白云凹陷陆坡峡谷沉积与迁移特征及其对天然气水合物成藏的影响

利用高分辨率地震资料,研究了南海北部白云凹陷中新世以来的陆坡峡谷沉积和迁移特征及其对动态似海底反射（BSR）的影响。白云凹陷陆坡区浊流和底流共同作用形成了大型单向迁移峡谷沉积体系。峡谷的沉积过程包括侵蚀为主阶段、侵蚀-沉积共同作用阶段及沉积为主阶段。峡谷沉积相主要包括峡谷侵蚀基底、谷底沉积、谷内滑塌块体搬运沉积及侧向倾斜沉积层等4个单元。峡谷的迁移造成含天然气水合物脊部两侧不同的侵蚀-沉积环境,因此,脊部两侧BSR反射特征也不同。随着峡谷迁移的进行,在峡谷侵蚀侧翼处,沉积物被侵蚀,天然气水合物稳定带底界将发生下移,BSR反射特征为多轴较连续反射;而峡谷沉积侧翼处,沉积物增厚,天然气水合物稳定带将发生上移,BSR反射特征为单轴连续反射。     

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。     

南海北部天然气水合物研究进展

天然气水合物是一种新型的储量巨大的绿色能源,是目前世界各国研究界的研究热点之一。我国以及美国、日本、印度、韩国等国家都采集到了天然气水合物的实物样品。虽然我国对天然气水合物的研究起步较晚,但近年来的研究已经取得了飞速的进步,而且也于2007年5月在南海北部陆坡的神狐海域成功采集到天然气水合物的实物样品,这是在南海海域首次获取天然气水合物实物样品,证实了南海北部蕴藏着丰富的天然气水合物资源,标志着我国天然气水合物调查研究水平又上了一个新的台阶。目前,南海北部陆坡已经作为我国天然气水合物未来开发的战略选区之一。在总结我国天然气水合物以往十几年研究工作的基础上,综述了我国天然气水合物近年来在南海北部的地质、地球物理、地球化学3个方面的研究进展,提出了未来天然气水合物勘探和研究的方向和建议。     

南海北部陆坡区沉积与天然气水合物成藏关系

截止2007年底,世界上已直接或间接发现水合物的矿点共有132处。另外,许多地方见有生物及碳酸盐结壳标志。世界上取得天然气水合物样品的地区表明,其形成与分布除了需要特定的温压条件外,还与沉积条件有较为密切的关系。2007年5月,我国在南海北部海域成功钻获天然气水合物实物样品,证实了南海北部蕴藏有丰富的水合物资源。通过对南海中北部陆坡区地层的地震相和沉积相分布特征、层序地层和沉积体系的综合分析,总结其沉积层序的特征,并对该区域沉积条件与水合物聚集成藏的关系进行了探讨。     

南海北部揭阳凹陷天然气水合物的地震异常特征分析

大量研究表明南海北部珠江口盆地是天然气水合物发育区,但是该盆地东部揭阳凹陷水合物研究较少。本文利用揭阳凹陷新采集三维地震资料,对该三维地震资料进行成像道集优化和叠前时间偏移处理,得到针对水合物的新处理地震数据体,并通过高精度网格层析反演得到层速度数据体。利用该数据开展叠后约束稀疏脉冲反演,获得含天然气水合物地层波阻抗异常,综合分析反演与地震属性识别水合物。从新处理地震资料看,该区域似海底反射(bottom simulation reflection,BSR)反射呈连续、不连续与地层斜交等特征,BSR发育在一个继承性小型水道上,且下部断裂和气烟囱发育。通过分析BSR特征及BSR上下地层的速度、波阻抗、振幅、频率、相干等属性异常,结合水合物成藏条件,发现了南海北部新的天然气水合物有利富集区,为该区域水合物勘探提供基础。     

南海北部陆缘天然气水合物初探

根据天然气水合物存在的温度－压力条件,研究了南海北部的地球物理资料,发现有些地方在地震剖面上出现的海底反射ＢＳＲ,而在另一些地方海底第一沉积的层速度偏高,比一般海洋沉积高０．２－０．６４ｋｍ／ｓ。将这些地方海底第一层沉积界面处的温度－压力参数投于甲烷形成天然气水合物的温度－压力图上,发现它们的出现于水合物存在之区域中。     

南海北部天然气水合物赋存带识别与深度预测

利用地球物理方法,在钻探前对水合物赋存带进行识别和深度预测,可以为钻探井位的选取及井深的规划提供直接的数据支持.在我国南海北部二维地震数据的基础上,以速度为主要判别依据,以波阻抗反演为佐证,识别水合物赋存带的顶、底界,并以叠加速度谱为基础,预测神狐海域A测线预测井位的水合物赋存深度.在此数据的指导下,经钻探证实,该钻位...     

南海北部陆坡深水区浅层天然气藏特征

南海北部陆坡深水区的浅层天然气藏是一种伴随天然气水合物的新型油气藏, 具有埋藏浅、规模大的特点, 其埋藏深度一般小于300m。浅层天然气藏由深部裂解气沿断裂上升被天然气水合物封盖而形成, 识别似海底反射(BSR)是寻找浅层天然气藏有效方法。浅层天然气藏的气源主要有热解气、生物气和混合气, 陆坡张性断裂是气体运移的主要通道, 水合物下部的砂层是浅层天然气藏的主要储集层, 水合物层则是封盖层。从南海发现的天然气水合物分布特征看, 浅层天然气藏在陆坡深水区广泛分布且气藏厚度大, 潜在资源量非常可观, 是一种新型的开采成本相对低廉的油气藏。     

东海天然气水合物的地震特征

使用中国科学院海洋研究所"科学一号"调查船于2001年以及20世纪80年代在东海地区采集的多道地震资料,以海域天然气水合物研究为目的,对这些资料进行了数据处理并获得了偏移地震剖面。通过对地震剖面的解释,在6条剖面上确定了6段异常反射为BSR,均有振幅强、与海底相位相反的特点。6段BSR基本上都没有出现和沉积地层相交的现象。分析认为,这与东海地区第四纪以来的沉积特征有关,并不能由此否认这些异常反射是BSR。6段BSR出现的水深为750~2 000 m,埋深在0.1~0.5 s (双程时间)之间。随着海底深度的增大,BSR埋深有增大的趋势。计算结果显示,6段BSR所处的温度和压力条件都满足水合物稳定赋存所需要的温度和压力条件。本文的BSR主要与北卡斯凯迪亚盆地以及智利海域水合物的温度、压力条件相似,而与日本南海海槽、美国布莱克海台等海域水合物的温度、压力条件相差比较大。在地震剖面上,6段BSR所处的局部构造位置都和挤压、断层有关,有利于水合物的发育;在空间上,它们主要分布在东海陆坡近槽底的位置以及与陆坡相近的槽底。在南北方向上,除分布在吐噶喇断裂和宫古断裂附近外,还与南奄西、伊平屋和八重山热液活动区相邻。热液活动和水合物虽然没有直接的成因关系,但岩浆活动为水合物气源的形成提供了热源条件,为流体和气体的运移、聚集提供了通道条件,从而有利于水合物的发育与赋存。根据地震剖面反射特征推断,剖面A1A2和A14A23发育BSR的位置应该有气体或者流体从海底流出,可能是海底冷泉发育的位置。剖面A14A23上BSR发育处,振幅比的异常增大和BSR埋深的降低是相关联的。这种关联支持该处发育海底冷泉的推测。     

南海北部东沙海域天然气水合物分解事件及其与海底滑塌的关系

东沙海域是南海北部一个重要的天然气水合物成藏区, 其陆坡广泛发育滑塌构造。文章对采自东沙陆坡中部973-4柱样和下部平坦区973-5柱样开展了沉积学粒度、底栖有孔虫种属特征和稳定同位素等的综合分析。研究结果表明: 两个柱样中底栖有孔虫的δ¹³C在末次冰期均出现明显负偏现象, 同时δ¹⁸O增高, 指示该时期东沙海域存在持续的天然气水合物分解事件; 末次冰消期以来, δ¹³C负偏现象逐渐消失, δ¹⁸O值降低, 可能是由于海平面上升阻止了天然气水合物分解。973-4柱样仅在末次盛冰期对应层位440~600cm段存在明显的滑塌沉积, 且该层段对应的特征底栖有孔虫Uvigerina spp.和Bulimina spp.的数量突增, 推测该区的海底滑塌可能是由于末次盛冰期海平面大幅度下降引起天然气水合物大量分解诱发所致; 973-5柱样同样记录到了海底滑塌现象, 但其滑塌沉积晚于973-4柱样的滑塌时间, 且其规模较小。     

南海北部白云凹陷及其邻区的岩石圈强度分析

选取纵穿南海北部陆缘的长排列多道地震剖面,利用挠曲回剥和重力异常模拟相结合的过程导向法(process-oriented gravity modelling,POGM),计算了研究区内不同构造单元同张裂及裂后阶段的岩石圈有效弹性厚度(effective elastic thickness,Te),并对其分布特征进行了详细分析。计算结果显示:张裂过程中岩石圈强度很弱;而裂后阶段岩石圈强度在不同构造单元并不相同,其中番禺低隆起和下陆坡区强度较高,Te约为15km,而在北部坳陷带为7km左右,白云凹陷地区强度最低,仅为5km左右。获得的岩石圈强度结果,加深了对南海北部大陆边缘盆地特征和岩石圈构造演化过程的认识,具有重要的意义。     

Seismic evidence and formation mechanism of gas hydrates in the Zhongjiannan Basin,Western margin of the South China Sea

An analysis of 3D seismic data from the Zhongjiannan Basin in the western margin of the South China Sea (SCS) reveals seismic evidence of gas hydrates and associated gases, including pockmarks, a bottom simulating reflector (BSR), enhanced reflection (ER), reverse polarity reflection (RPR), and a dim amplitude zone (DAZ). The BSR mainly surrounds Zhongjian Island, covering an area of 350 km² in this 3D survey area. The BSR area and pockmark area do not match each other; where there is a pockmark developed, there is no BSR. The gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m. A mature pockmark usually consists of an outside trough, a middle ridge, and one or more central pits, with a diameter of several kilometers and a depth of several hundreds of meters. The process of pockmark creation entails methane consumption. Dense faults in the study area efficiently transport fluid from large depths to the shallow layer, supporting the formation of gas hydrate and ultimately the pockmark.     

Depositional characteristics and accumulation model of gas hydrates in northern South China Sea

The South China Sea (SCS) shows favorable conditions for gas hydrate accumulation and exploration prospects. Bottom simulating reflectors (BSRs) are widely distributed in the SCS. Using seismic and sequence stratigraphy, the spatial distribution of BSRs has been determined in three sequences deposited since the Late Miocene. The features of gas hydrate accumulations in northern SCS were systematically analyzed by an integrated analysis of gas source conditions, migration pathways, heat flow values, occurrence characteristics, and depositional conditions (including depositional facies, rates of deposition, sand content, and lithological features) as well as some depositional bodies (structural slopes, slump blocks, and sediment waves). This research shows that particular geological controls are important for the presence of BSRs in the SCS, not so much the basic thermodynamic controls such as temperature, pressure and a gas source. Based on this, a typical depositional accumulation model has been established. This model summarizes the distribution of each depositional system in the continental shelf, continental slope, and continental rise, and also shows the typical elements of gas hydrate accumulations. BSRs appear to commonly occur more in slope-break zones, deep-water gravity flows, and contourites. The gas hydrate-bearing sediments in the Shenhu drilling area mostly contain silt or clay, with a silt content of about 70%. In the continental shelf, BSRs are laterally continuous, and the key to gas hydrate formation and accumulation lies in gas transportation and migration conditions. In the continental slope, a majority of the BSRs are associated with zones of steep and rough relief with long-term alternation of uplift and subsidence. Rapid sediment unloading can provide a favorable sedimentary reservoir for gas hydrates. In the continental rise, BSRs occur in the sediments of submarine fans, turbidity currents.     

Source and accumulation of gas hydrate in the northern margin of the South China Sea

The northern South China Sea (NSCS) experienced continuous evolution from an active continental margin in the late Mesozoic to a stable passive continental margin in the Cenozoic. It is generally believed that the basins in the NSCS evolved as a result of Paleocene–Oligocene crustal extension and associated rifting processes. This type of sedimentary environment provides a highly favourable prerequisite for formation of large-scale oil- and gas–fields as well as gas hydrate accumulation. Based on numerous collected data, combined with the tectonic and sedimentary evolution, a preliminary summary is that primitive coal-derived gas and reworked deep gas provided an ample gas source for thermogenic gas hydrate, but the gas source in the superficial layers is derived from humic genesis. In recent years, the exploration and development of the NSCS oil, gas and gas hydrate region has provided a basis for further study. A number of 2D and 3D seismic profiles, the synthetic comparison among bottom simulating reflector (BSR) coverage characteristics, the oil-gas area, the gas maturity and the favourable hydrate-related active structural zones have provided opportunities to study more closely the accumulation and distribution of gas hydrate. The BSR has a high amplitude, with high amplitude reflections below it, which is associated with gas chimneys and pockmarks. The high amplitude reflections immediately beneath the BSR are interpreted to indicate the presence of free gas and gas hydrate. The geological and geochemical data reveal that the Cenozoic northern margin of the NSCS has developed coal-derived gas which forms an abundant supply of thermogenic gas hydrate. Deep-seated faults and active tectonic structures facilitate the gas migration and release. The thermogenic gas hydrate and biogenic gas are located at different depths, have a different gas source genesis and should be separately exploited. Based on the proven gas hydrate distribution zone, we have encircled and predicted the potential hydrate zones. Finally, we propose a simple model for the gas hydrate accumulation system in the NSCS Basin.     

Low-amplitude BSRs and gas hydrate concentration on the northern margin of the South China Sea

The passive northern continental margin of the South China Sea is rich in gas hydrates, as inferred from the occurrence of bottom-simulating reflectors (BSR) and from well logging data at Ocean Drilling Program (ODP) drill sites. Nonetheless, BSRs on new 2D multichannel seismic reflection data from the area around the Dongsha Islands (the Dongsha Rise) are not ubiquitous. They are confined to complex diapiric structures and active fault zones located between the Dongsha Rise and the surrounding depressions, implying that gas hydrate occurrence is likewise limited to these areas. Most of the BSRs have low amplitude and are therefore not clearly recognizable. Acoustic impedance provides information on rock properties and has been used to estimate gas hydrate concentration. Gas hydrate-bearing sediments have acoustic impedance that is higher than that of the surrounding sediments devoid of hydrates. Based on well logging data, the relationship between acoustic impedance and porosity can be obtained by a linear regression, and the degree of gas hydrate saturation can be determined using Archie’s equation. By applying these methods to multichannel seismic data and well logging data from the northern South China Sea, the gas hydrate concentration is found to be 3–25% of the pore space at ODP Site 1148 depending on sub-surface depth, and is estimated to be less than values of 5% estimated along seismic profile 0101. Our results suggest that saturation of gas hydrate in the northern South China Sea is higher than that estimated from well resistivity log data in the gas hydrate stability zone, but that free gas is scarce beneath this zone. It is probably the scarcity of free gas that is responsible for the low amplitudes of the BSRs.     

神狐海域水合物沉积层稀土元素地球化学特征

利用ICP-MS对神狐海域天然气水合物SHW2和SHW7两钻孔柱状样品含水合物层沉积物中的稀土元素进行了分析和研究,结果表明:SHW2及SHW7两钻孔各12个样品的ΣREE丰度平均值与上陆壳的平均值接近;沉积物中轻稀土和重稀土存在显著分异,两钻孔稀土元素配分模式相近,与上陆壳和代表上地壳平均值的澳大利亚后太古代页岩(PAAS)的稀土元素分布形态相似,并继承了源区母岩性质,显示出物源主体为陆源岩。两钻孔稀土参数剖面图显示,稀土总量(ΣREE)和轻稀土总量(LREE)、重稀土总量(HREE)具有很好的正相关性,δCe与δEu基本显示负相关,推测其沉积物源具有相似性。     

Cenozoic magmatism in the northern continental margin of the South China Sea: evidence from seismic profiles

Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene–Mid Miocene) and after cessation of seafloor spreading (Mid Miocene–Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean–continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to suggest that the northern margin of the SCS is a magma-poor rifted margin.     

Geochemistry of pore waters from the Xisha Trough, northern South China Sea and their implications for gas hydrates

This paper reports all available geochemical data on sediments and pore waters from the Xisha Trough on the northern continental margin of the South China Sea. The methane concentrations in marine sediments display a downhole increasing trend and their carbon isotopic compositions (δ ¹³C = −25 to −51‰) indicate a thermogenic origin. Pore water Cl⁻ concentrations show a range from 537 to 730 mM, and the high Cl⁻ samples also have higher concentrations of Br⁻, Na⁺, K⁺, and Mg²⁺, implying mixing between normal seawater and brine in the basin. The SO₄ ²⁻ concentrations of pore waters vary from 19.9 to 36.8 mM, and show a downhole decreasing trend. Calculated SMI (sulfate-methane interfaces) depths and sulfate gradients are between 21 and 47 mbsf, and between −0.7 and −1.7 mM/m, respectively, which are similar to values in gas hydrate locations worldwide and suggest a high methane flux in the basin. Overall, the geochemical data, together with geological and geophysical evidence, such as the high sedimentation rates, high organic carbon contents, thick sediment piles, salt and mud diapirs, active faulting, abundant thermogenic gases, and occurrence of huge bottom simulating reflector (BSR), are suggestive of a favorable condition for occurrence of gas hydrates in this region.