陆上、水下喷发成因火山岩储层发育特征和成藏控制因素对比分析

火山岩油气藏的研究一般以陆上喷发沉积的火山岩体为主,事实上全世界四分之三的火山活动是在水下发生的,只是水下喷发沉积的火山岩在各个方面的研究还比较少,在勘探中尚未引起注意。本文以三塘湖盆地石炭系火山岩油藏为例,对比分析了陆上与水下喷发沉积形成的火山岩在岩性、颜色、结构构造以及储集空间特征和地震反射特征等方面的差异,指出了三塘湖盆地石炭系哈尔加乌组火山岩为水下喷发环境形成。通过对研究区两种类型的火山岩油藏系统的研究,总结出了水下喷发沉积的火山岩储层的形成机理以及其形成规模油气藏的条件,为今后的火山岩油气藏的勘探指出了新的方向。     

A Miocene coarse volcaniclastic mass-flow deposit in the Shimane Peninsula,SW Japan: product of a deep submarine eruption?

 A subaqueous volcaniclastic mass-flow deposit in the Miocene Josoji Formation, Shimane Peninsula, is 15–16 m thick, and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non- to poorly vesiculated rhyolite. It can be divided into four layers in ascending order. Layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0.3–6 m thick. Layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6–11 m thick. This layer bifurcates laterally into minor depositional units individually composed of a massive, lithic-rich lower part and a diffusely stratified, pumice-rich upper part with inverse to normal grading of both lithic and pumice clasts. Layer 3 is 2.5–3 m thick, and consists of interbedded fines-depleted pumice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse ash bed 1.5–2 m thick. The volcaniclastic deposit shows internal features of high-density turbidites and contains no evidence for emplacement at a high temperature. The mass-flow deposit is extremely coarse-grained, dominated by traction structures, and is interpreted as the product of a deep submarine, explosive eruption of vesicular magma or explosive collapse of lava. Received: 10 January 1996 / Accepted: 23 February 1996     

柴北缘托莫尔日特滩间山群碎屑岩组的沉积环境与构造背景分析SCIEI北大核心CSCD

柴北缘构造带断续出露的寒武-奥陶纪滩间山群火山-沉积岩系是早古生代时期原特提斯洋俯冲造山作用的产物,也是中国西部块状硫化物和造山型金矿床的重要含矿岩系。前人已对滩间山群火山岩组开展了大量的地球化学和同位素年代学研究,而碎屑岩组的沉积序列和沉积相研究相对薄弱。我们在柴北缘托莫尔日特地区地质填图过程中,对该地区的滩间山群碎屑岩组开展了系统的沉积组合序列和沉积相研究。托莫尔日特地区滩间山群碎屑岩组是由富含火山碎屑物质的含砾砂岩、砂岩、粉砂岩、泥岩、沉凝灰岩、硅质凝灰岩、硅质岩及少量砾岩共同组成。可分为下岩性段和上岩性段,其中下岩性段为一套形成于海底扇外扇环境以沉凝灰岩为主的沉积组合;上岩性段包含海底扇内扇、中扇和外扇沉积,且以中扇环境的沉积为主。垂向上,它们呈现出粒度向上逐渐变细特征;空间上,具有向北西方向火山物质成分逐渐增多、砾岩和砂岩厚度变薄且砾岩消失的变化特征。砂岩主要为岩屑长石杂砂岩,其中岩屑主要是安山岩和少量玄武岩、英安岩、凝灰岩;长石主要为斜长石;石英碎屑相对缺失。砾岩中砾石以安山岩和硅质岩为主,并含有少量灰岩和凝灰岩砾石。底冲刷面、正粒序、平行层理和波纹层理等沉积构造普遍发育,呈现出典型的浊流沉积特征;同时在局部露头见有滑塌构造和波痕。区域上,这套沉积组合序列位于寒武-奥陶纪岛弧火山岩的南侧;古水流分析显示,它们的碎屑物质主要来自于其北东方向。这些结果表明,柴北缘构造带东段的滩间山群碎屑岩组是一套成熟度极低且与寒武-奥陶纪岛弧火山活动密切相关的沉积组合。     

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO₂), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO₂). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt     

An Upper Pliocene coarse pumice breccia generated by a shallow submarine explosive eruption,Milos, Greece

The Filakopi Pumice Breccia (FPB) is a very well exposed, Pliocene volcaniclastic unit on Milos, Greece, and has a minimum bulk volume of 1 km³. It consists of three main units: (A) basal lithic breccia (4–8 m) mainly composed of angular to subangular, andesitic and dacitic clasts up to 2.6 m in diameter; (B) very thickly bedded, poorly sorted pumice breccia (16–17 m); and (C) very thick, reversely graded, grain-supported, coarse pumice breccia (6.5–20 m), at the top. The depositional setting is well constrained as shallow marine (up to a few hundred metres) by overlying fossiliferous and bioturbated mudstone. This large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because: (1) pumice clasts are the dominant component; (2) the coarse pumice clasts (>64 mm) have complete quenched margins; (3) very large (>1 m) pumice clasts are common; (4) overall, the formation shows good hydraulic sorting; and (5) a significant volume of ash was deposited together with the coarsest pyroclasts.The bed forms in units A and B suggest deposition from lithic-rich and pumiceous, respectively, submarine gravity currents. In unit C, the coarse (up to 6.5 m) pumice clasts are set in matrix that grades upwards from diffusely stratified, fine (1–2 cm) pumice clasts at the base to laminated shard rich mud at the top. The coarse pumice clasts in unit C were settled from suspension and the framework was progressively infilled by fine pumice clasts from waning traction currents and then by water-settled ash. The FPB displays important features of the products of submarine explosive eruptions that result from the ambient fluid being seawater, rather than volcanic gas or air. In particular, submarine pyroclastic deposits are characterised by the presence of very coarse juvenile pumice clasts, pumice clasts with complete quenched rims, and good hydraulic sorting.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: J. Donelly-Nolan     

Emplacement mechanisms of the South Kona slide complex,Hawaii Island: sampling and observations by remotely operated vehicle Kaiko

Emplacement of a giant submarine slide complex, offshore of South Kona, Hawaii Island, was investigated in 2001 by visual observation and in-situ sampling on the bench scarp and a megablock, during two dives utilizing the Remotely Operated Vehicle (ROV) Kaiko and its mother ship R/V Kairei. Topography of the bench scarp and megablocks were defined in 3-D perspective, using high-resolution digital bathymetric data acquired during the cruise. Compositions of 34 rock samples provide constraints on the landslide source regions and emplacement mechanisms. The bench scarp consists mainly of highly fractured, vesiculated, and oxidized aa lavas that slumped from the subaerial flank of ancestral Mauna Loa. The megablock contains three units: block facies, matrix facies, and draped sediment. The block facies contains hyaloclastite interbedded with massive lava, which slid from the shallow submarine flank of ancestral Mauna Loa, as indicated by glassy groundmass of the hyaloclastite, low oxidation state, and low sulfur content. The matrix facies, which directly overlies the block facies and is similar to a lahar deposit, is thought to have been deposited from the water column immediately after the South Kona slide event. The draped sediment is a thin high-density turbidite layer that may be a distal facies of the Alika-2 debris-avalanche deposit; its composition overlaps with rocks from subaerial Mauna Loa. The deposits generated by the South Kona slide vary from debris avalanche deposit to turbidite. Spatial distribution of the deposits is consistent with deposits related to large landslides adjacent to other Hawaiian volcanoes and the Canary Islands.     

The submarine landscape of the ‘Jurassic Coast’ World Heritage Site,Dorset, UK and its Setting

This paper reviews recent studies of the seabed offshore from the Jurassic Coast and discusses their significance for understanding landforms of the World Heritage Site’s Setting as well as adding to understanding of the structures exposed in the cliffs. The processes of coastal formation as sea-levels rose and fell during the Pleistocene and in the recent post-glacial period are critical for this as they reveal the landforms which preserve former shorelines. Detailed surveys of the seabed provide evidence of the low slope of much of this seabed and so enhance the potential to interpret older terrestrial landforms which have been attributed to uplift of former sea beds. Considering both the seabed and the coast from the Pleistocene to the Anthropocene means that it is possible to assess the human impact on these landforms and processes, as well as the natural timescale within which changes occur. The paper describes the seabed geomorphology, relating this, where appropriate, to both the coastal landforms and the exposed structures. Current interpretations of the seabed features and their implications for our understanding of the recent evolution of the Site and Setting are explored. The features of the coast and adjacent sea bed at different phases of the changing coastal environment are discussed to the extent that the available evidence makes this possible, but for much of the area this remains speculative.     

基于粒子群算法的浮筒配置优化问题的研究

针对传统的验证法在解决浮筒配置问题中存在的计算量大、计算时间长、工作重复性强等缺点,提出了一种将浮筒纵向位置配置和浮筒充气量综合考虑、一体优化的方法。文章将各种合理性校验准则作为约束条件,通过逼近沉艇起浮瞬间和浮起后的平衡状态,来求取各对浮筒的纵向位置和充气量,将浮筒配置问题转换为一个多目标、多约束的最优化问题。在此基础上,根据粒子群优化算法的特点,提出了一种双向搜索策略,来求解浮筒配置优化问题。这种方法简单实用,计算时间短,无需人工干预,算法运行一次可获得多组配置方案供决策人员选择。算例表明用这种方法来解决浮筒配置问题是可行的、有效的。     

Seafloor and buried mounds on the western slope of the Niger Delta

Seafloor mounds are potential geohazards to offshore rig emplacement and drilling operations and may contain evidence of underlying petroleum systems. Therefore, identifying and mapping them is crucial in de-risking exploration and production activities in offshore domains.A 738 km² high resolution three-dimensional seismic dataset was used to investigate the occurrence, seismic characteristics and distribution of features interpreted as seafloor and buried sediment mounds, at water-depths of 800–1600 m, on the western Niger Delta slope. Fifteen seafloor mounds and eighteen shallowly buried mounds were identified. The seafloor mounds are characterised by lower seismic amplitude anomalies than the surrounding seabed sediments, and overlie vertical zones of acoustic blanking. The buried mounds in contrast are characterised by high amplitude anomalies; they also directly overlie sub-vertical zones of acoustic blanking. Seismic evidences from the features, their distribution patterns and tectono-stratigraphic associations suggest that their formation was controlled by the juxtaposition of buried channels and structural highs and their formation caused by focused fluid flow and expulsion of entrained sediments at the seabed.Considering the acoustic and geometrical characteristics of the mounds and comparing them with mound-shaped features from around the world, we conclude that the mounds most likely comprise heterolithic seafloor extrusions of muds and sands from the Agbada Formation with gas and possibly oil in some of the pore space giving rise to the acoustic characteristics.     

模糊多属性决策法在海底电缆路由方案比选中的应用

海底电缆预选路由方案的比选需要参考多种指标,这些指标包含定量和定性两种,但各指标间的不可公度性,给路由方案的选择提出挑战。文章提出了一种基于离差思想和模糊向量投影的多属性决策法。首先引入三角模糊数,将定性指标合理量化,构建决策矩阵,规范化后运用离差思想求解各指标权重,然后计算各方案在正理想方案和负理想方案上的投影,最后求出相对贴近度,依据其大小比选出最佳方案。