松辽盆地徐深气田火山岩气藏地质约束-测井地震建模方法探讨

本文在借鉴徐深气田以往地质建模成功经验基础上,针对徐深气田B区块火山体规模小、横向变化快、开发井少、认识程度较低的实际,密切结合火山岩成因特点,探索性地提出体控地质建模的新思路,即以地震反射特征和地震属性特征为依据,结合钻井、测井资料,井震结合从三维空间上精细识别和刻画火山岩体,通过利用神经网络算法建立多种地震属性与测井参数的定量关系,实现了火山岩体内部多种地质属性的定量表征,建立的三维可视化地质模型比较真实地反映了火山机构及其内部属性空间发育和分布特征.     

Late Pleistocene to Holocene eruptive activity of Pico de Orizaba,Eastern Mexico

The Late Pleistocene to Holocene eruptive history of Pico de Orizaba can be divided into 11 eurptive episodes. Each eruptive episode lasted several hundred years, the longest recorded being about 1000 years (the Xilomich episode). Intervals of dormancy range from millenia during the late Pleistocene to about 500 years, the shortest interval recorded in the Holocene. This difference could reflect either changes in the volcano's activity or that the older stratigraphic record is less complete than the younger. Eruptive mechanisms during the late Pleistocene were characterized by dome extrusions, lava flows and ash-and-scoria-flow generating eruptive columns. However, in Holocene time plinian activity became increasingly important. The increase in dacitic plinian eruptions over time is related to increased volumes of dacitic magma beneath Pico de Orizaba. We suggest that the magma reservoir under Pico de Orizaba is stratified. The last eruptive episode, which lasted from about 690 years bp until ad 1687, was initiated by a dacitic plinian eruption and was followed by effusive lava-forming eruptions. For the last 5,000 years the activity of the volcano has been gradually evolving towards such a trend, underlining the increasing importance of dacitic magma and stratification of the magma reservoir. Independent observations of Pico de Orizaba's glacier early this century indicate that some increase in volcanic activity occurred between 1906 and 1947, and that it was probably fumarolic.     

深部流体与岩浆活动:兼论腾冲火山群的深部过程

深部流体强烈影响许多地质过程的发生和发展,然而对其行为的理解却甚少.在所有可能影响岩浆活动的因素中,流体是最重要的.流体的高度活动性及其在熔体中的溶解度随压力减小而降低,暗示岩浆系统必然是开放的动力系统,流体的丢失和获得可戏剧性地影响岩浆系统的整体行为.流体对岩浆系统的影响主要通过改变熔体的黏度来实现,也改变岩浆的平均密度,以及固相线和液相线温度.少量流体的注入即可以导致熔体黏度出现几个数量级的降低,这种戏剧性改变进而导致岩浆柱与通道壁摩擦力的快速减小,因而岩浆上升速度也可以呈现几个数量级的变化.当岩浆上升到流体相分离的深度以后,岩浆系统的行为更加不可预测.反之,流体的丢失将导致岩浆系统的行为向相反方向变化,岩浆将滞留在深部.值得注意的是,丢失到通道中的流体可以弱化上覆岩层的力学性质,改善岩浆上升的通道条件.因此,如果上升岩浆能够得到持续的深部流体补给,其补给量至少等于丢失量,岩浆必将以越来越快的速度上升.据此,岩浆系统是一种复杂性动力系统,岩浆作用是一种非线性过程.这种分析结果与流行的岩石学认识不一致,却与火山学观察和成矿学研究结果相同.腾冲火山岩中的聚斑结构暗示某些岩浆在喷发之前曾经在深部作过停留,它们曾经位于不同的深度水平上.同岩浆交代结构暗示岩浆房的活化有赖于深部流体的注入,因而火山监测过程中关注岩浆房之下的深部流体活动是必须的.将岩浆房上、下两部分的流体活动紧密结合在一起,可能是火山监测的一个新方向.     

Fluid geochemistry of Montserrat Island,West Indies

 Two geochemical surveys carried out in March 1991 and September 1992 revealed the existence of a hydrothermal system in the southern portion of Montserrat Island, below Soufrière Hills Volcano. This conclusion is supported by the presence of: (a) the thermal springs of Plymouth which are fed by deep Na–Cl waters (Cl concentration ∼25 000 mg/kg, temperature ca. 250  °C) mixed with shallow steam-heated waters; (b) the four fumarolic fields of Galway's Soufrière, Gages Upper Soufrière, Gages Lower Soufrière, and Tar River Soufrière, where acid to neutral, steam-heated waters are present together with several fumarolic vents, discharging vapors formed through boiling of hydrothermal aqueous solutions. Involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by: (a) the high ³He/⁴He ratios of fumarolic fluids, i.e., 8.2 R_A at Galway's Soufrière and 5.9 R_A at Gages Lower Soufrière; (b) the δD and δ¹⁸O values of Na–Cl thermal springs and steam condensates, indicating the involvement of arc-type magmatic water in the formation of deep geothermal liquids; and (c) the CH₄/CO₂ ratios of fumarolic fluids, which are lower than expected for equilibrium with the FeO–FeO_1.5 hydrothermal rock buffer, but being shifted towards the SO₂–H₂S magmatic gas buffer. Received: 26 March 1996 / Accepted: 19 July 1996     

Reappraisal of the 1982 eruptions of El Chichón Volcano, Chiapas, Mexico: new data from proximal deposits

 Additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases III and IV of the 1982 eruption of El Chichón Volcano. Phase III began on 4 April at 0135 GMT with a powerful hydromagmatic explosion that generated radially fast-moving (∼100 ms^–1) pyroclastic clouds that produced a surge deposit (S1). Due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source, causing a new explosion. The ejected juvenile material mixed with large amounts of fragmented dome and wall rock, which were dispersed laterally in several pulses as lithic-rich block-and-ash flow (F1). Partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture-rich surge clouds and small-volume block-and-ash flows (IU) up to distances of 3 km from the crater. The eruption continued by further decompression of the magmatic system, with the ensuing emission of smaller amounts of gas-rich magma which, with the strong erosion of the volcanic conduit, formed a lithic-rich Plinian column that deposited fallout layer B. Associated with the widening of the vent, an increase in the effective density of the uprising column took place, causing its collapse. Block-and-ash flows arising from the column collapse traveled along valleys as a dense laminar flow (F2). In some places, flow regime changes due to topographic obstacles promoted transformation into a turbulent surge (S2) which attained minimum velocities of approximately 77 ms^–1 near the volcano. The process continued with the formation of a new column on 4 April at 1135 GMT (phase IV) that emplaced fall deposit C and was followed by hydromagmatic explosions which produced pyroclastic surges (S3). Received: 13 May 1996 / Accepted: 12 November 1996     

新疆阿什库勒盆地大黑山火山地质及地球化学特征

大黑山火山是新疆阿什库勒火山群最大的火山,通过野外地质调查和遥感解译,火山锥体由渣锥、溅落锥和寄生火山锥组成。大黑山岩石类型分别为玄武粗安岩、粗面安山岩和粗面岩,钾玄岩系列。岩浆分离结晶特征明显,斜长石晶体析出导致残余熔体中δEu形成明显负异常。大黑山锥体南侧发育一套长约1.3km的碎屑熔岩。该套岩石显微薄片上表现为发育玻晶交织结构和溶蚀结构等熔岩结构特征,少许凝灰结构。碎屑主要为5~6cm的角砾,含少量的火山弹。角砾可能来自于大黑山火山较强的爆发,生成岩屑以及少量凝灰质碎屑。后期熔岩喷泉式喷发形成熔岩流,在流动过程中裹带大量的岩屑以及少量凝灰质碎屑。该套碎屑熔岩的发现,加深了对整个地区火山活动类型的了解。     

Geochemistry of the volcano-hydrothermal system of El Chichón Volcano, Chiapas, Mexico

 The 1982 eruption of El Chichón volcano ejected more than 1 km³ of anhydrite-bearing trachyandesite pyroclastic material to form a new 1-km-wide and 300-m-deep crater and uncovered the upper 500 m of an active volcano-hydrothermal system. Instead of the weak boiling-point temperature fumaroles of the former lava dome, a vigorously boiling crater spring now discharges  / 20 kg/s of Cl-rich (∼15 000 mg/kg) and sulphur-poor ( / 200 mg/kg of SO₄), almost neutral (pH up to 6.7) water with an isotopic composition close to that of subduction-type magmatic water (δD=–15‰, δ¹⁸O=+6.5‰). This spring, as well as numerous Cl-free boiling springs discharging a mixture of meteoric water with fumarolic condensates, feed the crater lake, which, compared with values in 1983, is now much more diluted (∼3000 mg/kg of Cl vs 24 030 mg/kg), less acidic (pH=2.6 vs 0.56) and contains much lower amounts of S ( / 200 mg/kg of SO₄, vs 3550 mg/kg) with δ³⁴S=0.5–4.2‰ (+17‰ in 1983). Agua Caliente thermal waters, on the southeast slope of the volcano, have an outflow rate of approximately 100 kg/s of 71  °C Na–Ca–Cl water and are five times more concentrated than before the eruption (B. R. Molina, unpublished data). Relative N₂, Ar and He gas concentrations suggest extensional tectonics for the El Chichón volcanic centre. The ³He/⁴He and ⁴He/²⁰Ne ratios in gases from the crater fumaroles (7.3R_a, 2560) and Agua Caliente hot springs (5.3R_a, 44) indicate a strong magmatic contribution. However, relative concentrations of reactive species are typical of equilibrium in a two-phase boiling aquifer. Sulphur and C isotopic data indicate highly reducing conditions within the system, probably associated with the presence of buried vegetation resulting from the 1982 eruption. All Cl-rich waters at El Chichón have a common source. This water has the appearence of a "partially matured" magmatic fluid: condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in S due to anhydrite precipitation. Shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits (Red waters) are Ca–SO₄–rich and have a negative oxygen isotopic shift, probably due to ongoing formation of clay at low temperatures. Received: 21 July 1997 / Accepted: 4 December 1997     

Seismo-acoustic signals associated with degassing explosions recorded at Shishaldin Volcano,Alaska, 2003–2004

In summer 2003, a Chaparral Model 2 microphone was deployed at Shishaldin Volcano, Aleutian Islands, Alaska. The pressure sensor was co-located with a short-period seismometer on the volcano’s north flank at a distance of 6.62 km from the active summit vent. The seismo-acoustic data exhibit a correlation between impulsive acoustic signals (1–2 Pa) and long-period (LP, 1–2 Hz) earthquakes. Since it last erupted in 1999, Shishaldin has been characterized by sustained seismicity consisting of many hundreds to two thousand LP events per day. The activity is accompanied by up to ∼200 m high discrete gas puffs exiting the small summit vent, but no significant eruptive activity has been confirmed. The acoustic waveforms possess similarity throughout the data set (July 2003–November 2004) indicating a repetitive source mechanism. The simplicity of the acoustic waveforms, the impulsive onsets with relatively short (∼10–20 s) gradually decaying codas and the waveform similarities suggest that the acoustic pulses are generated at the fluid–air interface within an open-vent system. SO₂ measurements have revealed a low SO₂ flux, suggesting a hydrothermal system with magmatic gases leaking through. This hypothesis is supported by the steady-state nature of Shishaldin’s volcanic system since 1999. Time delays between the seismic LP and infrasound onsets were acquired from a representative day of seismo-acoustic data. A simple model was used to estimate source depths. The short seismo-acoustic delay times have revealed that the seismic and acoustic sources are co-located at a depth of 240±200 m below the crater rim. This shallow depth is confirmed by resonance of the upper portion of the open conduit, which produces standing waves with f=0.3 Hz in the acoustic waveform codas. The infrasound data has allowed us to relate Shishaldin’s LP earthquakes to degassing explosions, created by gas volume ruptures from a fluid–air interface.     

Estimation by photogrammetry of the glacier recession on the Cotopaxi Volcano (Ecuador) between 1956 and 1997 / Estimation par photogrammétrie de la récession glaciaire sur le Volcan Cotopaxi (Equateur) entre 1956 et 1997

Abstract

Aerial photographs of the Cotopaxi Volcano ice cap dating from 1956 to 1997 were used to quantify the evolution of the surface area. Results were obtained using precise stereoscopic methods that give the most accurate information. In addition four specific glacier tongues were investigated in detail to measure the ice mass lost between 1976 and 1997. Surprisingly, the bedrock morphology is shown to be very irregular and this explains a large extent of the variability found in the ice losses. The results show that glaciers stagnated from 1956 to 1976 and lost about 30% of their surface area between 1976 and 1997. Slope exposure did not seem to have any significant effect since all the glaciers of the volcano retreated in the same proportion. In accordance with specific measurements performed on the nearby Antizana 15 Glacier, it is suggested that the strong recession observed after 1976 was associated with increasing melting conditions which have occurred repeatedly during the intense/long-duration warm ENSO phases.     

藏北高原现代火山的航空象片与地图分析

本文应用航空象片和地形图分析了藏北地区现代火山和分布,对研究藏北高原火山活动有一定参考价值。