Composition and exhalation flux of gases from mud volcanoes in Taiwan

Many mud volcanoes are distributed along the tectonic sutures in southern Taiwan and can be divided into five zones based on their relative positions in different tectonic domains. Most active mud volcanoes are exhaling methane-dominated gases. Nevertheless, some gases show unusual carbon dioxide-dominated and/or nitrogen-excess compositions. This implies that there are multiple sources for the gas compositions of mud volcanoes in Taiwan.For better understanding the total amount of exhalation gases and its flux, the gas flow and compositions were continuously measured in the interval of two minutes at Chung-lun (CL) bubbling mud pool for a few months. The major compositions of gases exhaling from this site were 75~90% of CO₂ and 5~12% of CH₄. The amount of gases exhaling from the mud pool can be estimated to be about 1.4 ton/year for CH₄ and 28 ton/year for CO₂, respectively. The preliminary results of exhaling gas flux from the major vents of representative active mud volcanoes, yielded an estimated total CH₄ output of the mud volcanoes in Taiwan of ca. 29 ton/year during quiescent period.     

The mud volcanoes of Pakistan

Marine-geologic investigations on the Arabian Sea by Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) in 1995 and 1998, and land expeditions in 1998 and 1999 to the coastal regions of the Makran Desert/Pakistan have extended the knowledge of the aerial distribution of mud volcanoes. These structures rise from under-compacted formations within the regional accretionary prism, which is built by the subduction of the oceanic crust of the Arabian Sea and its km-thick sedimentary load. The occurrence of mud volcanoes is limited to the abyssal plain near the accretionary front, to the coastal region of the Makran Desert and to a region in the interior of the Desert to the south to southeast of the so-called Hinglay Synform. The location of mud volcanoes in Pakistan is clearly tied to fault systems. Mud volcanoes are conspicuously absent on the lower slope of the accretionary prism, where thick gas hydrate layers have developed. The presence of large gas plumes emerging from the seafloor landward of the gas hydrate stability zone at water depths of less than 800 m points to a redirection of fluids from depth, which might explain the absence of mud volcanoes along the lower slope.     

Distribution and formation conditions of salt diapirs and mud volcanoes

Analysis of the distribution of salt diapirs and mud volcanoes in the Earth’s continental block reveals their frequent spatial coincidence. These structures are also characterized by similar morphological and geological structures indicating their elisional origin. The appearance of zones with anomalously high formation pressures (AHFP) is the main factor responsible for the development of mud volcanoes and salt diapirs. The anomalously high formation pressure reflects transformation of the phase composition of salts, clay minerals, and dispersed organic matter under the influence of thermolysis and thermocatalysis in closed physicochemical systems.     

Diapir structure and its constraint on gas hydrate accumulation in the Makran accretionary prism,offshore Pakistan

The Makran accretionary prism is located at the junction of the Eurasian Plate, Arabian Plate and Indian Plate and is rich in natural gas hydrate (NGH) resources. It consists of a narrow continental shelf, a broad continental slope, and a deformation front. The continental slope can be further divided into the upper slope, middle slope, and lower slope. There are three types of diapir structure in the accretionary prism, namely mud diapir, mud volcano, and gas chimney. (1) The mud diapirs can be grouped into two types, namely the ones with low arching amplitude and weak-medium activity energy and the ones with high arching amplitude and medium-strong activity energy. The mud diapirs increase from offshore areas towards onshore areas in general, while the ones favorable for the formation of NGH are mainly distributed on the middle slope in the central and western parts of the accretionary prism. (2) The mud volcanoes are mainly concentrated along the anticline ridges in the southern part of the lower slope and the deformation front. (3) The gas chimneys can be grouped into three types, which are located in piggyback basins, active anticline ridges, and inactive anticline ridges, respectively. They are mainly distributed on the middle slope in the central and western parts of the accretionary prism and most of them are accompanied with thrust faults. The gas chimneys located at different tectonic locations started to be active at different time and pierced different horizons. The mud diapirs, mud volcanoes, and gas chimneys and thrust faults serve as the main pathways of gas migration, and thus are the important factors that control the formation, accumulation, and distribution of NGH in the Makran accretionary prism. Mud diapir/gas chimney type hydrate develop in the middle slope, mud volcano type hydrate develop in the southern lower slope and the deformation front, and stepped accretionary prism type hydrate develop on the central and northern lower slope. The middle slope, lower slope and deformation front in the central and western parts of the Makran accretionary prism jointly constitute the NGH prospect area.     

The formation mechanism of mud diapirs and gas chimneys and their relationship with natural gas hydrates: insights from the deep-water area of Qiongdongnan Basin,northern South China Sea

ABSTRACT

Mud diapirs and gas chimneys are widely developed in continental slope areas, which can provide sufficient gas for hydrate formation, and they are important for finding natural gas hydrates. Based on the interpretation and analysis of high-resolution 2D and 3D seismic data covering the deep-water area in the Qiongdongnan Basin (QDNB), northern South China Sea, we studied the formation mechanism of mud diapirs and gas chimneys and their relationship with natural gas hydrates. Mud diapirs and gas chimneys are columnar and domelike in shape and the internal regions of these bodies have abnormal reflections characterized by fuzzy, chaotic, and blanking zones. The reflection events terminate at the rims of mud diapirs and gas chimneys with pull-up reflections and pull-down reflections, respectively. In addition, ‘bright spots’ and diapiric-associated faults occur adjacent to mud diapirs and gas chimneys. The rapidly deposited and deeply buried fine sediments filling in the Tertiary in deep-water areas of the QDNB and overpressure potential derived from undercompacted mudstones, as well as from the pressurization of organic matter and hydrocarbon generation, provide abundant materials and intensive driving forces for the formation of mud diapirs and gas chimneys. Bottom simulating reflectors (BSRs) with strong amplitude and high or poor continuity were recognized atop the mud diapirs and gas chimneys and in the structural highs within the same region, indicating that they have a close relationship with each other. The mud diapirs and gas chimneys and associated high-angle faults provide favourable vertical pathways for the hydrocarbons migrating from deep strata to shallow natural gas hydrate stability zones where natural gas hydrates accumulate; however, some BSRs are characterized by weak amplitude and poor continuity, which can be affected by high temperature and overpressure in the process of the mud diapir and gas chimney activities. This mutually restricting relationship must be taken into consideration in the process of gas hydrate exploration in QDNB.     

台湾西南部乌山顶泥火山的成因机制初探

乌山顶泥火山位于台湾西南部旗山断层沿线.由于台湾西南部具有独特的地质构造背景,沿着古亭坑背斜和旗山断层沿线分布了许多泥火山.本文以乌山顶泥火山为例,研究泥火山的成因.在了解泥火山地质背景、活动现状的基础上,分析了喷出物的矿物组成、古生物组合、流体地球化学特征.喷出的主要矿物有石英、伊利石、高岭石-蒙脱石混层矿物、伊利石-蒙脱石混层矿物,古生物有Reticulofenestra minuta、 Sphenolithus abies等,流体中的离子主要是Cl-、Na+等.由此探讨了泥浆来源和成因机制,认为泥火山的活动与深部古亭坑组地层存在异常高压有关,构造活动是泥火山活动的主要诱发因素.     

莺歌海盆地泥底辟成因、展布特征及其与天然气运聚成藏关系

泥底辟是南海北部边缘西区莺歌海盆地颇具特色的地震地质异常体，具有欠压实异常、高温高压特点及低密低速的地球物理特征，其展布则具有明显分带性且沿盆地北西走向呈近南北向雁行式排列。泥底辟成因及分布，主要受控于盆地快速沉降及高速沉积充填压实与排出流体不均衡所产生的巨大异常高温高压潜能．和区域控盆断裂晚期右旋走滑伸展活动。而晚期泥底辟热流体上侵活动及其他成藏条件的有效配置，则有效地控制了天然气及CO2运聚成藏规律。     

Provenance of Cored Sediments from Active Margin off Southwestern Taiwan Deduced from Geochemical Constraints

The cored sediments sampled by R/V Marion Dufrense in the active margin off southwestern Taiwan on the east side of the Manila Trench are mostly muds consisting mainly of quartz, feldspar, illite, chlorite ＋ kaolinite with a trace amount of montmorillonite. The cored sediments have higher Fe, Ti, Cr, Cs, Li, Ni, and V but lower Ca, Na, Mn, Hf, Nb, Sr, Ta, U and Zr contents when compared with the upper continental crust（UCC, Taylor and McLennan, 1985）. The Eu/Eu＊ ratios of the sediments averaging 0.69 indicate that the recycling effect on the Eu anomaly may be minor and the contribution of mafic source rock was unimportant, which can also be depicted in the La-Th-Sc plot（Cullers, 1994） showing a ＂mixed origin＂ close to the felsic end member. The chemistry of the cored sediment can be explained by a mixing model designed by Ho and Chen（1996） with four end members i.e., greywacke, shale, quartzite and limestone, which are widely distributed in the Cenozoic formations of southern Taiwan with greywacke as the dominant contributor followed by shale.     

Global methane emission through mud volcanoes and its past and present impact on the Earth's climate

Mud volcanism is an abundant, global phenomenon whereby fluid-rich, low-density sediments extrude both on land and offshore. Methane, which generally exceeds 90 vol% of the gas phase, is emitted at high rates during and after emplacement of the mud domes and is known for its high global warming potential (GWP). This comprehensive estimate of the annual contribution of mud volcano degassing assesses the significance of mud volcanism for the accumulation of greenhouse gases in the atmosphere. A first-order estimate for the earlier, pre-anthropogenic volume of methane released through mud volcanoes further supports their profound effect on the Earth's climate since at least the Paleozoic (570 Ma).     

A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere

A new estimate of global methane emission into the atmosphere from mud volcanoes (MVs) on land and shallow seafloor is presented. The estimate, considered a lower limit, is based on 1) new direct measurements of flux, including both venting of methane and diffuse microseepage around craters and vents, and 2) a classification of MV sizes in terms of area (km²) based on a compilation of data from 120 MVs. The methane flux to the atmosphere is conservatively estimated between 6 and 9 Mt y^–1. This emission from MVs is 3–6% of the natural methane sources and is comparable with ocean and hydrate sources, officially considered in the atmospheric methane budget. The total geologic source, including MVs, seepage from seafloor, microseepage in hydrocarbon-prone areas and geothermal sources, would amount to 35–45 Mt y^–1. The authors believe it is time to add this parameter in the Intergovernmental Panel on Climate Change official tables of atmospheric methane sources.GEM     

东海与泥底辟构造有关的天然气水合物初探

根据所获得的高分辨率地震资料分析,发现冲绳海槽南部西侧槽坡附近以及海槽内部发育有一系列泥火山（底辟）构造,在地形上表现为泥火山地貌,在穿过泥火山的地震剖面上,表现出典型的泥底辟构造。对穿过泥底辟构造的DMS01-5地震剖面进一步的处理和解释发现,泥底辟构造顶部存在明显的似海底反射（BSR）,其与海底反射波组极性相反,在BSR之上存在振幅空白带,在速度谱上出现速度异常,指示存在与泥火山有关的天然气水合物。从世界广泛发现的与泥底辟构造有关的天然气水合物来看,天然气水合物既可以在泥底辟构造的丘状外围成藏,也可以在其外围的海底沉积物中产出。在泥底辟构造的丘状外围附近,天然气水合物的形成机制类似于传统的矿物低温热液的形成;在泥底辟构造外围海底沉积物中,其形成过程类似于传统的矿物交代形成机制。冲绳海槽泥底辟构造的发育与很高的沉积速率和槽坡的活动断层有关。在冰期期间,长江携带大量的陆源物质直接输送到大陆坡地区,沉积速率达300 m/Ma,产生异常高压,同时张性断层极为发育,为流体的迁移提供了良好的通道,在异常压力以及上覆地层压力作用下大量流体向上运移,从而发育大量的泥底辟构造。富含甲烷的流体易在其外围及外围海底沉积物中形成天然气水合物藏。     

Isotopic evidence (He,B, C) for deep fluid and mud mobilization from mud volcanoes in the Caucasus continental collision zone

The Caucasian orogenic wedge formed as a consequence of the closure of the Tethyan Ocean, and numerous fields of active mud volcanoes pepper the area adjacent to the Black and Caspian Seas. Stable isotope ratios of boron, helium, and carbon have been measured for gas, fluid and sediment samples from active mud volcanoes of Taman Peninsula and Georgia to estimate the sources and mobilization depths of the fluid phase and mud. Boron concentrations in mud volcano fluids were found to be 5–35× higher than seawater. Fluid isotope ratios vary between ¹¹B=22 and 39, while isotope ratios of the smectite- and illite-rich extruded mud are considerably depleted in heavy ¹¹B (¹¹B=–8 to +7). B contents of these muds are ~8× higher than modern marine sediments. This suggests that liquefaction prior to mud volcanism was accompanied by both B enrichment and isotope fractionation, most likely at an intermediate depth mud reservoir at 2–4 km.The hydrocarbon-generating source beds to the mud volcanoes are located at 7 to >10 km depth in the folded Maikop Formation and are of proposed Oligocene–Miocene age. The most likely mechanism is re-hydration of these shales by both hydrocarbons and a geochemically mature fluid from greater depth within the orogenic wedge. Such a deep fluid source is supported by our results from gas analyses, which imply an admixture of minor amounts (less than 1%vol) of ³He (Georgia), thermogenic ¹³C in methane as well as "ultraheavy" ¹³C in CO₂ (both Taman and Georgia). The overall results attest active local flow of geochemically different fluids along deep-seated faults penetrating the two study areas in the Caucasian orogenic wedge, with the waters as well as the gases coming from below the Maikop Formation.     

Authigenesis of vivianite as influenced by methane-induced sulfidization in cold-seep sediments off southwestern Taiwan

Authigenesis of iron-rich phosphate nodules occurs in iron-rich cold-seep sediments (MD052911 core) at Yung-An Ridge offshore southwestern Taiwan. Raman, FTIR, and quantitative X-ray energy-dispersive spectroscopic analyses indicate that the phosphate mineral is vivianite (or barićite) and shows Fe/Mg molar ratios spanning from ca. 0.6 to 4.0 and a general down core trend of increasing Fe/Mg ratios. The formation of vivianite is limited to a depth interval of 13–17 mbsf (meters below seafloor) and is most prominent at ∼16 mbsf in association with high dissolved iron concentrations and depleted dissolved sulfide below a peak sulfidization zone (enriched in mackinawite and greigite). Alternate growths of vivianite and iron monosulfides and compositional zoning with Mg enriched towards the peripheries of individual nodules occur in the transition from the zone of vivianite mineralization to the sulfidization zone. The crystallization of vivianite below the sulfidization front could have been favored by scavenging of downward diffusive dissolved sulfide from pore waters in the sulfidization zone. Alternate growths and overlapping of the zones of iron monosulfides and vivianite can be attributed to fluctuations of the sulfidization front and methane flux. The discovery of vivianite in the Yung-An Ridge sediments implies that authigenic vivianite can be an important sink for phosphorus burial in cold-seep sediments that have high reactive-iron contents and high sedimentation rates.     

羌塘盆地现代泥火山的发现及其油气地质意义

在羌塘盆地唢呐湖地区发现现代泥火山,通过采集泥浆样、水样等样品,开展水化学离子、水溶烃类气体、碳氢氧稳定同位素等测试分析,对现代泥火山的油气地质意义进行了探讨。水化学类型和氢氧同位素表明,该地区地下可能存在油气藏;水溶烃类气体测试结果表明,气体成分为C1~C5,以甲烷为主,与常规气藏的气体成分一致;碳同位素判别结果表明,其为热解成因的油成气。结合泥火山的地质背景,表明泥火山为油气藏破坏渗漏的产物,在保存良好的未泄露地区,油气勘探具有良好前景。     

Geochemical characteristics of mud volcano fluids in the southern margin of the Junggar basin,NW China: implications for fluid origin and mud volcano formation mechanisms

Mud volcanoes can provide important information about the underlying strata, hydrocarbon accumulation, and recent neotectonic movements in an area. The fluids erupting from mud volcanoes provide important information about their formation and evolution. The ion concentration and the hydrogen and oxygen isotopes of the fluids that were erupted from the three mud volcano groups, Baiyanggou, Aiqigou, and Dushanzi, and nearby rivers in the southern margin of the Junggar basin, northwestern China, are studied. The concentrations of Na and Cl in mud volcano fluids are clearly elevated, displayed as the Na-Cl type. The δD and δ18O values of the fluids are similar between the Baiyanggou and Dushanzi mud volcanoes, which are mainly from ancient sedimentary pore water. However, the Aiqigou mud volcano is depleted in dissolved Cl and shows lower δ18O values with mixed sources, including deep pore and local meteoric water. Two types of mud volcanoes are proposed in this study. One type is low-energy mud volcanoes with a low volume of fluid of deep origin on the hillcrest, which display as mud pool/pie/hole. The other type is high-energy mud volcanoes having mixed fluid origin in the valley and formed in the shape of a mud cone (dome).     

羌塘盆地戈木错地区泥火山群沉积及浅表地球化学特征

本文对羌塘盆地戈木错地区的泥火山群进行了沉积学及地球化学的研究。研究区泥火山沉积物主要为第四系风化沉积物及被流体带到地表的下伏地层岩块组成。酸解烃法、蚀变碳酸盐法(△C)对土壤样品的研究结果表明,样品含有的有机酸解烃成分具有完整C1-C5序列;烃类各组分间、烃类和碳酸盐指标间具有显著的正相关性;烃类气体来源为原油伴生气和凝析油气,显示出深部热解成因气的特征。     

Regional and local geothermal conditions in the northern Black Sea

More than 100 new heat flow measurements have been collected in recent years (2002–2004) in different tectonic environments of the northern Black Sea. The northern periphery of the Black Sea is characterized by strong geodynamic and seismic activity, high sedimentation rates, diapiric structures, mud volcanism, and fluid and gas escape at the sea floor. We present new thermal data from the shelf, continental slope and deep-water basin, measured off-shore using a marine thermo-probe and on-shore in drill holes. Heat flow density ranges from 20 to more than 2,000 mW/m². For two local areas (the Dnieper gas seeps and the Dvurechenski mud volcano area), we discuss the relation between heat flow variability and the geological and physical processes in the near-bottom sediment layer. The Dnieper gas seeps area is characterized by strong small-scale heat flow variability and is controlled by fluid and gas migration. In the Dvurechenski active mud volcano, the near-bottom temperature in sediments is anomalously elevated because additional heat is carried out by mass flows of fluids and clay minerals. Away from the mud volcano heat flow quickly decreases to background values.     

Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes

The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.