Geological characteristics of large gas provinces and large gas fields in China

Based on the examination of the global researches on oil and gas provinces and large gas fields and the analysis of the features,attributes and distribution of large gas provinces and gas fields,this paper puts forward three indicators of determining large oil and gas provinces:spatial indicator,reservoir-forming indicator and resource indicator.It classifies the gas accumulated areas and large gas provinces in China and analyzes the controlling factors on the distribution of large gas provinces and large gas fields:the lateral distribution is mainly controlled by high-energy sedimentary facies and constructive diagenetic facies,palaeo-highs and their periclinal zones,deep faults,etc,and the vertical distribution is mainly controlled by unconformities,series of evaporates and deep low-velocity highly-conductive beds,etc.It also reveals the main geological characteristics of large gas provinces and large gas fields in China.Large gas fields in four-type basins have their own characteristics and onland large gas fields are dominantly developed in foreland basins and craton basins;there are three types of gas sources,of which,coal is the main source with high gas generating intensity and varying origins;reservoir rocks of the large gas fields(provinces)are of various types and dominated generally by low-middle permeability and porosity pore-type reservoirs;structural traps and litho-stratigraphic traps coexist in Chinese large gas fields and form dense high abundance and large-area low and middle-abundance large gas fields;most of the large gas fields have late hydrocarbon-generation peaks and reservoir formation,and experienced the process of multiple-stage charging and late finalization; large gas provinces(fields)have good sealing and preservation conditions,and evaporates seals are largely developed in large and extra-large gas fields.This paper intends to shed light on the exploration and development of large gas fields(provinces)through analyzing their geological characteristics.     

Natural gas origins of large and medium-scale gas fields in China sedimentary basins

China sedimentary basins present abundant natural gas resource thanks to its unique geological settings.Marine highly-matured hydrocarbon source rocks,widespread coal-measure strata and low temperature Quaternary saline strata,etc.,indicate the wide foreground of China natural gas resources. Up to now,most of the petroliferous basins have been discovered to have wholesale natural gas accumulation from Precambrian,Paleozoic,Mesozoic to Cenozoic in the east,the central,the west and the coast of China.These large and medium-scale gas reservoirs are mainly composed of hydrocarbon gas with big dry coefficient,tiny non-hydrocarbon,wide carbon isotope distribution and varying origin types,the hydrocarbon gas includes coal-formed gas,oil-formed gas,biogenic gas and inorganic gas, etc.Coal-formed gas is the main type of China natural gas resources,in particular several explored large-scale gas fields(>100 billion cubic meter)of Kela 2,Sulige and Daniudi,etc.,they all belong to coal-formed gas fields or the gas fields consisting mostly of coal-formed gas.Oil-formed gas is also abundant in China marine basins,for example marine natural gas of Sichuan Basin generated from crude oil cracking gas.Primary and secondary biogenic gas fields were discovered respectively in the Qaidam Basin and Western Slope of Songliao Basin.In addition,inorganic gases are mainly distributed in the eastern China,in particular the Songliao Basin with abundant carbon dioxide accumulation,indicating that the eastern China present large exploration potential of inorganic gas.     

Development of the 1990 Kalapana Flow Field,Kilauea Volcano,Hawaii

The 1990 Kalapana flow field is a complex patchwork of tube-fed pahoehoe flows erupted from the Kupaianaha vent at a low effusion rate (approximately 3.5 m³/s). These flows accumulated over an 11-month period on the coastal plain of Kilauea Volcano, where the pre-eruption slope angle was less than 2°. the composite field thickened by the addition of new flows to its surface, as well as by inflation of these flows and flows emplaced earlier. Two major flow types were identified during the development of the flow field: large primary flows and smaller breakouts that extruded from inflated primary flows. Primary flows advanced more quickly and covered new land at a much higher rate than breakouts. The cumulative area covered by breakouts exceeded that of primary flows, although breakouts frequently covered areas already buried by recent flows. Lava tubes established within primary flows were longer-lived than those formed within breakouts and were often reoccupied by lava after a brief hiatus in supply; tubes within breakouts were never reoccupied once the supply was interrupted. During intervals of steady supply from the vent, the daily areal coverage by lava in Kalapana was constant, whereas the forward advance of the flows was sporadic. This implies that planimetric area, rather than flow length, provides the best indicator of effusion rate for pahoehoe flow fields that form on lowangle slopes.     

Preliminary study on the origin identification of natural gas by the parameters of light hydrocarbon

The light hydrocarbon composition of 209 natural gas samples and individual light hydrocarbon carbon isotopes of 53 natural gas samples from typical humic-sourced gas and sapropelic-sourced gas in the four basins of China have been determined and analyzed.Some identification parameters for humic-sourced gas and sapropelic-sourced gas are proposed or corrected.The differences of compound-specificδ13C value of individual light hydrocarbon between humic-sourced gas and sapropelic-sourced gas have been founded.The humic-sourced gas has the distribution ofδ13C benzene > -24‰,δ13C toluene >-23‰,δ13C cyclohexane >-24‰andδ13C methyl cyclohexane >-24‰,while the sapropelicsourced gas has the distribution ofδ13C benzene <-24‰,δ13C toluene <-24‰,δ13C cyclohexane <-24‰and δ13C methyl cyclohexane <-24‰.Among the components of C7 light hydrocarbon compound,such as normal heptane(nC7),methyl cyclohexane(MCH)and dimethyl cyclopentane(ΣDMCP),etc,relative contents of nC7 and MCH are influenced mainly by the source organic matter type of natural gas.Therefore,it is suggested that the gas with relative content of nC7 of more than 30%and relative content of MCH of less than 70%is sapropelic-sourced gas,while gas with relative content of nC7 of less than 35%and relative content of MCH of more than 50%is humic-sourced gas.Among components of C5-7 aliphatics, the gas with relative content of C5-7 normal alkane of more than 30%is sapropelic-sourced gas,while the gas with relative content of C5-7 normal alkane of less than 30%is humic-sourced gas.These paremeters have been suggested to identify humic-sourced gas and sapropelic-sourced gas.     

Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano,USA

The pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, OR, USA, contain minor amounts of obsidian (1–6 wt.%). The volatile (H₂O and CO₂) contents and textures of these clasts vary considerably. FTIR measurements of H₂O in obsidian pyroclasts range from 0.1 to 1.5 wt.% indicating equilibration pressures ≤20 MPa. CO₂ contents are low (<10 ppm) except in clasts that also contain xenolith powder that provided a local CO₂ source. Obsidian clasts exhibit a range of color and textural types that changed in relative proportion as the eruption progressed. Together these data indicate that there were multiple origins of obsidian and that the dominant source changed during the eruption. Early in the eruption, obsidian was almost entirely black or grey (microlite-bearing) and probably derived from dikes or wall rock fractures filled with vanguard magma or tuffisite that, together with wall rocks, were eroded and incorporated into the eruption column as the vent widened. Later in the eruption, following a brief cessation of activity, the proportion of obsidian to wallrock lithic clasts increased and new types of obsidian dominated, types that represent remnants of a shallow conduit plug, welded fallback material from within the conduit, and sheared and degassed magma from near the conduit walls. Analysis of bubble shapes preserved within obsidian indicates that shear stresses and shear rates varied by over two orders of magnitude, with maxima of 88 kPa and 10^−2.3 s⁻¹, respectively, based on an assumed magma temperature of 850°C. Furthermore, the highest shear rates and stresses, and the shortest flow times (10–20 min), are preserved in clasts that also contain wall rock. The longest deformation times (5 and 8 h) correspond to two microlite-rich clasts, suggesting that the higher microlite content results from slower ascent rates and/or longer magma residence times at shallow levels. Differences between obsidian pyroclasts from the Newberry eruption and those of the Mono Craters may relate to the nature of the conduit feeding the two events. From this comparison, we conclude that obsidian can provide information on time scales and mechanisms of pre-fragmentation magma ascent.     

Stable carbon and hydrogen isotopes of gases from the large tight gas fields in China

By the end of the year 2010,a total of 15 large tight gas fields have been found in China,located in the Ordos,Sichuan,and Tarim basins.The annual production and total reserves of these fields in 2010 were 222.5×108and 28657×108m3,respectively,accounting for 23.5%and 37.3%,respectively,of the total annual production and reserves of natural gases in China.They took a major part of all natural gas production and reserves in China.According to the analyses of 81 gas samples,the stable carbon and hydrogen isotopic compositions of tight gases in China have following characteristics:(1)Plots of 13C1-13C2-13C3,13C1-C1/C2+3and 13C1-13C2demonstrate the coal-derived origin of tight gases in China;(2)For the primary alkane gases,both carbon and hydrogen isotopic values increase with increasing molecular mass,i.e.,13C113C213C313C4and2H12H22H3;(3)The isotopic differences of 13C2-13C1,13C3-13C1,2H2-2H1and 2H3-2H1decrease with increasing Ro(%)and C1/C1–4;(4)There are seven causes for the carbon and hydrogen isotopic reversal,however,the carbon and hydrogen isotopic reversal of tight gases in China is caused mainly by multiple stages of gas charge and accumulation.     

Characterisation of the 1997 Vulcanian explosions of Soufrière Hills Volcano,Montserrat, by video analysis

The activity of Convention at Montserrat Soufrière Hills Volcano, Montserrat, during the period 1995–1999 included numerous violent explosions. Two major cycles of Vulcanian explosions occurred in 1997: a first of 13 explosions between 4 and 12 August and a second of 75 between 22 September and 21 October. The explosions were short-lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows, and buoyant plumes ascended 3–15 km into the atmosphere. Each explosion discharged on average 3×10⁵ m³ (dense-rock equivalent, DRE) of magma, draining the conduit to depths of 1–2 km. The paper focuses on the first few seconds of three explosions of the 75 that occurred in September/October 1997: 6 October 1997 at 17:50, 7 October 1997 at 16:02 and 9 October 1997 at 12:32. Physical parameters such as exit velocities, magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum-dominated finger jets visible on videos during the initial stages of each explosion. The model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent, and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters. Each explosion reveals a progressive increase in eruptive intensity with time, jet exit velocities increasing from 40 m s^–1 at the beginning of the explosion up to 140 m s^–1 after a few seconds. Modelling suggests that the first magma to exit was largely degassed, whereas that discharged after a few seconds contained up to 2 wt% water. Magma overpressures up to ~10 MPa are estimated to have existed in the conduit immediately prior to each explosion. Progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre-eruptive gradients in water content and magma pressure in the upper reaches (probably 100–500 m) of the conduit. Fountain collapse occurred during the first 10–20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant. Such high eruptive densities were due to the presence of partially degassed magma in the conduit.Editorial responsibility: A. Woods     

Geochemical characteristics and origin of light hydrocarbons in biogenic gas

The light hydrocarbon geochemical characteristics of biogenic gases from Sebei 1 gas field in the Qaidam Basin, Baoshan gas field in the Baoshan Basin and Alaxin gas field, Puqian gas pool, Aonan gas pool in the Songliao Basin are studied and the origin is discussed based on the composition and isotope data of gases. The isoalkane contents among light hydrocarbons in natural gas show a negative relationship with δ13C1 values. The isoalkane contents of the gases with δ13C1 values of less than ?60‰ are also h...     

Temporal variations of gas compositions of fumaroles in the Tatun Volcano Group,northern Taiwan

Hydrothermal activity is common in the Tatun Volcano Group of northern Taiwan. Helium isotopic compositions of fumarolic samples show that mantle component occupies more than 60% in the previous study. Along with recent seismic results, a magma reservoir is inferred to have existed beneath the area of Da-you-keng, where fumarolic venting is the most active in Tatun Volcano Group. Progressive increases of HCl concentrations and SO₂/H₂S ratio in fumaroles from Da-you-keng have been observed since August 2004. The HCl concentration changed from almost the detection limit to thousands of ppm, even up to 30,000 ppm. SO₂/H₂S ratios varied from almost 0 to 3; hence SO₂ became the dominated S species in this area. These variations were accompanied by rising temperature of fumaroles in the Tatun Volcano Group, especially in the area of Da-you-keng (from boiling point to 131 °C). Meanwhile, ³He/⁴He ratios showed a decreasing trend but returned to normal values shortly thereafter. We propose two possible processes, 1) new magma supply and 2) recent opening of fractures in local area, to explain these observations. Based on the change of ³He/⁴He ratio and lack of ground deformation, we consider the latter might be more plausible.     

Variation in noble gas isotopic composition of gas samples from the Aegean arc, Greece

In contrast to most other arcs with oceanic plate subduction, the Aegean arc is characterized by continent–continent subduction. Noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc, 2 islands in the back-arc region and 7 sites in the surrounding areas. The ³He/⁴He ratios of the samples ranged from 0.027R_A to 6.2R_A (R_A denotes the atmospheric ³He/⁴He ratio of 1.4×10⁻⁶), demonstrating that even the maximum ³He/⁴He ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems, which are equal to the MORB value of 8±1 R_A. Regional variations in the ³He/⁴He ratio were observed both along and across the arc. The maximum ³He/⁴He ratio was obtained from Nisyros volcano located in the eastern end of the arc, and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano. Across the volcanic arc, the ³He/⁴He ratio decreased with an increasing distance from the arc front, reaching a low ratio of 0.063R_A in Macedonia, which suggested a major contribution of radiogenic helium derived from the continental crust. At Nisyros, a temporal increase in ³He/⁴He ratio due to ascending subsurface magma was observed after the seismic crisis of 1995–1998 and mantle neon was possibly detected. The maximum ³He/⁴He ratio (6.2R_A) in the Aegean region, which is significantly lower than the MORB value, is not probably due to crustal assimilation at shallow depth or addition of slab-derived helium to MORB-like mantle wedge, but inherent characteristics of the subcontinental lithospheric mantle (SCLM) beneath the Aegean arc.     

Natural gas leakage of Mizhi gas reservoir in Ordos Basin, recorded by natural gas fluid inclusion

Abundant natural gas inclusions were found in calcite veins filled in fractures of Central Fault Belt across the centre of Ordos Basin. Time of the calcite veins and characteristics of natural gas fluid inclusion were investigated by means of dating of thermolum luminescence (TL) and analyzing stable isotope of fluid inclusion. Results show that natural gas inclusion formed at 130―140℃ with salinity of 5.5 wt%―6.0 wt% NaCl. It indicates that natural gas inclusion is a kind of thermal hydrocarbon fluid formed within the basin. Method of opening inclusion by heating was used to analyze composition of fluid inclusion online, of which the maximal hydrocarbon gas content of fluid inclusion contained in veins is 2.4219 m3/t rock and the maximal C1/Σci ratio is 91%. Laser Raman spectroscopy (LRS) was used to analyze chemistry of individual fluid inclusion in which the maximal hydrocarbon gas content is 91.6% compared with little inorganic composition. Isotope analysis results of calcite veins show that they were deposited in fresh water, in which the δ13CPDB of calcite veins is from -5.75‰ to 15.23‰ andδ18OSMOW of calcite veins is from 21.33‰ to 21.67‰. Isotope results show thatδ13C1 PDB of natural gas fluid inclusion is from -21.36‰ to -29.06‰ and δDSMOW of that is from -70.89‰ to -111.03‰. It indicates that the gas of fluid inclusion formed from coal source rocks and it is the same as that of natural gas of Mizhi gas reservoir. Results of TL dating show that time of calcite vein is (32.4±3.42)×104 a, which is thought to be formation time of gas inclusion. It indicated that natural gas inclusion contained in calcite veins recorded natural gas leakage from Mizhi gas reservoir through the Central Fault Belt due to Himalayan tectonic movement.     

Eruptive history and tectonic setting of Medicine Lake Volcano,a large rear-arc volcano in the southern Cascades

Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that < 6% of the ∼ 2000 km² of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km³, giving an overall effusion rate of ∼ 1.2 km³ per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H₂O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.     

沉积盆地、结晶基底和油、气成因理念与第二深度空间勘探和开发

在石油、天然气的勘探和开发研究中,涉及到深部空间的介质、结构和属性,在沉积建造上涉及到陆相沉积、海相沉积和火山岩相,在油、气成因理论上涉及到有机成因和无机成因,在机制上涉及到深部流体的分异、调整、运移和动力作用.这些本质性问题的研究和探讨是人们十分关注的科学领域.基于油、气生、储和形成的理论、应用和成效的研究提出,有几个基础性的理念问题必须重新认识,特别是沉积建造与古老变质岩结晶基底;双相（海相和陆相）沉积建造与盆地的内涵和双基（有机和无机）混合油、气成因理念.清晰地厘定这些认识不仅有益于对油、气形成、聚集和勘探及开发给出一个更为科学的深部空间,即第二深度空间（5000~10000 m）的油、气勘探,而更为重要的是基于对这些理念的重新认识和深化研究可构建新的思路或理论,且在新的理念导向下,强化油、气深部勘探,以期能在理论研究和实践应用中取得新的成效,发现大型与超大型油、气田.     

Measurement of radon gas on major faults in California,USA

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Source mechanism of a large scale gas outburst at Sunagawa Coal Mine in Japan

On January 29ty, 1986, the third largest gas outburst in Japan took place at Sunagawa Coal Mine, which is the only hydraulic mine in Japan. It occurred at a face of a cross-cut, just after a coal seam was outcropped by blasting for drivage of the cross-cut. The site of the gas outburst was located 1,180 m below the surface. No workers were injured, but the cross-cut was plugged with 1,600 m³ of coal fragments extending over 100 m behind the face and 60,000 m³ of methane gas was emitted.The site of the gas outburst was investigated in detail to clarify the geological features. A normal and a reverse fault existed at the site. The area of the ejected zone was about 400 m² and extended upward along the normal fault. The shape of the ejected zone suggests a great role of the normal fault on the gas outburst.Digital seismograms, recorded by a mine-wide seismic array at the coal mine, consisting of 27 microseismic events were used to investigate the gas outburst. Magnitude, seismic energy release, distribution of hypocenter and focal mechanism were analyzed. Taking the shape of the ejected zone together with results of the seismological investigation into consideration, it appears that the seismicity started with left-lateral faulting of the reverse fault and then right-lateral faulting of the normal fault followed. The faulting of the normal fault might be the direct cause and be the predominant mechanism of the gas outburst.Presented at the Fred Leighton Memorial Workshop on Mining Induced Seismicity, Montreal, Canada, August 30, 1987.     

Discovery of native aluminum and its possible origin from prospective gas hydrate areas in the South China Sea

Aluminum is an active amphoteric metal element, but native aluminum (Al°) can be formed and preserved in special condi- tions so that its occurrence has unique geological significance. In this paper, the coarse fractions ( 63 μm) of 269 samples of surface sediments and of 165 sediment samples in four cores from the South China Sea (SCS) were picked out and analyzed. Al° particles were found in only five surface samples and three samples in two sedimentary cores. The particles are grayish or silver white in color with a strongly metallic luster and high plasticity, and they occur as irregular plats, elongated shapes and spherules with botryoidal structures. The identified Al° particles belong to cubical system, Fm3m space group with a varying from 4.028×10?1 nm to 4.059×10?1 nm. Their chemical compositions consist mainly of Al (95.07%–99.84%) and minor amounts of Si, Fe, Ti, S, Zn, Mg, Ca and trace amounts of K, Na, Cu, Co, P and Mn. Our results show that the identified Al° is authigenic, formed in situ in the prospective areas of gas hydrates, and that it is an unreported type of Al° occurrence. A possi- ble mechanism of Al° formation is proposed: under the strongly reducing micro environments at cold seeps, dissolved Al3+ in sediments is reduced to its metallic state by the strongly reducing agent of H2 through microbial processes and enzyme action. As a result, the Al° particles from the prospective gas hydrate areas in the SCS are the special products, representing the first report on Al° from the sediments at cold seeps. The proposed mechanism provides a theoretical basis for further studies on the special biogeochemical processes at cold seeps.     

Geochemical Features of the Hubin Spring Gases from the Northern Caldera Lake of the Tianchi Volcano,Changbai Mountains

This paper deals with geochemical features of gas emitted from the Hubin Springs.The Hubin Springs zone,a strong thermal emission zone,is are locatedd at the north edge of the Tianchi caldera lake.Very young deposits with uncertain eruption date are found on the top area of the Tianwenfeng,which might have been formed in one of the recent eruptions or the Millennium Eruption.It is of significance to study the geochemistry features of the emitting gas from the Hubin Springs to understand the activities of the Tianchi Volcano.This paper systematically sampled and analyzed the gases emitted from the Hubin Springs and discussed their geochemistry features.The results show that there is a high content of deep derived gases,such as CO2,He,CH4 and Ar in Hubin Springs zone.The isotopic ratio of He lies between 4.18 and 5.95 Ra.The averaged mantle derived gas content calculated from the 4He/20Ne ratio and He content reaches 67.1%.All these show that the Hubin Springs are located on a special belt of deep gases released in high intensity and large scale.The spatial distribution of Helium isotope is characterized by concavity,showing that this special area may be related to the volcanic edifice.It is highly possible that the released gases represent the residual gas samples of the latest eruptions from the Tianchi Volcano.However more detailed studies are demanded.     

Structural control and origin of volcanism in the Taupo volcanic zone,New Zealand

Taupor volcanic zone (TVZ) is the currently active volcanic arc and back-arc basin of the Taupo-Hikurangi arc-trench system, North Island, New Zealand. The volcanic arc is best developed at the southern (Tongariro volcanic centre) end of the TVZ, while on the eastern side of the TVZ it is represented mainly by dacite volcanoes, and in the Bay of Plenty andesite/dacite volcanoes occur on either side of the Whakatane graben. The back-arc basin is best developed in the central part of the TVZ and comprises bimodal rhyolite and high-alumina basalt volcanism. Widespread ignimbrite eruptions have occurred from this area in the past 0.6 Ma. Normal faults occur in both arc and back-arc basin. They are generally steeply dipping (>40°) and strike between 040° and 080°. In the back-arc basin, fault zones are en echelon and have the same trend as alignments of rhyolite domes and basalt vents. Open fissures have formed during historic earthquakes along some of the faults, and geodetic measurements on the north side of Lake Taupo suggest extension of 14±4 mm/year. In the Bay of Plenty and M_L=6.3 earthquake occurred on 2 March 1987. Modelling of known structure in the area together with data derived from this earthquake suggests block faulting with faults dipping 45°±10° NW and a similar dip is suggested by seismic profiling of faults offshore of the Bay of Plenty where extension is estimated to be 5±2 mm/year. To the east of the TVZ, the North Island shear belt (NISB) is a zone of reverse-dextral, strike-slip faults, the surface expression of which terminates at the eastern end of the TVZ. On the opposite side of the TVZ in the offshore western Bay of Plenty and on line with the NISB is the Mayor Island fault belt. If the two fault belts were once continuous, as seems likely, strike-slip faults probably extend through the basement of the TVZ. When extension associated with the arc and back-arc basin is combined with these strike-slip faults, the resulting transtension provides a suitable tectonic environment for caldera formation and voluminous ignimbrite eruptions in the back-arc basin. The types of volcano in the TVZ are considered to be related to the source of magma and overlying crustal structure. Lavas of the arc are probably formed by a multistage process involving (1) subsolidus slab dehydration, (2) anatexis of the mantle wedge, (3) fractionation and minor crustal assimilation and (4) magma mixing. High-alumina basalts of the back-arc basin may be derived by partial melting of peridotite at the top of the mantle wedge, while rhyolitic magmas are thought to come from partial melting of lavas and subvolcanic reservoirs associated with the southern end of the Coromandel volcanic zone. Extreme thinning associated with transtension in the back-arc basin will favour the eruption of large-volume, gas-rich ignimbrites accompanied by caldera formation.     

汶川地震前后四川中坝气田的气井井口压力异常变化

分析了龙门山断裂带附近的四川中坝气田气井井口压力数据,发现汶川地震前,多口气井出现井口压力异常升降的变化.开采层深3 100 m的7口雷三储气层的气井压力,在4月8日-5月5日井压同步出现了大于35%-130%的上升;开采层深2400m的24口须二储气层的气井中,多数井压在震前出现缓慢下降的异常变化,幅度最大为45%,...     

Characteristics,genesis and accumulation history of natural gas in Hetianhe gasfield,Tarim Basin,China

Hetianhe gasfield in Bachu region of the Tarim Basin is mainly composed of three reservoir-caprock assembly,namely regional caprock of upper mudstone,middle mudstone and lower mudstone of the Carboniferous and reservoir of Bachu bioclastic limestone,glutenite and the Ordovician carbonate buried hill.Natural gas in Hetianhe gasfield sourced from the Cambrian source rock.It is thought that gases in Ma4 well block in the east of Hetianhe gasfield are mainly crude-oil cracked gases,while those in Ma3 and Ma8 well blocks in the west are the mixture gases of kerogen cracked gases and crude-oil cracked gases.Natural gas is rich in H2S and accumulated in multiply stages as the result of TSR.The accumulation history is divided into three stages,namely accumulation and breakage in the late Caledonian-early Hercynian,migration and dissipation in the late Hercynian and accumulation in Himalayan. The main accumulation of reformed gas reservoir is in Himalayan.