琼东南盆地北礁地区梅山组丘形反射特征

目前琼东南盆地北礁凹陷中中新统梅山组顶部丘形反射引起广泛关注,但对其成因有不同认识。本文通过高精度二维、三维地震、钻井资料,研究丘形反射的特征。研究表明北礁地区梅山组顶部发育近东西向展布的长条形丘体,丘间为水道,丘内为中-弱振幅的地震反射,与西南部强振幅水道砂岩形成鲜明的对比,波阻抗反演揭示丘内为低波阻抗,属泥岩范畴。梅山组塑性丘内地层发生重力扩展,在其上覆的脆性地层（强振幅砂岩和弱振幅泥岩）发育多边形断层,反推出梅山组形成于深水环境,丘为泥丘,沉积环境分析也认为北礁凹陷中中新世为半深海沉积,梅山组的丘-谷分别对应上覆地层的谷-丘,认为是底流剥蚀/沉积成因。本文的研究对南海北部丘形反射的认识有重要意义,并可降低油气探勘风险。     

南海北部深水区中新世生物礁发育特征

基于近些年南海北部深水区采集的地震资料,对南海北部深水区中新世生物礁发育特点进行分析、对比和研究,认为西沙隆起地区发育典型的生物礁,具有丘状反射、强振幅、中频、中连和杂乱地震相,发现琼东南盆地北礁地区在中新世梅山组也有似礁相发育.通过对琼东南盆地深水区中新世生物礁层序地层学分析,认为生物礁在中新世梅山组海侵体系域和高位体系域发育.从对北礁地区典型生物礁剖面进行的波阻抗反演来看,其与LH11-1生物礁油田的波阻非常相似,波阻抗值为8×106-9×106kg/(m2·s).古地理分析认为,中新世西沙隆起区与北礁地区处于滨、浅海沉积环境,梅山组时期的陆缘碎屑供给量比较少,适于生物礁发育.     

琼东南盆地北礁凹陷梅山组单向迁移水道特征及成因探讨

深水区重力流与底流交互作用的过程、响应及动力学机制是海洋沉积学研究的前沿和薄弱环节。本文通过三维地震资料,在深水区北礁凹陷南西部梅山组发现多条相间分布的长条形顺直强振幅水道,垂直于西沙隆起(南部隆起)北斜坡走向,向南西方向单向迁移,水道具有南西陡(凹岸或陡岸)北东缓(凸岸或缓岸)的特征,该类水道分为侵蚀界面和水道砂-堤岸泥过渡复合体系两个单元,侵蚀界面在凹岸的削截反射明显多于凸岸,水道砂-堤岸泥过渡复合体振幅强度由凹岸强振幅逐渐过渡为凸岸弱振幅。分析认为,该类水道发育于中中新世半深海环境,不同于向底流下游方向单向迁移的峡谷,它们向底流上游方向发生单向迁移,并提出其成因模式:前期来自南部的浊流下切形成负向地貌单元(水道),底流对这一地貌单元进行改造,形成迎流面缓(凸岸)背流面陡(凹岸)的地貌,同时驱使浊流上部顺底流方向偏移,形成溢岸浊流沉积,致凹岸沉积速率低,凸岸沉积速率高,这样就迫使水道逆底流方向偏移。沉积物源、中层水相关底流、古气候和海平面的变化、北礁凸起古地形控制是该区单向迁移强振幅水道发育的因素。本研究在南海首次发现这种向底流上游方向单向迁移的水道,是底流与重力流交互作用的新型类型,对古海洋、古气候研究,深水油气勘探有着重要的意义,希望引起地质学家的重视。     

琼东南盆地南部中新统“丘”形反射成因探讨

在琼东南盆地南部中新统梅山组广泛发育“丘”形反射, 对其识别分析具有重要的意义。这些“丘”形反射主要分布在北礁凹陷及周缘斜坡带上, 在顶底界面呈强反射, 在内部成层、杂乱或为空白反射, 有时在顶部见披覆沉积, 从盆地中心北礁凹陷向边缘斜坡带迁移生长。通过对“丘”形反射的古构造和古地理背景、几何学特征及地震响应特征等方面综合分析, 对其成因进行了探讨, 排除了其为生物礁、泥底辟以及火山丘的可能, 认为其可能为深水环境底流作用下形成的等深流沉积或某种沉积物波。     

琼东南盆地北礁凹陷上新统等深流影响的水道沉积体系

等深流影响的水道沉积体系的沉积特征及其沉积过程是当前深水沉积学研究的热点、难点和前沿科学问题,但研究程度较为薄弱。该文以北礁凹陷上新统（地震反射T20?T30）为研究对象,利用覆盖北礁凹陷局部的三维地震资料,采用均方根属性、相干属性、时间域构造,再结合地震切片等方法,研究北礁凹陷深水区上新统斜交斜坡（走向）的特殊水道沉积体系特征及其沉积过程。研究发现,该水道沉积体系分为早、晚两期,早期发育水道和片状、扇状溢堤沉积,晚期仅发育水道和片状溢堤沉积,其中扇状溢堤沉积仅发育在水道右侧弯曲处,片状溢堤沉积仅分布在水道左侧,水道始终与区域斜坡斜交,水道对称分布且无明显迁移现象。结合该时期北礁凸起发育等深流相关的丘状漂积体和环槽,认为该水道沉积体系特殊的形态主要受控于等深流与浊流交互作用的沉积结果:浊流流经水道,其上覆浊流溢出水道,形成溢岸浊流,在水道左侧,该溢岸浊流与等深流发生相向运动,被等深流“吹拂”到单侧,大面积分布,延伸千米,形成片状溢堤沉积;而在水道弯曲处（右侧）,溢岸浊流与等深流发生相对运动,抑制溢岸浊流进一步扩展,形成相对小范围扇状溢堤沉积,该沉积结果与前人水槽实验结果相一致。     

琼东南盆地陆坡区深水浊积水道的地震相特征

水道-天然堤体系作为油气储集圈闭日益引起沉积学家和勘探家的重视。地震相特征是识别深水水道的有效途径,本文基于高分辨率2D、3D地震资料的地震相分析,在琼东南盆地陆坡区深水盆地中识别出早中新世、上新世和第四纪多期深水水道体系。早中新世深水水道在地震剖面上具有强、弱振幅交替反射和相互叠置的地震反射特征,局部具有杂乱反射特点;上新世水道整体表现为强振幅,横向上连续或者半连续,纵向上为强振幅的叠加;第四纪水道在地震剖面上具有典型下切反射特点,该水道整体振幅相对较弱,但其水道轴部充填具有典型的强反射特征,这与世界典型地区的水道轴部粗粒充填强振反射一致。这几期深水水道都发育于低水位时期,为上部物源搬运引起的浊流事件而形成。     

下刚果盆地A区块中新统深水水道沉积特征

重力流分支水道是下刚果盆地中新统发育的典型深水沉积单元之一.利用相干时间切片、RMS均方根振幅和3D振幅可视化等地球物理手段识别出工区内发育的深水弯曲水道,论述了复合水道砂体内部充填结构,精细刻画了深水水道砂体的内幕结构,并利用地质异常体处理与三维可视化技术相结合追踪出工区内发育的水道砂体,描述了其平面分布特征和储层特征.工区内主要发育高弯度重力流分支水道,根据深水水道充填成因分类将其进一步划分为侵蚀充填型和侵蚀—加积型水道复合体;大型侵蚀水道内部由多期充填,主要由滑塌形成的旋转滑块和碎屑流、叠置水道及水道—天然堤沉积组成;并在三维可视化中识别出了多期水道砂体,探讨了水道砂体的地震反射特征和测井响应特征.     

莺歌海盆地中新统轴向重力流沉积特征及勘探潜力

轴向重力流沉积是一种重要的深水储层,其形成的岩性油气藏也是目前莺歌海盆地重点勘探领域。通过钻井、测井、地震和区域地质等资料的综合研究,分析莺歌海盆地中新统轴向重力流沉积特征和演化规律,探讨轴向重力流岩性油气藏的成藏条件和控制因素。结果表明,中新统储层为重力流沉积成因的厚层细砂岩,主水道和朵叶复合体是重力流沉积有利的沉积微相;中新世海南物源供给充足、断裂坡折带发育以及盆地轴向负向地形是该区形成轴向重力流沉积的宏观地质条件,在中新世各个时期形成了一系列沿盆地轴向分布、具有前积反射结构特征的轴向重力流沉积。研究区中新统轴向重力流储层厚度大、沉积规模广、临近烃源岩、构造脊微裂隙发育、圈闭保存好,具备优越的岩性油气藏成藏条件,是盆地下一步勘探的重要领域。     

西沙海区碳酸盐台地地震反射特征及沉积模式

随着南海海盆的持续扩张,西沙海区整体沉降,从早中新世起西沙碳酸盐台地开始发育,而且在台地之上生长了不同类型的生物礁。通过地震识别认为,西沙海区生物礁在地震剖面上表现为顶底强振幅的丘形连续反射、内部弱振幅杂乱反射;碳酸盐台地表现为顶部强振幅连续平行反射、底部界面局部模糊、内部强弱相间亚平行连续反射。通过对西沙海区地层层序的分析以及大量地震资料的解释认为,在西沙碳酸盐台地的发育早期它受基底构造的控制,而在后期主要受多期海平面变化的影响,其发育演化经历了初始生长—加积—出露—二次生长—淹没等一系列阶段,复杂的演化过程也使西沙碳酸盐岩具有较好的储集条件和油气远景。     

南海北部西沙海槽盆地新生代层序地层及地震相北大核心CSCD

西沙海槽盆地是南海北部陆坡西段的一个勘探程度较低的大型新生代深水沉积盆地。基于新采集的高精度多道地震资料并结合周边地区地质特征对盆地进行了层序地层分析,在研究区内识别出8个地震反射界面,结合地震剖面振幅旋回性变化,将研究区新生代地层划分了3个超层序和8个层序,并进一步论述了各层序的顶底接触关系、地震反射特征、地层厚度、层速度及砂岩含量等。在层序格架内识别出5类典型的地震相:平行-亚平行相、楔状发散相、前积相、杂乱相及水道充填相。在地震相划分和沉积相分析的基础上,通过对各层序沉积特征和沉积发育史的分析,认为始新世研究区呈现出陆相湖盆沉积体系特征;渐新世,盆地遭受海侵,研究区接受滨海相和浅海相沉积;早中新世,盆地中部沉积大规模半深海相沉积;中中新世末海平面出现下降,陆坡半深海环境的范围有所减小,西沙海槽浊积水道的雏形形成;晚中新世之后,研究区进入稳定的区域沉降阶段,主要发育了一套半深海-深海相泥岩沉积。此外,由研究区南北缘隆起区提供物源在层序内部还发育有近岸水下扇、三角洲、扇三角洲等沉积体,由地形高差控制作用在陡坡带或断层下降盘还形成了斜坡扇、浊积体。     

Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea

Numerous elongated mounds and channels were found at the top of the middle Miocene strata using 2D/3D seismic data in the Liwan Sag of Zhujiang River Mouth Basin(ZRMB) and the Beijiao Sag of Qiongdongnan Basin(QDNB). They occur at intervals and are rarely revealed by drilling wells in the deepwater areas. Origins of the mounds and channels are controversial and poorly understood. Based on an integrated analysis of the seismic attribute, palaeotectonics and palaeogeography, and drilling well encountering a mound, research results show that these mounds are dominantly distributed on the depression centres and/or slopes of the Liwan and Beijiao sags and developed in a bathyal sedimentary environment. In the Liwan and Beijiao sags, the mounds between channels(sub) parallel to one another are 1.0–1.5 km and 1.5–2.0 km wide, 150–300 m and 150–200 m high, and extend straightly from west to east for 5–15 km and 8–20 km, respectively. Mounds and channels in the Liwan Sag are parallel with the regional slope. Mounds and channels in the Beijiao Sag, however, are at a small angle to the regional slope. According to internal geometry, texture and external morphology of mounds, the mounds in Beijiao Sag are divided into weak amplitude parallel reflections(mound type I), blank or chaotic reflections(mound type II), and internal mounded reflections(mound type Ⅲ). The mounds in Liwan Sag, however, have the sole type, i.e., mound type I. Mound type I originates from the incision of bottom currents and/or gravity flows. Mound type II results from gravity-driven sediments such as turbidite. Mound type Ⅲ is a result of deposition and incision of bottom currents simultaneously. The channels with high amplitude between mounds in the Beijiao and Liwan sags are a result of gravity-flow sediments and it is suggested they are filled by sandstone.Whereas channels with low-mediate amplitudes are filled by bottom-current sediments only in the Beijiao Sag,where they are dominantly composed of mudstone. This study provides new insights into the origins of the mounds and channels worldwide.     

Seismic characteristics of a reef carbonate reservoir and implications for hydrocarbon exploration in deepwater of the Qiongdongnan Basin,northern South China Sea

Our analysis of approximately 40,000 km of multichannel 2-D seismic data, reef oil-field seismic data, and data from several boreholes led to the identification of two areas of reef carbonate reservoirs in deepwater areas (water depth ≥ 500 m) of the Qiongdongnan Basin (QDNB), northern South China Sea. High-resolution sequence stratigraphic analysis revealed that the transgressive and highstand system tracts of the mid-Miocene Meishan Formation in the Beijiao and Ledong–Lingshui Depressions developed reef carbonates. The seismic features of the reef carbonates in these two areas include chaotic bedding, intermittent internal reflections, chaotic or blank reflections, mounded reflections, and apparent amplitude anomalies, similar to the seismic characteristics of the LH11-1 reef reservoir in the Dongsha Uplift and Island Reef of the Salawati Basin, Indonesia, which house large oil fields. The impedance values of reefs in the Beijiao and Ledong–Lingshui Depressions are 8000–9000 g/cc × m/s. Impedance sections reveal that the impedance of the LH11-1 reef reservoir in the northern South China Sea is 8000–10000 g/cc × m/s, whereas that of pure limestone in BD23-1-1 is >10000 g/cc × m/s. The mid-Miocene paleogeography of the Beijiao Depression was dominated by offshore and neritic environments, with only part of the southern Beijiao uplift emergent at that time. The input of terrigenous sediments was relatively minor in this area, meaning that terrigenous source areas were insignificant in terms of the Beijiao Depression; reef carbonates were probably widely distributed throughout the depression, as with the Ledong–Lingshui Depression. The combined geological and geophysical data indicate that shelf margin atolls were well developed in the Beijiao Depression, as in the Ledong–Lingshui Depression where small-scale patch or pinnacle reefs developed. These reef carbonates are promising reservoirs, representing important targets for deepwater hydrocarbon exploration.     

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.     

Contourite identification along Italian margins: The case of the Portofino drift (Ligurian Sea)

A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s⁻¹ in winter 2013 and calculated bottom shear stress exceeding 0.2 N m⁻² in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.     

Variations in bottom water activity at the southern Argentine margin: indications from a seismic analysis of a continental slope terrace

Continental slope terraces at the southern Argentine margin are part of a significant contourite depositional system composed of a variety of drifts, channels, and sediment waves. Here, a refined seismostratigraphic model for the sedimentary development of the Valentin Feilberg Terrace located in ~4.1?km water depth is presented. Analyzing multichannel seismic profiles across and along this terrace, significant changes in terrace morphology and seismic reflection character are identified and interpreted to reflect variations in deep water hydrography from Late Miocene to recent times, involving variable flow of Antarctic Bottom Water and Circumpolar Deep Water. A prominent basin-wide aggradational seismic unit is interpreted to represent the Mid-Miocene climatic optimum (~17?C14?Ma). A major current reorganization can be inferred for the time ~14?C12?Ma when the Valentin Feilberg Terrace started growing due to the deposition of sheeted and mounded drifts. After ~12?Ma, bottom water flow remained vigorous at both margins of the terrace. Another intensification of bottom flow occurred at ~5?C6?Ma when a mounded drift, moats, and sediment waves developed on the terrace. This may have been caused by a general change in deep water mass organization following the closure of the Panamanian gateway, and a subsequent stronger southward flow of North Atlantic Deep Water.     

Sequence stratigraphy of the Mississippi fan (Plio-Pleistocene), Gulf of Mexico

The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick, deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. Analysis of 19,000 km of multifold seismic data defined 17 seismic sequences, each characterized by channel, levee, and associated overbank deposits, as well as mass transport deposits. At the base of nine sequences are a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10–20% of the sediments in each the sequences. These facies are externally mounded and occur in two general regions of the fan: (1) in the upper and middle fan they are elongate in shape and mimic the channel's distribution; (2) in the middle fan to lower fan they are characterized by a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites (informally called “mass transport complexes”). Overlying this basal interval, characteristic of all sequences, are well-developed channel-levee systems that constitute 80–90% of the fan's sediments. Channels consist of high amplitude, subparallel reflections, whereas the flanking levee sediments appear as subparallel reflections that have high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in grain size and bed thicknesses. Overbank sediments are characterized by interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites from the channel, as well as slides and debris flows derived both locally and from the slope updip.     

Reconstructions of the Mediterranean Outflow Water during the quaternary based on the study of changes in buried mounded drift stacking pattern in the Gulf of Cadiz

Contourite deposits in the central sector of the middle slope of the Gulf of Cadiz have been studied using a comprehensive acoustic, seismic and core database. Buried, mounded, elongated and separated drifts developed under the influence of the lower core of the Mediterranean Outflow Water are preserved in the sedimentary record. These are characterised by depositional features in an area where strong tectonic and erosive processes are now dominant. The general stacking pattern of the depositional system is mainly influenced by climatic changes through the Quaternary, whereas changes in the depositional style observed in two, buried, mounded drifts, the Guadalquivir and Huelva Drifts, are evidence of a tectonic control. In the western Guadalquivir Drift, the onset of the sheeted drift construction (aggrading QII unit) above a mounded drift (prograding QI unit) resulted from a new Lower Mediterranean Core Water hydrodynamic regime. This change is correlated with a tectonic event coeval with the Mid Pleistocene Revolution (MPR) discontinuity that produced new irregularities of the seafloor during the Mid- to Late-Pleistocene. Changes in the Huelva Drift from a mounded to a sheeted drift geometry during the Late-Pleistocene, and from a prograding drift (QI and most part of QII) to an aggrading one (upper seismic unit of QII), highlight a new change in oceanographic conditions. This depositional and then oceanographic change is associated with a tectonic event, coeval with the Marine Isotope Stage (MIS) 6 discontinuity, in which a redistribution of the diapiric ridges led to the development of new local gateways, three principal branches of the Mediterranean Lower Core Water, and associated contourite channels. As a result, these buried contourite drifts hold a key palaeoceanographic record of the evolution of Mediterranean Lower Core Water, influenced by both neotectonic activity and climatic changes during the Quaternary. This study is an example of how contourite deposits and erosive elements in the marine environment can provide evidence for the reconstruction of palaeoceanographic and recent tectonic changes.