南海北部天然气水合物成藏地质条件及成因模式探讨

中国南海北部陆坡区是天然气水合物成藏的理想场所,资源潜力巨大。文章基于天然气水合物勘探成果,结合南海北部天然气水合物成藏地质背景,从天然气水合物成藏的温压稳定条件、气源形成条件、构造输导条件和沉积储集条件4方面,系统分析了南海北部天然气水合物成藏的基本地质条件,探讨了南海北部陆坡中部神狐海域、南海北部陆坡西部海域和南海北部陆坡东北部海域天然气水合物的成藏类型与成因模式。结果认为,南海北部陆坡中部神狐海域主要发育扩散型水合物,而南海北部陆坡西部海域主要发育渗漏型水合物,南海北部陆坡东北部海域则发育兼具扩散型与渗漏型特征的复合型水合物。     

基于天然气水合物地震数据计算南海北部陆坡海底热流

天然气水合物是一种由水的冰晶格架及其间吸附的气体分子（以甲烷为主）组成的固态化合物,地震剖面上的似海底反射BSR是天然气水合物赋存的重要地球物理标志。相同气体成分水合物的相对稳定的温压关系是根据BSR的赋存深度计算海底热流的理论基础。选择南海北部陆坡有典型BSR反射的地震剖面,计算了南海北部陆坡天然气水合物发育区的压力、温度、地温梯度、热导率及热流等地热参数。通过计算热流值与实测热流值的对比可以大致推测,在南海北部陆坡海底运用该方法计算的热流值误差可能在12％以内。本研究不仅可以为海底热流等理论研究提供一定信度的数据资料,而且通过实测热流值校正后的热流数据以及经验公式,可以反过来用于BSR深度的计算以及天然气水合物稳定域的预测,具有重要的实践意义。     

南海北部东沙陆坡主要灾害地质因素特征

我国深水区海洋灾害地质研究成果较少,而南海北部东沙陆坡深水区天然气水合物资源的进一步勘探急需弄清海底地质环境。研究区缺乏浅地层剖面资料,因此,本文基于高分辨率2D地震资料等实测资料,率先在东沙陆坡深水区开展灾害地质研究。文中识别出地震、活动断层、海底滑坡、浅层气、泥火山、陡坎及侵蚀沟(谷)共7种具有活动能力的破坏性灾害地质因素,还识别出浅埋基岩面、海底火山、岩浆底辟共3种不具有活动能力的限制性灾害地质因素,同时总结了各灾害地质因素的平面展布特征并研究了其地震反射特征及灾害性。基于以上研究,本文认为陆坡深水区应重点避让海底滑坡、活动断裂、浅层气、侵蚀沟等与海洋工程关系密切的灾害地质类型,研究成果虽然精度有限,但可初步为该区天然气水合物的进一步勘探及防灾减灾提供参考。     

珠江口盆地白云深水区海底重力滑脱构造地震地质综合识别

综合利用南海北部陆坡珠江口盆地白云深水区水深数据、海底多波束地形资料及高分辨率多道地震资料,对该区海底重力滑脱构造的内部结构、外部形态及地震反射特征进行识别,并探讨了其主要控制因素。研究发现,白云深水区海底重力滑脱构造发育,在陆架边缘、陆坡和深海平原均有分布,海底重力滑脱构造与珠江海底峡谷共同塑造了该区的海底地形地貌特征,多期发育造成的复杂结构使其外部形态和内部地震反射呈现多样性。结合该区构造沉积背景及天然气水合物勘探研究现状,可以发现该区构造格局和上新世末珠江口海平面升降是控制其发育的必要条件,天然气水合物的多期次分解可能与其相互作用,后期底流的影响进一步改造了局部微观地貌。     

南海北部陆坡坳隆断裂带中水合物赋存的温压场环境

天然气水合物的发育和赋存明显受构造条件控制,一些活动和非活动大陆边缘的特殊构造单元,如增生楔、泥火山、泥底辟、海底滑塌体均有利于水合物的发育和赋存。南海北部陆坡天然气水合物调查研究表明:南海北部陆坡的坳隆断裂带是有利于天然气水合物发育和赋存的重要构造之一。选择穿过坳隆断裂带的地震剖面A,根据南海北部陆坡海水深度和海底温度的关系曲线,计算了沉积物的温度;并在拟合沉积物深度与体密度关系曲线的基础上,计算了沉积物中的净岩压力,进一步探讨了测线A的温压场环境。结果表明:坳隆断裂带中存在连续变化的温压场环境,当气体在其中运移时,总能遇到有利的温压场环境,从而形成天然气水合物;另一方面,坳隆断裂带中也不是处处均有利于水合物的发育和赋存,需要进行具体的温压条件分析,从而对水合物稳定域进行准确定位。     

西沙海槽研究区天然气水合物地球化学勘探及成藏模式研究

依据ODP204航次1250C站位钻井样品酸解烃数据结果,以及作者在南海西沙海槽研究区天然气水合物地球化学现场勘查中得到的多种烃类指标数据、异常点上微量甲烷碳同位素数值等资料,对海洋水合物地球化学勘探的依据和研究区气态烃异常特征、气体成因、天然气水合物成藏模式等相关问题进行了研究探讨。结果表明:气态烃指标地球化学异常主要分布在工区北部斜坡地带,并与BSR等地震标志及深部断裂关系密切;西沙海槽研究区海底沉积物气态烃甲烷以热解成因为主,但也有混合成因;推测该区天然气水合物为断层渗滤综合成藏模式。研究成果比较合理地解释了BSR分布和海底沉积物甲烷局部异常并非完全一致的原因;评价预测了该区天然气水合物有利勘探目标。成果为该区天然气水合物勘探、天然气水合物成因机制研究和天然气水合物远景预测,提供了地球化学方面的证据。     

南海天然气水合物稳定带厚度分布特征

天然气水合物在未来能源、自然环境和灾害等方面具有重要的研究意义,其形成除需要充足的气源外,还与温度、压力密切相关。天然气水合物稳定带表明该地区水合物发育与分布的可能范围。以Dickens和Quinby Hunt的甲烷水合物相平衡公式为基础,从地热学角度分析南海甲烷水合物稳定带厚度及其分布特征。研究表明,南海大部分海域均具备形成天然气水合物的条件。由于受海底深度、海底温度、热流等参数的影响,在不同位置发育的水合物稳定带厚度变化较大,最大厚度可达1 100 m,位于吕宋海槽内。水合物稳定带厚度较大的区域主要呈条带状分布在南海中部和东部,大陆边缘水深500 m左右即可形成水合物,说明南海地区具有广泛的天然气水合物形成环境。天然气水合物稳定带厚度仅是水合物厚度的理论值,地层中实际的水合物发育厚度和分布特征还受到气源、构造、沉积等因素的影响。此外,岩石热导率、海底温度、热流和水深等对南海水合物稳定带厚度及其分布有影响。     

南海北部陆坡构造坡折带中的天然气水合物

构造坡折带是由同沉积构造长期活动引起的沉积斜坡明显突变的地带,对盆地充填的可容纳空间和沉积作用可产生重要的影响,构造坡折带和油气资源分布有着较为密切的关系。本文首次把“构造坡折带”概念引入到天然气水合物调查研究中,认为:构造坡折带是天然气水合物发育和赋存的重要构造单元。论文从海底坡降、断层发育以及温压场环境等角度讨论了构造坡折带对天然气水合物的控制作用,发现BSR发育位置的海底坡降一般在10×10-2～30×10-2之间;利用论文中设计的相对构造强度的算法计算了南海北部陆坡D区的相对构造强度,发现相对构造强度的次高值区有利于水合物赋存;根据高分辨率地震数据计算的构造坡折带的温压场环境,发现构造坡折带中连续变化的温压场环境能够使游离气运移到适宜的温压场中并形成水合物。在对上述研究进行总结归纳的基础上,初步提出了南海北部陆坡构造坡折带中天然气水合物的成藏模式。     

东沙海域SMI与甲烷通量的关系及对水合物的指示

硫酸根甲烷界面(SMI)是识别海洋沉积物中天然气水合物赋存(甲烷通量)的一个重要生物地球化学标志.通过对南海北部陆坡东沙海域37个站位浅表层沉积物中孔隙水的SO42-和H2S含量变化和沉积物顶空气甲烷含量的变化等地球化学特性进行分析,研究南海北部东沙海域硫酸根甲烷界面(SMI)的分布情况,通过硫酸根变化梯度估算甲烷通量.研究结果显示,东沙海域存在南部深水区"海洋四号"沉积体和北部浅水区九龙甲烷礁两个水合物有利区域,SMI埋深普遍较浅,指示较高的甲烷通量(3.8×10-3~5.9×10-3 mmol/(cm2·a)),与国际上已发现天然气水合物区的地球化学特征相类似.这种高甲烷通量很可能是由下伏的天然气水合物所引起的,暗示着该区海底之下可能有天然气水合物层赋存.     

南海东北部陆坡区天然气水合物成矿作用

南海东北部陆坡区具有准被动大陆边缘构造特征,新生代含油气沉积盆地发育,具备天然气水合物发育的一系列有利地质构造背景。基于该区已有的调查资料,从构造、沉积、成矿气体来源和水合物相平衡及其热力学特征等方面,综合分析了南海东北陆坡区有利的天然气水合物成矿条件。结果表明,研究区完全具备形成水合物的海底温度、地温梯度和压力条件,存在底辟和泥火山构造、斜坡-断层构造组合等特殊地质构造环境及构造部位,浅部沉积层具有合适的岩石学条件和较高的沉积速率,并存在丰富的生物气和热解气。因此,南海东北部陆坡区具有潜力极大的天然气水合物成矿远景。     

南海天然气水合物稳定带的影响因素

文章利用南海所积累的大量热流、海底温度和地温梯度数据，针对地温梯度的变化，对地温梯度数据进行了初步校正。分情况研究了纯甲烷，甲烷、乙烷、丙烷混合物分别在纯水、海水条件下形成的天然气水合物在南海的可能分布范围；进而对影响天然气水合物分布的影响因素进行了讨论。研究表明，随着天然气中重烃含量的增加，孔隙水盐度的降低，水合物稳定带在平面上的分布范围越来越大，水合物稳定带的厚度也越来越大。比较而言，气体组成的影响要比孔隙水盐度的大。同时，天然气水合物稳定带的厚度与热流有一定的负相关关系。在南海2000m水深范围之内，由于受海底温度的控制，水合物稳定带的厚度与水深呈明显的正相关关系。     

青藏高原天然气水合物的形成与多年冻土的关系

天然气水合物是一种新型清洁能源,赋存在多年冻土区和海洋沉积物等低温高压环境中.青藏高原多年冻土面积占高原总面积的一半以上,是可能的天然气水合物赋存区.根据青藏高原多年冻土条件和天然气水合物形成的热力学条件,讨论了多年冻土地温梯度、冻土厚度与天然气水合物形成的热力学条件之间的关系和青藏高原存在天然气水合物的可能性.结果表明,青藏高原多年冻土区基本具备形成天然气水合物的热力学条件,最适宜的热力学条件是多年冻土地温梯度接近或略大于多年冻土底板附近融土的地温梯度,且融土地温梯度越小,越容易形成天然气水合物.估算得到天然气水合物最浅的顶界埋深为74 m左右,最深的底界埋深达上千米.     

Centrifuge modelling of laccolith intrusion

Laccolith intrusion has been investigated by centrifuge modelling. Silicone putty, representing magma with Bingham rheology, is intruded by overburden pressure through a circular conduit into a stack of paraffin wax layers, which represent sedimentary strata. The model intrusions evolve through a sequence of geometric forms similar to those exhibited by natural laccoliths; the results are also in accord with published mathematical analyses of laccolith formation. The model magma first flows laterally between overburden layers to form a sill. As the sill increases in area, the cover strata are arched upwards into a sinusoidal, ‘bending’ form. Subsequently, the overburden fails abruptly by kinking above the periphery of the intrusion, and the form changes to a cupola separated from surrounding horizontal strata by a sharp inflection in the layering. Finally, strata on the flanks of the cupola undergo a second episode of localized failure. The top of the dome becomes flat, and the dipping flanks become a monoclinal flexure surrounding this ‘kink’ form.A number of variables influence this evolution. Increased overburden strength, thickness of overburden strata, and total thickness of overburden all prolong the bending form to larger intrusion size, delay the localized failure which marks the cupola and kink stages, and reduce the aspect ratio of the intrusion at all stages. Effectively bonded overburden interfaces have a similar effect, but interfaces with sharply defined yield strengths enhance development of the monoclinal flexure. Magmatic driving pressure affects the rate of intrusion, but not the evolution of its form. Intrusion rate is also influenced by magma flow properties, and probably by the form of the feeder conduit. The range of final forms preserved by natural laccoliths probably results from competition between rate of intrusion, controlled by driving pressure and conduit geometry, and rate of chilling of magma in the laccoliths. The model laccoliths form in times that scale to between one and a few tens of years, in agreement with available estimates for natural laccoliths.     

Geology of the ultrabasic to basic Uitkomst complex, eastern Transvaal, South Africa: an overview

The Uitkomst complex in eastern Transvaal, South Africa, is a mineralized, layered ultrabasic to basic intrusion of Bushveld complex age (2.05–2.06 Ga) that intruded into the sedimentary rocks of the Lower Transvaal Supergroup. The complex is situated 20 km north of Badplaas. It is elongated in a northwesterly direction and is exposed over a total distance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enveloped by metamorphosed and, in places, brecciated country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of teh complex.The complex consists of six lithological units (from bottom to top): Basal Gabbro, Lower Harzburgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite and Gabbronorite. The Basal Gabbro Unit, developed at the base of the intrusion and showing a narrow chilled margin of 0.2 to 1.5 m against the floor rocks, has an average thickness of 6 m and grades upwards into the sulphide-rich and xenolith-bearing sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich harzburgite of the Chromitiferous Harzburgite Unit (average thickness 60 m). Following on from the Chromitiferous Harzburgite Unit is the 330 m thick Main Harzburgite Unit. The Pyroxenite and Gabbronorite Units (total combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromitiferous Harzburgite, are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, steatitization, saussuritization and uralitization.Field relations, petrography and mineral and whole rock chemistry suggest the following sequence of events, The original emplacement of magma took place from northwest to southeast. The intrusion was bounded between two major fracture zones that gave rise to an elongated body, which acted as a conduit for later magma heaves. The first magma pulses, forming the chilled margin of the intrusion, show chemical affinities to a micropyroxenite described from the Bushveld complex. The Lower Harzburgite and Chromitiferous Harzburgite Units, judged from the abundance of xenoliths, originated by crystal settling from a contaminated basic magma. The Main Harzburgite crystallized from a magma of constant, probably also basic, composition, which flowed through the conduit after formation of the lower three lithological units. At a late stage of emplacement, after replenishment in the conduit came to a standstill, closed system conditions developed in the upper part of the complex, resulting in a magma fractionation trend of increasing incompatible elements contents towards the top of the intrusion.The mineralization in the lower three rock units and at the base was most probably caused by a segregating sulphide liquid forced to precipitate by the oxidative degassing of dolomite. Sulphur isotope ratios indicate various degrees of contamination of the magma by the enveloping sedimentary rocks, which provided the necessary amounts of S to reach S saturation.     

白云凹陷浅成岩浆侵入体发育特征、成因及油气地质意义

珠江口盆地白云凹陷内发育大量早中新世(约15.5 Ma)浅成岩浆侵入体(侵入深度<3 km).目前对其形态特征、相互关系以及岩浆侵入诱发的强制褶皱等尚缺乏系统研究.通过利用高精度三维地震资料,细致刻画了侵入体的形态、岩浆运移通道以及岩床复合体内各岩床的空间关系,定量分析了强制褶皱与侵入体的几何关系.白云凹陷内岩浆侵入体...     

http://www.sciencedirect.com/science/article/pii/S1674987112000096

According to the metallogenic theory by transmagmatic fluid(TMF).one magmalic intrusion is a channel of ore-bearing fluids,but not their source.Therefore,it is possible to use TMF’s ability for injection into and for escaping from the magmatic intrusion to evaluate its ore-forming potential.As the ore-bearing fluids cannot effectively inject into the magmatic intrusion when the magma fully crystallized, the cooling time and rates viscosity varied can be used to estimate the minimum critical thickness of the intrusion.One dimensional heat transfer model is used to determine the cooling time for three representative dikes of different composition(granite porphyry,quartz diorite and diabase) in the Shihu gold deposit.It also estimated the rates viscosity varied in these lime interval.We took the thickness of dike at the intersection of the cooling time—thickness curve and the rates viscosity varied versus thickness curve as the minimum critical thickness.For the ore-bearing fluids effectively injecting into the magma,the minimum critical thicknesses for the three representative dikes are 33.45 m for granite porphyry,8.22 m for quartz diorite and 1.02 m for diabase,indicating that ore-bearing dikes must be thicker than each value.These results are consistent with the occurrence of ore bodies,and thus they could be applied in practice.Based on the statistical relationship between the length and the width of dikes.these critical thicknesses are used to compute critical areas:0.0003—0.0016 km~2 for diabase. 0.014—0.068 km~2 for quartz diorite and 0.011—0.034 km~2 for granite porphyry.This implies that orebearing minor intrusions have varied areas corresponding to their composition.The numerical simulation has provided the theoretical threshold of exposed thickness and area of the ore-bearing intrusion.These values can be used to determine the ore-forming potentials of dikes.     

青藏高原多年冻土区天然气水合物的研究前景和建议

天然气将是２１世纪的主要能源品种,作为非常规天然气水合物被誉为２１世纪的新能源。它的研究受到世界上许多国家的高度重视。青藏高原羌塘多年冻土区具备形成重烃类天然气水合物的温度和压力条件,同时可以存在由“自保护效应”引起民的轻烃类天然怕事物。开展青藏高原多年冻土区天然气水合物研究,对我国宏观能源战略决策、开拓新学科 领域和保持人类社会可持续发展的均有重要理论意义和广阔的应用前景。从目前人才结构、资料积     

Mechanism of Crystal Redistribution in a Sheet-like Magma Body: Constraints from the Nosappumisaki and Other Shoshonite Intrusions in the Nemuro Peninsula, Northern Japan

Processes of crystal separation in a magma heavily laden withcrystals without phase change are investigated from observationson frozen magma systems: Nosappumisaki and other shoshoniteintrusions in the Nemuro peninsula, Japan, for which the originof the crystals and the initial conditions are well constrained.The Nosappumisaki intrusion is 120 m in thickness and extendsfor more than 1·5 km. It exhibits a wide range of lithologicalvariation, principally as a result of crystal redistributionafter intrusion. Crystals in each lithology can be clearly dividedinto two kinds according to their composition and texture: thosepresent before the intrusion of the magma (‘phenocrysts’)and those that crystallized in situ after intrusion. From thevertical change in mode and size of ‘phenocrysts’,it is shown that (1) augite ‘phenocrysts’ were rapidlydeposited, with little overgrowth after intrusion, by significantcoagulation or clustering on a time-scale of more than a fewyears, and (2) plagioclase ‘phenocrysts’, definitelydenser than the melt but concentrated in the upper level, floatedby counter flow of massive deposition of augite ‘phenocrysts’.These results indicate that in a magma heavily laden with crystalsof a few millimeters in size (>20 vol. %), crystal–crystaland crystal–melt interaction play an important role inthe separation of crystals from the host melt. KEY WORDS: magma chamber; sill; crystal settling; plagioclase flotation; Nosappumisaki     

Mechanics of growth of some laccolithic intrusions in the Henry mountains, Utah, II: Bending and failure of overburden layers and sill formation

Deformation of host rocks during growth of a laccolithic intrusion is analyzed using the theory of bending a stack of thin elastic plates. The theoretical model suggests that magma spreading laterally in the form of a sill will eventually gain sufficient leverage on the overlying strata to deflect them upward and form a laccolith. The amount of bending increases as the fourth power of the distance the magma spreads, whereas the overburden resists bending as the third power of its effective thickness. Effective thickness is the thickness of a single layer which has the same resistance to bending as a multilayer of similar length and elastic modulus. The effective thickness of overburden in the Henry Mountains is estimated as between and of the actual thickness. The form of bending is similar for Newtonian, pseudoplastic, and Bingham magmas. The magnitude of the bending depends upon the total upward force and its distribution and is not simply related to magma viscosity as has been suggested by several previous investigators.After elastic bending strata should fail over the periphery of an intrusion, the site of maximum bending strain and differential stress predicted by the theory. Field observations described in Part I correlate well with these predictions. Because bending strains are proportional to layer thickness, strata of comparable strength but different thicknesses fail at different stages of laccolith development. This leads to the different cross-sectional forms of laccoliths observed in the field.The effect of host rocks on sill form and growth is analyzed using the elastic solution for an elliptical hole under uniform pressure. The theory suggests that sill thickness increases in proportion to length. The concentration of high stresses near the sill termination should induce permanent deformation and account for the blunt terminations described in Part I. This blunting is most likely to occur in relatively ductile rocks whereas sills simply split brittle rocks and maintain sharp terminations. The driving pressure in sills can be calculated from measurements of length and termination radius of curvature, if the yield strength of the host rocks can be estimated. This driving pressure must be greater than the overburden pressure, but sills apparently do not form or propagate by lifting their overburdens. Instead they propagate by locally deforming the host rock. After spreading over a distance about three times the effective overburden thickness, the overlying layers begin to bend upward significantly. This stage marks the transition from a sill to a laccolithic intrusion.