed sill spacing and sediment grading on the potential erosion by jets forming over the sills. A new formula is derived, applicable to high‐gradient streams (slope > 0·04), which can be applied to the special case of scour holes developed by interfering sills. Sediment size gradation, not accounted for in previous formulae, is found to have a signi?cant effect on the scour dimensions and is included in the new predictive formula. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Effects of Tectonic Force on Hydrostatic Pressure in Crust   总被引：1，自引：0，他引：1  

Lü Guxian 《中国地质大学学报(英文版)》2004,15(2):155-161

The research into the hydrostatic pressure in the crust has been previously conducted from the viewpoint that the hydrostatic pressure is equal to the gravity, based on the fact that the hydrostatic pressure is derived mainly from the gravity of its overlying rocks. In this paper, the stress state of any point in the crust is suggested to have been caused by both the gravity and the tectonic force. The author proposes that the hydrostatic pressure is a combination or superposition of two isotropic stresses in the tectonic force and gravity stress fields. The results obtained with a finite-element simulation indicate that the additional hydrostatic pressure borne by rocks decreases gradually from the compression zone (pc^s),the shear zone (psh^s) to the tensile zone (pt^s), and that the difference in the additional tectonic hydrostatic pressure between these deformed zones tends to increase, following the increase in the absolute value and/or the difference in external forces between different directions. This paper presents the foundation for the research into the tectonic physicochemistry.  相似文献   

Geobarmal Gradient in Orogenic Belt and Metamorphism Caused by Ultrahigh Pressure     

Lü Guxian 《中国地质大学学报(英文版)》2004,15(2):178-182

The components and structures of lithosphere,inhomogeneous,are changing incessantly in different periods.Therefore,the state of load,called the pressure,in lithosphere is also incessantly changing.When the lithosphere volume remains the same,namely on the premise of isovolumes,the geobarmal gradient is:(dp/dh)v=(p/(e)h)v (p/(e)T)v(dT/dh)v.If β=(dp/dh)v/(p/(e)h)v is supposed,then β=1 rg-1Cv (dT/dh)v.When the geothermal degree in (dT/dh)v is zero,then the pressure grade of lithosphere is equal to the lithostatic grade,which is the minimum value in the pressure grade of lithosphere.Suppose that the lithosphere is made only up of quartz,Cv≌Cp=0.782 5.718×10-4 T-1.883×104T-2 (J·g-1·K-1) is obtained,which is the minimum one in lithospheric rock,and then the geothermal grade value of 20 ℃/km is calculated according to the geophysical transection data of Qinling Mountain orogeny.The results show that the high-pressure and ultrahigh-pressure eclogites in Jiangsu Province and Huangzhen,Dabie may,on the condition of incompletely isovolumes,occur in the depths of 17-40 km with the increase in geothermal temperature,whose values of β do not correspond to the theoretic value of 3.08.  相似文献