南海北部天然气渗漏系统地球物理初探

本文主要针对南海北部大陆边缘发育的5个沉积盆地——台西南盆地、珠江口盆地、琼东南盆地、莺歌海盆地和中建南盆地,分析了近年来利用地球物理方法研究南海北部天然气渗漏系统的成果,重点包括3个方面:天然气水合物的储藏、流体运移通道以及海底表面渗漏特征。其中表征天然气水合物存在的似海底反射BSR在台西南和珠江口盆地发育明显,莺歌海盆地发现有大型气田;5个盆地流体运移活跃,其内发现了多样的运移通道:断层、底辟、气烟囱、多边形断层及水道(峡谷)等破裂结构;海底表面渗漏特征也在台西南、珠江口、莺歌海和中建南盆地均有发现。南海北部大陆边缘天然气渗漏系统广泛发育,值得进一步深入研究。     

珠江口盆地海底稳定性分析

本文根据大量野外调查资料,系统地整理了珠江口盆地各种海底不稳定的地质现象,通过分析和讨论,划分出海底比较不稳定区、稳定区、比较稳定区、不稳定区,并进行了工程地质评价,给海洋开发提供了依据。     

海底滑坡作用下滩海管道结构安全分析

海底滑坡作为常见的海洋地质灾害,对海洋油气工程安全产生巨大威胁。海床土体失稳引起滑坡体滑动,会对海底管道产生拖曳作用。基于计算流体动力学方法(CFD)建立海底滑坡体对管道作用的评估模型,采用H-B模型描述块状滑坡体并与试验比较验证,分析不同海床倾斜度滑坡对管道的作用并拟合表达式;研究了海底管道在滑坡作用下的力学响应,并采用极限状态方法开展海底滑坡作用下管道结构极限安全分析,探讨了管道埋地状态时的极限安全界限,建立滑坡作用下管道结构安全分析方法。研究表明:滑坡对管道作用力与海床倾角呈现正相关,而覆土层厚度对作用力影响较小;随着不排水抗剪强度的减小,允许的滑坡宽度和速度均增加,表明土体不排水抗剪强度与引起的拖曳力呈正相关;滑坡土体宽度对极限安全速度影响较大。     

High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone

We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data.We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km²) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as ‘chimney’ features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes.     

Hydrocarbon plumbing systems above the Snøhvit gas field: Structural control and implications for thermogenic methane leakage in the Hammerfest Basin,SW Barents Sea

Based on the analysis of the high-resolution 3D seismic data from the SW Barents Sea we study the hydrocarbon plumbing system above the Snøhvit and Albatross gas field to investigate the geo-morphological manifestation and the dynamics of leakage from the reservoir. Fluid and gas escape to the seafloor is manifested in this area as mega-pockmarks 1–2 km-wide, large pockmarks (<100 m wide) and giant pockmarks 100–300 m-wide. The size of the mega pockmarks to the south of the study area may indicate more vigorous venting, whilst the northern fluid flow regime is probably characterised by a widespread fluid and gas release. Buried mega depressions and large-to-giant pockmarks are also identified on the base Quaternary and linked to deep and shallow faults as well as to seismic pipes. A high density of buried and seafloor giant pockmarks occur above a network of faults overlying an interpreted Bottom Simulating Reflector (BSR), whose depth coincides with the estimated base of the hydrate stability zone for a thermogenically derived gas hydrate with around 90 mol% methane. Deep regional faults provide a direct route for the ascending thermogenic fluids from the reservoir, which then leaked through the shallow faults linked to seismic pipes. It is proposed that the last episodic hydrocarbon leakage from the reservoir was responsible for providing a methane source for the formation of gas hydrates. We inferred that at least two temporally and dynamically different fluid and gas venting events took place in the study area: (1) prior to late Weichselian and recorded on the Upper Regional Unconformity (URU) and (2) following the Last Glacial Maximum between ∼17 and 16 cal ka BP and recorded on the present-day seafloor.     

Pockmark-like depressions near the Goliat hydrocarbon field,Barents Sea: Morphology and genesis

Pockmarks are observed worldwide along the continental margins and are inferred to be indicators of fluid expulsion. In the present study, we have analysed multibeam bathymetry and 2D/3D seismic data from the south-western Barents Sea, in relation to gas hydrate stability field and sediment type, to examine pockmark genesis. Seismic attributes of the sediments at and beneath the seafloor have been analysed to study the factors related to pockmark formation. The seabed depths in the study area are just outside the methane hydrate stability field, but the presence of higher order hydrocarbon gases such as ethane and/or propane in the expelled fluids may cause localised gas hydrate formation. The selective occurrence of pockmarks in regions of specific seabed sediment types indicates that their formation is more closely related to the type of seabed sediment than the source path of fluid venting such as faults. The presence of high acoustic backscatter amplitudes at the centre of the pockmarks indicates harder/coarser sediments, likely linked to removal of soft material. The pockmarks show high seismic reflection amplitudes along their fringes indicating deposition of carbonates precipitated from upwelling fluids. High seismic amplitude gas anomalies underlying the region away from the pockmarks indicate active fluid flow from hydrocarbon source rocks beneath, which is blocked by overlying less permeable formations. In areas of consolidated sediments, the upward flow is limited to open fault locations, while soft sediment areas allow diffused flow of fluids and hence formation of pockmarks over a wider region, through removal of fine-grained material.     

Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.     

南海神狐水合物钻探区不同形态流体地震反射特征与水合物产出的关系

天然气水合物的分布在很大程度上受到含气流体运移的影响。南海北部陆坡区,尤其是珠江口盆地的白云凹陷,普遍存在流体渗漏的现象,暗示了水合物赋存的良好前景。神狐海域水合物钻探区内的高分辨率地震资料显示,区域内发育大量流体运移通道,在地震剖面上表现为不同形态的地震反射模糊带,根据其形态特征,可以划分为花冠状和穹顶状两大类模糊反射带。模糊反射带的存在意味着研究区内具有良好的含气流体运移条件,能够为甲烷气体的垂向运移提供通道。神狐海域水合物的钻探结果表明,水合物的分布与模糊反射带的分布范围具有良好的空间匹配关系,其中,花冠状地震反射模糊带侧翼部与中尺度正断层相连,促进了含气流体的侧向运移,顶部与可能的微裂隙相通,气体可向上运移至水合物稳定带,形成了水合物藏;而穹顶状地震反射模糊带顶部则通过疑似流体通道与海底沟通,这种结构极易形成气体逃逸而无法形成水合物。因此,不同形态特征的模糊反射带可能对水合物的分布具有一定的指示意义。     

琼东南盆地深水区流体势分析及其对天然气水合物成藏的意义

针对深水区缺乏钻井资料的情况,从可以广泛获取的叠加速度出发建立了一套计算地下流体势(气势)的方法,并以此对琼东南盆地深水区进行了实例分析,以此来获得天然气运移和水合物成藏的有益信息。盆地广泛发育断裂、底辟构造,影响着水合物的分布。BSR发育区与气体势场强度的汇聚区域有着较好的对应关系,水合物根据成因可分为两类:一类是以浅部地层生物成因气为主,另一类是以深部热成因气为主。大部分断层对应于相对低势区,反映断层的开启性,可以作为气体运移的通道,其上部发育深部热成因气为主的水合物藏。在构造隆起附近发育的底辟具有相对高气势的特征,这类底辟携带大量的深部热成因气运移至浅部,为水合物的形成提供充足的气源,在其附近剖面上常具有BSR的显示。     

天然气水合物的分解导致海底沉积层滑坡的力学机理及相关分析

Gas bydrate is a form of natural gas and water,similar to ice-like solids,mainly in low-temperature occurrence in the context of high voltage submarine and terrestrial and terrestrial sediments permafrost zone.Since 1970s marine lil and gas exploration in the course of a certain depth of the seabed found that the stability of gas hydrate layer and collected samples in kind,its social concem will be on the increase. Landslide Harbor is a major cause of the destructive consequences of the marine geological disaters.In recent years,researchers continue to find that as the undersea gas hydrate decomposition led to the seabed to reduce the stability of rocks,the seabed is a major reason for the decline. The research on submarine landslide,related with natural gas Hydrate is the study of a new direction. This paper analyzes the gas hydrate formation and decomposition fo the seabed sediment. Fron the mechanical aspects of the mechanism,it analyzes of the decomposition of gas hydrate deposits related to property caused by the impact of changes in the landslide,and look to the future hydrate research.     

Flash-flood hyperpycnal flows generating shallow-water landslides at Fiumara mouths in Western Messina Strait (Italy)

On October 1, 2009, a small area along the Sicilian coast between the villages of Scaletta Zanclea and Giampilieri was struck by intense and concentrated rainfall causing countless small landslides widespread over the catchment area of steep subaerial creeks, locally known as Fiumara. Dense and quick debris flows were channelized within the Fiumara and destructively hit the villages and entered the sea, where they likely transformed into hyperpycnal flows. The availability of pre- and post-flood high-resolution bathymetry allows us to recognize the main features and the most significant morphological variations related to the impact of the flows on the seafloor. The passage of hyperpycnal flows on the seafloor possibly produced a suite of mass-wasting events, encompassing sheet landslides (i.e. erosive scours), retrogressive slope failure on loose sediment at the canyon headwall and rock-falls on the conglomeratic bedrock along canyon sidewall. The possible causes of these events are discussed on the basis of available morphological evidence and geotechnical considerations. Finally, the widespread occurrence of mass-wasting features (i.e. submarine landslide scars) morphologically similar to those generated by the 2009 flash flood allows us to hypothesize, from one side, a strong correlation between this catastrophic event and the evolution of submarine canyons, and from the other side, the possible use of these features for the assessment of flash-flood occurrence.     

南海东北陆坡海底微地貌特征及其天然气渗漏模式

海底天然气渗漏是海洋环境中广泛分布的自然现象,在世界各大洋中都有发现。海底渗漏可以极大地改变海底地貌特征,形成多种与之相关的微地貌类型。海底渗漏和天然气水合物的赋存具有密切的关系,海底渗漏区常伴有埋藏浅、饱和度高的天然气水合物。对南海东北部陆坡海域浅地层剖面、多波束测深和地震反射剖面等资料进行综合研究,识别出海底麻坑、海底丘状体、大型海底圆丘、泥火山等与海底天然气渗漏有关的微地貌类型,且麻坑、海底丘状体／大型海底圆丘、泥火山微地貌分别代表了浅覆盖层快速天然气渗漏、浅覆盖层中等速度天然气渗漏和厚覆盖层快速天然气渗漏3种天然气渗漏模式。以海底丘状体微地貌及声空白反射特征的浅层天然气聚集带,成为块状水合物最理想的发育场所,这可能成为南海北部陆坡勘察块状水合物的重要识别标志。     

Fluid migration directions inferred from gradient of time surfaces of the sub seabed

The role of sub seabed topographically controlled fluid migration is assessed to improve our understanding of distributions of acoustic chimneys at the Nyegga pockmark field on the mid-Norwegian continental margin. 3D seismic data interpretations resulted in topographic gradients of seismic time surfaces and RMS amplitude maps. Topographical gradient maps and flow tracing allowed identifying migration pathways and trapping locations for free gas within the shallow sub seabed. The occurrence of acoustic chimneys, pockmarks and mounds correlate with identified fluid migration pathways and gas trapping locations. An important factor that controls the trapping locations and the lateral distribution of seeps on the seabed at Nyegga is the variation through time of the depth of the base of the gas hydrate stability zone (BGHSZ). Fluids can derive from gas hydrate systems that are suspected of being a biogenic source and/or Tertiary domes that are considered to show leakage of thermogenic fluids to the shallow geosphere.     

南海中建海域麻坑发育特征及成因机制

海底麻坑在圈定潜在的天然气水合物发育区和指示海底地质灾害方面都具有重要意义。基于南海中建海域的高密度三维地震资料,采用自动追踪技术对研究区海底地貌特征进行了刻画,发现了众多形态各异、大小不一的麻坑,可分为圆形麻坑、椭圆形麻坑、拉长形麻坑、新月形麻坑和复合型麻坑等5类。中建海域的海底麻坑主要发育在海底地形坡折的位置,成群、成带分布,在地形平坦的位置麻坑不发育。影响中建海域麻坑形成的因素主要有火山活动、断裂活动、水合物分解、海底底流等。引起中建海域海底形成麻坑的流体主要有4种,分别是火山热液、天然气水合物分解的气体、沿断裂向上运移的深部油气及火山热液与天然气水合物分解气体的混合。     

Distribution and expression of gas seeps in a gas hydrate province of the northeastern Sakhalin continental slope, Sea of Okhotsk

Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km² area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage.     

An insight into shallow gas hydrates in the Dongsha area,South China Sea

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential, but cared little about the shallow gas hydrates occurrences. Shallow gas hydrates have been confirmed by drill cores at three sites(GMGS2 08, GMGS2 09 and GMGS2 16) during the GMGS2 cruise, which occur as veins, blocky nodules or massive layers, at 8–30 m below the seafloor. Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways. The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas. The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates.Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor, although without resulting in gas plumes easily detectable by acoustic methods.The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the subbottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area. Active cold seeps,indicated by either gas plume or seepage vent, can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.     

天然气水合物赋存区麻坑地貌特征及其地质灾害意义

文章通过对天然气水合物资源及其赋存区典型地貌体特征的分析,梳理了天然气水合物赋存区和麻坑地貌体的地质灾害类型,阐述了由于天然气水合物分解释放造成海底滑坡、滑塌和甲烷气体大规模逸散,继而引起海底输电或通信电缆毁坏、海洋石油钻井平台垮塌,甚至海啸和全球气候变暖等严重灾害。为天然气水合物勘探和开采提供有益参考。     

Numerical analysis of the impact forces exerted by submarine landslides on pipelines

Submarine pipelines that transport crude oil and natural gas are often in a complex marine geological environment and may become unstable and fail upon impact by submarine landslides. Previous research has mostly focused on the impact forces exerted by submarine landslides on suspended pipelines, but the impact of submarine landslides on pipelines laid on the seafloor at various impact angles, θ, have been relatively infrequently discussed, and the effects of suspended height, H, on the impact forces exerted by submarine landslides on pipelines have not been thoroughly investigated. In this study, based on the Herschel–Bulkley model, the impact forces exerted by a submarine landslide on laid-on or suspended pipelines at various impact angles θ were simulated using the computational fluid dynamics (CFD) approach. Equations for calculating the axial and normal drag coefficients of a submarine pipeline were proposed. The CFD numerical simulation results were rearranged based on the soil mechanics approach. By comparing the parameters, an essentially corresponding relationship was found between the soil mechanics and CFD approaches when the equations were used to calculate the impact forces exerted by a submarine landslide on a pipeline. In addition, a semi-analytical expression for the failure envelope was provided. Furthermore, the effects of H on the forces on a pipeline were discussed, and an equation for calculating the acting forces on a pipeline along the flow direction of a submarine landslide that comprehensively accounts for the effects of θ and H was proposed. The lift force was discussed preliminarily and the results provide a basis for further investigation. The achievement of this study is applicable for selecting locations of submarine pipeline routes and for designing submarine pipelines.     

冲绳海槽天然气水合物与地质构造的关系

海底天然气水合物大多与通过切穿沉积盖层的断裂的上升烃类流体相关,这些高渗透带包括底辟和泥火山等侵入构造,所以海底断裂、底辟和泥火山等构造周围可能赋存天然气水合物;其次,高沉积速率和巨厚沉积层可使有机质迅速掩埋而保存起来,为天然气水合物的生成提供充足物源,因此,邻近陆坡河谷口的海底沉积扇也是天然气水合物赋存的有利地区;另外,由于陆坡区的水合物沉积层比海盆更容易受外界温压变化的影响发生失稳分解,诱发海底滑坡,所以滑坡与天然气水合物赋存之间的关系也非常密切。冲绳海槽邻近海域具有覆水深、沉积厚度大、沉积速率高和有机质丰富等有利赋存条件,目前的研究已经在该海域发现了天然气水合物赋存的地球物理证据BSR,因此,在现有研究基础上开展断裂、泥火山、海底扇、海底滑坡等与天然气水合物相关的构造研究,可以深入了解天然气水合物在不同地质构造中的分布特征与演化,为更精确地评估其资源潜力提供参考。     

南海东北部陆坡天然气水合物区的滑塌和泥火山活动

本文研究了南海东北部陆坡天然气水合物区滑塌和泥火山活动的特征及表现形式,探讨了滑塌和泥火山活动对天然气水合物成藏的影响,提出了滑塌主导和滑塌、泥火山共同作用两种控制模式。根据地震数据、浅层剖面和海底地形数据解释,将研究区划分为规则滑塌区和泥火山活动影响区,并识别出泥火山、泥火山脊、凹槽、凹坑等特征地形。滑塌和泥火山活动是陆坡天然气水合物发育区重要的地形控制因素,两种活动共同作用产生复杂的地形特征。综合多条地震测线中似海底反射层(BSR)形态、连续性和滑塌、泥火山活动的关系,认为滑塌控制的区域,BSR连续,天然气水合物储藏较完整,泥火山活动区天然气水合物储藏也仅受到局部破坏。同时指出滑塌和泥火山活动对研究区长期天然气渗漏活动具有重要作用。