New high-resolution 2D seismic imaging of fluid escape structures in the Makran subduction zone,Arabian Sea

Seabed fluid escape is active in the Makran subduction zone, Arabian Sea. Based on the new high-resolution 2D seismic data, acoustic blanking zones and seafloor mounds are identified. Acoustic blanking zones include three kinds of geometries: Bell-shaped, vertically columnar and tilted zones. The bell-shaped blanking zone is characterized by weak and discontinuous reflections in the interior and up-bending reflections on the top, interpreted as gas chimneys. Vertically columnar blanking zone is interpreted as side-imaged gas chimneys associated with focused fluid flow and topped by a seafloor anomaly expressed as a localized reflection discontinuity, which may together serve as a vent structure. Tilted acoustic blanking zone could be induced by accretionary thrust activity and rapid sedimentation surrounding slope. Seafloor mounds occur at the sites of bell-shaped acoustic blanking zone and may be associated with the material intrusion. Bottom simulating refectors (BSRs) are widely distributed and exhibit a series of characteristics including diminished amplitude, low continuity as well as local shoaling overlapping with these acoustic blanking zones. The large amount of gases dissociated from the gas hydrates migrated upwards and then arrived at the near-seafloor sediments, followed by the formation of the gas hydrates and hence the seafloor mound.     

Evaluation of clayed silt properties on the behavior of hydrate production in South China Sea

Gas hydrate is one kind of potential energy resources that is buried under deep seafloor or frozen areas. The first trial offshore production from the silty reservoir was conducted in the South China Sea by the China Geological Survey (CGS). During this test, there were many unique characteristics different from the sand reservoir, which was believed to be related to the clayed silt physical properties. In this paper, simulation experiments, facilities analysis, and theoretical calculation were used to confirm the hydrate structure, reservoir thermo-physical property, and bond water movement rule. And the behavior of how they affected production efficiency was analyzed. The results showed that: It was reasonable to use the structure I rather than structure II methane hydrate phase equilibrium data to make the production plan; the dissociation heat absorbed by hydrate was large enough to cause hydrate self-protection or reformation depend on the reservoir thermal transfer and gas supply; clayed silt got better thermal conductivity compared to coarse grain, but poor thermal convection especially with hydrate; clayed silt sediment was easy to bond water, but the irreducible water can be exchanged to free water under high production pressure, and the most obvious pressure range of water increment was 1.9–4.9 MPa.     

钻采天然气水合物的初步设想

天然气水合物在一定温度压力条件下能以固体形式存在,但由于其特殊的热物理力学性质,难以像开采其它固体矿产资源的＂矿山＂型式来开发。在现有技术条件下,钻探技术是解决开采天然气水合物的技术、经济可行和避免可能由开采带来的环境问题的关键。鉴于通过钻探手段将天然气水合物在地下转化为可控的资源而后开采的现实,提出了采用化学浆液注入和降压相结合的开采模式。     

祁连山冻土区天然气水合物地球化学迁移机理探讨

陆域冻土区天然气水合物成矿机制较为复杂,水合物横向难以对比,形成机理不清楚,急需对天然气水合物迁移机理进行研究。文章根据祁连山冻土区天然气水合物发现区钻井揭示的地质和地球化学资料以及岩芯样品分析测试结果进行了综合分析。结果显示,研究区中侏罗统和上三叠统均为较好烃源岩,天然水合物气源以热解气为主,主要由上三叠烃源岩迁移和中侏罗统木里组烃源岩扩散提供,显示了多源多期次的特点。根据地质和地球化学分析,祁连山天然气水合物的形成经历了晚侏罗世—早白垩世的气体运移与聚集、中新世中晚期—上新世整体抬升、第四纪游离气体转化成天然气水合物矿藏3个阶段,经历了"先聚集-再抬升-后成藏"等过程,是构造-气候耦合作用的结果,初步建立了祁连山冻土区天然气水合物迁移机理。     

多功能水合物沉积物三轴试验系统的研制与应用

研制了新型多功能水合物沉积物三轴试验系统,系统由供-排气模块、应力加载模块、温控模块、数据采集模块和辅助模块组成。利用泵驱动高压液体施加围压和轴压,加载方式有恒压、恒流、梯度增压、跟踪气压模式,可以实现不同条件下的水合物合成、三轴剪切。围压跟踪气压的功能使水合物生成过程中净围压不变,保证了试样初始条件一致性。根据围压液体积变化测量试样体积变形,通过设置回压泵压力变化方式控制水合物气压,可以模拟不同降压速率条件下的水合物沉积物分解变形。以泥质粉细砂和二氧化碳为材料制作了三轴试样,进行了水合物合成、三轴剪切和降压分解试验,检验了仪器的可靠性。     

天然气水合物区多相流体运移模型：以南海神狐海域为例

天然气水合物是重要的海洋油气资源,近年来利用孔隙水地球化学手段结合流体反应运移模型对水合物进行勘探已 成为研究热点。文章针对南海神狐海域钻探GMGS-1航次的数据资料,用孔隙水溴碘摩尔数比值拟合全新世流体运移速率, 构建了二期次非稳态水合物多相流体运移模型,并以SH7站位作为应用,发现自更新世以来研究区水合物成藏系统处于衰 退状态。通过模型参数敏感性分析,了解到影响模型输出的参数权重从高到低依次是沉积物孔隙度、有机质含量和沉积埋 藏速率。进一步数值分析表明,在水合物成藏系统中,有机质含量与水合物饱和度满足线性相关,其他主要参数与饱和度 的关系并非单调函数,且主导水合物稳定区内的甲烷物源是深部外源流体所携带的甲烷。多相流体运移模型对水合物资源 勘查具有一定的理论意义。     

海洋三维VC观测系统优化设计

观测系统设计在地震勘探中起着至关重要的作用,最佳观测系统不仅可以提高资料品质,还能够降低采集成本。通过对海洋垂直缆（vertical cable,VC）进行正演模拟我们发现,随着偏移距的增大,同相轴会出现交叉、合并现象,地层顺序也会发生变化。我们针对单VC和多VC的特点,分别设计了相应的观测系统。然后对所设计的观测系统进行评价与优化,并得出了一些有益的结论：采用增加激发点密度的方法对面元覆盖次数的改善效果要好于采用增加激发面积的方法对面元覆盖次数的改善效果;当目标层存在倾角时,通过在构造走向上增加激发线条数,同时在下倾方向上增加激发线长度可以补偿照明损失;当目标层为背斜或向斜时,通过增大最大环半径来增加背斜和向斜照明范围的方法奏效甚微;当激发面积和接收面积相等时,通过同时增大激发面积和接收面积来提高中心区域面元覆盖次数的方法行不通,而当激发面积大于接收面积时则该问题得以解决。     

天然气水合物开采过程甲烷气泄漏海床基高密度电阻率法监测效果模拟与分析

为实现天然气水合物开采过程沉积物内甲烷气泄漏过程有效监测,设计了一种海床基原位电学监测方法。为界定该方法对不同模式泄露甲烷气的探测能力,以南海神狐海域天然气水合物试采区为研究区,构建相应地质及电阻率模型,模拟利用设计电学系统对其进行监测,计算得到不同采集参数的电阻率剖面,并对其进行对比分析。研究结果表明：海水层会在探测剖面上某深度区间内形成层状低阻异常带,对该深度区间有效电信号形成压制。将该异常带顶界深度定义为系统有效探测深度后,发现该深度受电极距直接影响,10 m极距偶极装置有效探测深度约为50 m。电学探测剖面对有效探测深度内分布的层状和团状甲烷气聚集区、慢速甲烷气泄露区、沿断层泄露的甲烷气区具有良好反映能力,数据处理得到的相对电阻率剖面与电学探测剖面相比能更好地反映甲烷气聚集区边界。该监测方法能够实时监测含气区空间变化。     

The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain,Northwest China

Muri Basin in the Qilian Mountain is the only permafrost area in China where gas hydrate samples have been obtained through scientific drilling. Fracture-filling hydrate is the main type of gas hydrate found in the Qilian Mountain permafrost. Most of gas hydrate samples had been found in a thin-layer-like, flake and block group in a fracture of Jurassic mudstone and oil shale, although some pore-filling hydrate was found in porous sandstone. The mechanism for gas hydrate formation in the Qilian Mountain permafrost is as follows: gas generation from source rock was controlled by tectonic subsidence and uplift--gas migration and accumulation was controlled by fault and tight formation--gas hydrate formation and accumulation was controlled by permafrost. Some control factors for gas hydrate formation in the Qilian Mountain permafrost were analyzed and validated through numerical analysis and laboratory experiments. CSMGem was used to estimate the gas hydrate stability zone in the Qilian permafrost at a depth of 100–400 m. This method was used to analyze the gas composition of gas hydrate to determine the gas composition before gas hydrate formation. When the overlying formation of gas accumulation zone had a permeability of 0.05 × 10⁻¹⁵ m² and water saturation of more than 0.8, gas from deep source rocks was sealed up to form the gas accumulation zone. Fracture-filling hydrate was formed in the overlap area of gas hydrate stability zone and gas accumulation zone. The experimental results showed that the lithology of reservoir played a key role in controlling the occurrence and distribution of gas hydrate in the Qilian Mountain permafrost.     

南祁连盆地木里冻土区天然气水合物烃源岩特征及评价

中国地质调查局2008年在南祁连盆地木里冻土区采集到了中国陆域第一例天然气水合物实物样品,对于天然气水合物的气体来源存在不同的认识。利用新完钻3口井样品的化验分析结果,分析了中侏罗统和上三叠统烃源岩有机地球化学特征。结果表明,中侏罗统江仓组、木里组煤系地层烃源岩有机质丰度较高,TOC(总有机碳含量)大于1%的样品占78.9%;氯仿沥青"A"含量大于0.1%的占总数的72.2%;有机质类型以Ⅱ_2、Ⅱ_1型为主;镜质体反射率R_o多介于0.7%~1.2%之间;71个样品生烃潜量平均值为8.8mg/g,总体处于生烃高峰期的生油阶段或凝析油阶段,属于好、很好烃源岩,为天然气水合物主力生烃层系。上三叠统尕勒得寺组亦为煤系地层烃源岩,TOC含量大于1%的样品占76.9%;氯仿沥青"A"含量小于0.05%;有机质类型以Ⅲ、Ⅱ_1型为主;镜质体反射率Ro介于1.1%~1.77%之间;总体处于生湿气或干气阶段,但由于构造抬升影响其生烃潜量,平均值仅为0.35mg/g,当前整体生烃能力较差,为非烃源岩或较差烃源岩,对天然气水合物成藏贡献不大。