地球科学进展, 2020, 35(6): 632-642 DOI: 10.11867/j.issn.1001-8166.2020.047

研究简报

浅谈深水块体搬运复合体的结构、成因分类以及识别方法

秦磊,1,2, 毛金昕1,2, 倪凤玲1,2,3, 徐少华,1,2, 李小刚1,2, 蔡长娥1,2, 尚文亮1,2, 刘家恺1,2

1.重庆科技学院复杂油气田勘探开发重庆市重点实验室,重庆 401331

2.重庆科技学院石油与天然气 工程学院,重庆 401331

3.中国石油化工股份有限公司江汉油田分公司,湖北 潜江 430050

A Brief Introduction to Deep-Water Mass-Transport Complexes: Structures, Genetic Classifications and Identification Methods

Qin Lei,1,2, Mao Jinxin1,2, Ni Fengling1,2,3, Xu Shaohua,1,2, Li Xiaogang1,2, Cai Change1,2, Shang Wenliang1,2, Liu Jiakai1,2

1.Chongqing Key Laboratory of Complex Oil and Gas Exploration and Development, Chongqing University of Science and Technology, Chongqing 401331, China

2.School of Petroleum Engineering, Chongqing University of Science Technology, Chongqing 401331, China

3.China Petroleum & Chemical Corporation Jianghan Oilfield Branch, Qianjiang Hubei 430050, China

通讯作者: 徐少华(1987-),男,湖北潜江人,讲师,主要从事层序地层学与沉积学研究. E-mail:xsh_xu@163.com

收稿日期: 2020-03-11   修回日期: 2020-05-08   网络出版日期: 2020-07-03

基金资助: 国家自然科学基金项目“陆架边缘三角洲体系供源速率的侧向差异对地层叠加样式的控制——以珠江口盆地SQ13.8为例”.  41902114
重庆科技学院研究生科技创新训练计划项目“强制海退体系域的地震响应临界条件及其控制因素”.  YKJCX1920113

Corresponding authors: Xu Shaohua (1987-), male, Qianjiang City, Hubei Province, Lecturer. Research areas include sequence stratigraphy and sedimentology. E-mail:xsh_xu@163.com

Received: 2020-03-11   Revised: 2020-05-08   Online: 2020-07-03

作者简介 About authors

秦磊(1995-),男,重庆人,硕士研究生,主要从事油气勘探地质工程研究.E-mail:ql_stone@163.com

QinLei(1995-),male,ChongqingCity,Masterstudent.Researchareasincludeoilandgasexplorationgeologicalengineering.E-mail:ql_stone@163.com

摘要

全球普遍发育的块体搬运复合体是深水地层的重要组成单元,广泛分布在大陆边缘和陆相湖盆中。系统认识块体搬运复合体有助于拓展深水油气勘探领域和评价海底工程地质风险。典型的块体搬运复合体由头部拉张区、体部滑移区和趾部挤压区3个部分组成,三者在顺坡搬运过程中,存在由头部到趾部依次演变的关系。基于沉积物搬运过程和不同阶段流体性质,将块体搬运复合体分为滑动岩、滑塌岩和碎屑岩3类,分别对应搬运过程的弹性形变、弹性和塑性形变以及塑性形变阶段。块体搬运复合体在三维地震平面图通常可根据其沉积物的流动方向(顺坡方向)依次识别出头部陡崖、体部滑塌褶皱和趾部挤压脊等构造单元,剖面上以弱—中振幅反射和丘状—长条状外形为特征,具有不规则顶底界面。测井上块体搬运复合体具有顶底界面突变的曲线响应。露头上块体搬运复合体通常具有一种或多种沉积构造,比如块状构造、滑塌褶皱和泥质撕裂屑等。块体搬运复合体与浊积岩的主要区别在于块体搬运复合体整体以“冻结式”搬运沉积,具有“长条形—似扇形”的斜坡组合形态,垂向上沉积厚度大,无明显粒序分异特征,地层倾角常大于浊积岩沉积地层。

关键词: 深水块体搬运复合体 ; 沉积结构 ; 成因分类 ; 识别标志

Abstract

Mass-Transport Complexes (MTCs) are important components of deep-water systems, and widely distributed in continental margins and lake basins. Understanding MTCs is helpful for expanding the targets of deep-water hydrocarbon exploration and for evaluating geological risks of subsea engineering. Typical MTCs consist of three parts: the head tension area, body slip area, and toe compression area. During the transportation of MTCs, these three domains come into being successively. MTCs can be classified into three types: slide rock, slump rock, and debris rock, based on the sediment transport process and fluid properties, and they respectively correspond to elastic deformation, elastic and plastic deformation and plastic deformation stages. In 3D seismic plan, according to the direction of sediment flow (the slope direction), some depositional elements of MTCs, such as head cliffs, body folds, and toe squeeze ridges, can be clearly identified in proper order, and in seismic profiles MTCs are characterized by weak-medium amplitude reflections, mound-like shapes, and irregular top/bottom interfaces. In conventional log data, MTCs are commonly responded as abrupt shifts at their top/bottom interfaces. In outcrop data, MTCs have several kinds of sedimentary structures, such as massive textures, sliding folds, and mud lumps. Compared with turbidites, MTCs have many distinct characteristics. For example, MTCs are deposited from sediments transported through a "frozen" manner and have an "elongate-fan-like" slope morphology, a relative large thickness. Furthermore, MTCs can be characterized by no obvious features of grading differentiation and relatively large stratigraphic dips.

Keywords: Deep-water mass-transport complexes ; Sedimentary structure ; Genetic classification ; Identification marks

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本文引用格式

秦磊, 毛金昕, 倪凤玲, 徐少华, 李小刚, 蔡长娥, 尚文亮, 刘家恺. 浅谈深水块体搬运复合体的结构、成因分类以及识别方法. 地球科学进展[J], 2020, 35(6): 632-642 DOI:10.11867/j.issn.1001-8166.2020.047

Qin Lei, Mao Jinxin, Ni Fengling, Xu Shaohua, Li Xiaogang, Cai Change, Shang Wenliang, Liu Jiakai. A Brief Introduction to Deep-Water Mass-Transport Complexes: Structures, Genetic Classifications and Identification Methods. Advances in Earth Science[J], 2020, 35(6): 632-642 DOI:10.11867/j.issn.1001-8166.2020.047

1 引 言

深水环境下沉积类型是多样的,包括远洋披覆沉积或湖相细粒沉积、等深流沉积、浊流沉积以及块体搬运沉积等。作为块体搬运沉积(Mass-Transport Deposits,MTDs)的多期次复合体——“块状搬运复合体”(Mass-Transport Complexes,MTCs),是大陆斜坡、峡谷或水道、隆起构造翼部以及水道侧壁等附近的失稳地质体在重力作用下,以整体“冻结”的方式顺坡搬运而形成的多期沉积体,其广泛分布于海洋(大陆边缘)和陆相湖盆(半深湖—深湖)中[1,2,3,4,5,6,7,8]。作为深水地层的重要组成单元,MTCs通常占据地层层序的50%以上,具有重要的油气勘探意义和研究价值[9,10,11]。1999年由美国等国家发起的全球“源—汇”研究计划中,MTCs被列为重点研究对象;中国科学院以及美国华盛顿大学等高校纷纷成立MTCs的研究团队[12],有关MTCs沉积方面的研究得到国际上众多国家和高校的普遍关注。目前,随着地球物理勘探手段的不断创新发展以及综合大洋钻探计划(Integrated Ocean Drilling Program,IODP)、深海钻探计划(Deep Sea Drilling Program,DSDP)的实施,已在北海、墨西哥湾、尼日尼亚和印度东部海域等地区发现了与MTCs相关的油气田[13,14,15]。同时,MTCs具有巨大的破坏性,大规模发生的MTCs不仅对海底隧道、海底输油管道和钻井平台等构成潜在的破坏,而且还可以产生海啸,极大地威胁到沿海人民的生命财产安全[16]。因此,深入研究MTCs对拓展深水油气勘探领域和评价地质风险具有重要的应用价值。

由于深水环境下MTCs的搬运和分布极少能被直接观测,观测手段有赖于地球物理技术的发展。有效表征MTCs的分类、结构以及成因仍然困难,MTCs的研究目前依旧欠缺。体现在:MTCs类型的划分不清楚;MTCs的识别依据不够明确,常与浊积岩混淆;MTCs的储盖特征不清楚。本文基于上述存在问题,通过梳理国内外的研究成果,重点阐述MTCs的分类、结构及识别方法,讨论MTCs与浊积岩的区别以及对深水油气勘探研究的意义。

2 MTCs沉积单元的构成

MTCs沉积是持续的重力流活动,根据沉积物顺坡流动过程和不同阶段的沉积物特征来划分MTCs结构,即典型的MTCs可以划分为头部拉张区、体部滑移区和趾部挤压区3个结构单元(图1[17]

图1

图1   MTCs沉积模式图及其对应岩性特征

(a)MTCs沉积模式图[17];(b)~(d)MTCs岩心照片,显示MTCs不同沉积部位的岩性特征[12,13]

Fig.1   The sedimentary model and litholoyical characteristics of MTCs

(a)A sedimentary model of MTCs[17]; (b)~(d)The core photographs of MTCs, displaying lithological characteristics of different sedimentary parts[12,13]


2.1 头部拉张区域

MTCs的头部拉张区是弹性应力释放区域,属于地质薄弱带。在外界触发机制的作用下,斜坡地质体失稳形成断块体,并沿断裂面、主滑移面或次滑移面向下滑移,形成一系列的滑动块体或旋转块体[16],同时形成陡崖(后壁)和侧壁等海底地貌单元,并与犁式正断层伴生(图1[17]。头部拉张区的后壁坡度陡,面积大,平面上呈“新月”形;侧壁较缓,面积小,终止于顺坡方向;犁式正断层呈阶梯状分布。该区域的MTCs的侵蚀能力弱,整体以滑动搬运为主,内部沉积结构极少变形(表1)。

表1   MTCs不同沉积阶段的特征

Table 1  The features of different deposition stages of MTCs

部位力学性质流体性质构造特征平面形态平面展布侵蚀能力变形结构坡度
头部弹力释放滑动后壁、侧壁、犁式正断层新月形不发育
体部重力卸载滑塌滑塌褶皱、剪切槽长条形中等发育较陡
趾部挤压应力汇聚碎屑流挤压脊、逆冲断层似扇形较弱发育

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2.2 体部滑移区域

体部滑移区域是MTCs的主体发育区,MTCs体部属于重力卸载集中区域。失稳沉积物在自身重力作用下沿基底剪切面继续向下坡方向搬运,形成一系列以滑塌作用为主的沉积体[18],易发育剪切槽和滑塌褶皱等构造单元(图1[17]。其中剪切槽倾向与陆坡走向近似直交;滑塌褶皱顺坡方向分布。该区域的MTCs平面上呈长条形分布,以滑塌搬运为主,对周围地层的侵蚀作用强,内部通常夹杂大量不等粒级的沉积物,变形结构发育(常见地层弯曲变形)。地形坡度较头部拉张区域明显变缓,沉积厚度增大(表1)。

2.3 趾部挤压区域

MTCs趾部是挤压应力的主要集中区域。受海底古隆起或其他正向地貌的控制,顺坡搬运的沉积物以连续弧形向下坡方向扩展,形成多期推挤和叠置的沉积体(常见叠合面,图1[17]。一般发育挤压脊和叠瓦状逆冲断层,呈平行或亚平行分布,并近似平行陆坡走向,趾部挤压区前端可见倒梯形状的底部侵蚀擦痕和外逸块体[19]。该区域的MTCs以碎屑流沉积为主,平面上呈似扇形,侵蚀能力弱,但内部变形结构发育,顶部通常漂浮大量泥质碎屑(表1)。地形坡度相对平缓,碎屑流最终向深水中心区消亡或转换成浊流继续搬运[20]

3 MTCs的触发机制

MTCs的形成需要一定的触发机制,通常包括沉积物的堆积失稳、较大的地形坡度、海平面下降和天然气水合物的分解等。例如,渤海湾盆地东营凹陷胜坨地区断陷湖盆坡折带MTCs的触发机制主要是边界活动断层[21];尼日尔三角洲盆地陆坡逆冲构造区MTCs的形成主要受陆坡坡度的影响[22];东非鲁武马盆地渐新统MTCs的触发机制主要是海平面变化[23];南海北部陆坡MTCs的触发机制主要是天然气水合物的分解[16]。所以,不同沉积背景下MTCs的主要触发机制并不相同,主控因素因地而异。

4 MTCs的分类

目前,针对MTCs的分类已有多名学者进行过研究,分类方式概括来说包括:按颗粒支撑机制、外部形态、物源位置、流体力学性质和沉积物浓度等5种方式[24,25,26,27,28,29]表2)。Middleton等[24]基于沉积物颗粒支撑机制,将MTCs划分为3类,即分散压力支撑的颗粒流、基质强度支撑的碎屑流和逃逸流体支撑的液化沉积物流。这种分类由于专业性太强,不便于推广。潘树新[25]依据MTCs的外部形态,将其划分为扇形体系和非扇形体系;Moscardelli等[26]根据MTCs的触发机制和形态规模,把MTCs从物源位置上分为附属型和非附属型两种。Nemec[27]基于MTCs的搬运过程及流体力学性质,将MTCs分为蠕滑、滑动、滑塌、塑性流。Shanmugam[28]和Posamentier等[29]在系统总结Nemec[27]研究成果的基础上,依据MTCs搬运过程的流体力学性质和不同阶段的沉积浓度,将MTCs分为滑动、滑塌和碎屑流。

表2   MTCs的分类

Table 2  The classifications of MTCs

分类标准分类结果参考文献
颗粒支撑机制颗粒流、碎屑流、液化沉积物流[24]
外部形态扇形体系、非扇形体系[25]
物源位置附属型、非附属型[26]
搬运过程及流体力学性质蠕滑、滑动、滑塌、塑性流[27]
流体力学性质和沉积物浓度滑动、滑塌、碎屑流[28,29]

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本文更倾向于采用Shanmugam[28]和Posamentier等[29]的分类方式(图2),因为蠕滑是一种极其缓慢的搬运机制,蠕滑地质体每年移动仅数毫米至数厘米,这种搬运过程在深水环境中很难被观测到。此外,Silva等[30]通过试验分析认为,当深水环境中斜坡坡度大于20°时,地质体通常会发生蠕滑作用。但吴嘉鹏等[31]的统计表明,目前全球已发现的绝大部分MTCs沉积坡度普遍较小,平均地形坡度仅在1.5°左右。较小的地形坡度难以促使MTCs发生蠕滑作用。Shanmugam等[28]和Posamentier等[29]的分类基本遵循了Nemec[27]对MTCs不同沉积部位的流体力学性质的原始定义,即认为按不同阶段的流体力学特征,MTCs的搬运过程可被分为:弹性形变(滑动)、弹性和塑性形变(滑塌)及塑性形变(碎屑流)3个阶段。由此可见,塑性流包含了滑塌和碎屑流两种不同力学性质的流体。因此,Shanmugam等[28]基于MTCs不同阶段沉积物浓度的差异,认为当塑性形变阶段的沉积物浓度等于100%时,MTCs的流体性质为滑塌,否则为碎屑流。

图2

图2   基于搬运过程和流体性质的MTCs分类(据参考文献[28,29]修改)

Fig.2   The classifications of MTCs based on transport process and fluid properties(modified after references2829])


5 MTCs的识别方法

基于MTCs沉积物的搬运过程、不同阶段流体的力学性质和浓度,将其分为滑动、滑塌和碎屑流3种,经历后期的成岩作用分别形成滑动岩、滑塌岩和碎屑岩。由于深水区钻井成本高,取心资料少,目前地震资料是作为识别MTCs最主要的方法,露头资料和测井等资料是重要的补充[14,15,32]

5.1 三维地震资料识别

(1)平面上,不同沉积背景下MTCs的规模差异较大[12]。通常MTCs的平面形态表现为顺坡方向呈“长条形—似扇形”的组合特征。此外,MTCs在三维地震平面图上具有清晰的地貌特征(图3a)[33]:可依次划分出头部拉张区、体部滑移区和趾部挤压区,其中陡崖、滑塌褶皱以及亚平行、平行排列的挤压脊依次发育。这明显区别于浊积岩在地震属性图上可识别的沉积物波和旋回坎等地貌特征(图3b)[34]

图3

图3   MTCs的地震资料识别特征

(a)巴西东南部圣埃斯皮里图盆地MTCs三维地震平面图,MTCs以头部陡崖、体部滑塌褶皱和趾部挤压脊为特征[33];(b)某典型峡谷水深及波列分布图,非对称的浊流波状底形向上游迁移[34];(c)摩洛哥海域下古新统地震剖面,内部见保存完好的一个单独滑动块体[35];(d)特立尼达海域上更新统MTCs地震剖面图,图中出现一些挤压逆冲,指示了MTCs内部的局部挤压,垂直比例放大约10倍[36]

Fig.3   The recognition characteristics of seismic data of MTCs

(a)The three-dimensional seismic plan of MTCs in Espirito Santo Basin on the SE Brazilian margin, characterized by head cliffs, body folds, and toe squeeze ridges[33]; (b)Bathymetric image and cyclic steps along typical canyon, characterized by unsymmetrical turbidity wave bottom shape migrates upstream[34]; (c)The seismic profile of the Lower Paleozoic in the Moroccan waters, with a single well-preserved sliding block inside[35]; (d)Seismic profile across an uppermost Pleistocene MTCs, offshore Trinidad, several thrust slides are present, indicating local contraction within the MTCs, vertical exaggeration is about 10∶1[36]


(2)地震剖面上,MTCs多表现为丘形—长条形特征,其内部地震反射特征差异大,一般来说至少可见3种不同反射类型:滑动块体或旋转块体的反射特征。这类块体常见于MTCs的头部拉张区域,整体呈长条形块体,保存了原始地层的基本形态。内部呈规则的平行、亚平行短轴状弱振幅反射特征,底部可见侵蚀面,四周为杂乱反射(图3c)[35]逆冲块体或推覆体的反射特征。常发生在顺坡搬运过程中的地貌高点处,保存了部分原始地层,剖面上可见多个似叠瓦状分布的逆冲断层和挤压脊(图3d)[36]杂乱反射。可以发生在MTCs内部任何一个地方,但在趾部挤压区域更容易出现,反映了沉积物分布的不连续性和性质的差异性(图3d)[36]

5.2 MTCs常规测井资料识别

单一的MTD在常规测井曲线响应上以漏斗形为主要特征,顶底界面均表现为突变的接触关系,曲线多呈齿形—弱齿形(图4a)[37]。需要强调的是,在利用测井资料对MTCs进行评价时,应采用井—震联合的方法,在地震剖面相带解释的宏观控制下,对MTCs顶底界面进行常规测井曲线的识别,并研究其内部样式和垂向叠置关系。地层倾角测井方面,MTCs表现出比浊积岩更高的度数(图4b)[38]。常规测井资料与特殊测井方法的综合利用,是识别MTCs的有效方法。

图4

图4   MTCs的测井资料识别特征

(a)亚马孙扇的ODP155航次933A站位块体搬运沉积的自然伽马、电阻率、速度、密度和孔隙度曲线[37];(b)亚马孙扇ODP155航次933A站位的地层倾角方位图[38]

Fig.4   The identification characteristics of logging data

(a)The natural gamma, resistivity, velocity, density, and porosity curves of the Amazon Fan's ODP155 voyage 933A block transport deposit[37]; (b)The azimuth map of the Amazon fan ODP155 voyage 933A[38]


5.3 MTCs露头资料识别

MTCs的滑动、滑塌和碎屑流在露头上能够区别。其中滑动岩沉积物以整体固结搬运的形式进行沉积,搬运距离短,沉积粒序特征与原位沉积体有关。滑动岩具块状构造,内部岩石发生破裂,顶部被后期河道沉积物覆盖,内部偶尔也可见外来侵入体(图5a)[39]。滑塌岩主要由褶皱地层和变形结构组成(图5b),地层横向延伸范围多为50~1 000 m,垂向沉积厚度差异大,粒序分异特征不明显[40]。通常其顶部不平整表面被深水细粒沉积、水道、漫滩或席状砂所覆盖,充填不同时期的沉积物。碎屑岩主要沉积物的粒度变化范围大,常见比重、粒度、硬度和形状相差较大的石英颗粒和泥质撕裂屑等碎屑颗粒混杂分布在泥岩或砂岩中[41]。其中薄—中层、分选性较好的块状砂岩是砂质碎屑岩底部的重要组成部分,顶部漂浮有大量碎屑颗粒,表现出逆粒序的沉积特征(图5c)[42]

图5

图5   MTCs露头资料识别特征

(a)西爱尔兰上石炭统Ross海底,滑动体上覆平坦的水道充填[39];(b)西班牙南部Tabernas盆地上中新统,滑塌岩地层发生一系列褶皱变形且被倾角很缓的水道充填[40];(c)西班牙东北部始新世大陆斜坡地层中的碎屑岩[42]

Fig.5   The outcrop identification features of MTCs

(a)The sliding body is filled with a flat channel in the upper Carboniferous Ross seafloor of Western Ireland[39]; (b)A series of folds and deformations occurred in the slump rock formation and were filled with water channels with very low dips in the Upper Miocene of the Tabernas Basin[40]; (c)Phototgraph of floating clasts in a thin debrite, Eoceneslope strata of Sant Llorenc del Munt, northeastern Spain[42]


6 讨 论

6.1 MTCs与浊积岩的区别

深水重力流通常包括块体流和浊流两种,其中块体流形成了MTCs,即滑动岩、滑塌岩和碎屑岩,浊流形成了浊积岩。关于MTCs与浊积岩的区别一直以来都是深水重力流沉积研究的难点。本文通过梳理前人研究成果,将两者的主要识别标志列入表3

表3   MTCs与浊积岩识别标志(据参考文献[6]修改)

Table 3  The identification marks of MTCs and turbidite (modified after reference[6])

搬运

方式

岩石

类型

形成过程

示意图

沉积物

浓度/%

力学、流

变学特征

识别标志
沉积特征粒度分析地震特征地震剖面

100弹性形变无明显形变,伴随强烈旋转C-M图具多段式的分布特征弱—中振幅、杂乱反射;不规则顶界面,底部侵蚀发育

弹性形变

塑性形变

同沉积变形非常发育

>25塑性形变块状砂岩为主,见泥岩漂砾

<23黏性流变发育正粒序层理平行C-M基线中—强振幅、短轴状反射轴,底部侵蚀较发育

注:块体流浓度据参考文献[28]修改;浊流浓度据参考文献[43]修改;MTCs地震剖面据参考文献[35]修改;浊积岩地震剖面据参考文献[44] 修改

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基于上述MTCs与浊积岩的差异特征,两者在地震、测井和露头等资料表现出截然不同的识别特征。地震属性图上,浊积岩的平面形态一般呈水道—朵体的组合形态特征,常见于盆底环境中。剖面上以中—强振幅反射和透镜状外形为特征,连续性优于MTCs,杂乱反射少见。测井上,单一的浊积岩在常规测井曲线响应上以钟形,底界面突变接触和顶界面渐变接触为特征[45]。露头上,浊积岩成层性好,沉积厚度较薄,通常无法保存原始地层的基本形态,常见平行层理、波状层理和水平层理等沉积构造。其主要沉积物粒度变化范围小,垂向上具有明显的正粒序沉积特征[46]

浊积岩是一种由湍流支撑的具黏性流变学的浊流沉积而成,以机械分异沉降为主,单层厚度小(数厘米至数米),泥岩含量相对较高。MTCs是一种由杂基强度、分散压力和浮力支撑的具有弹性和塑性流变学特征的块体流沉积而成,整体以“块状”的形式沿基底剪切面进行搬运,具有单层厚度大(数米至数千米),砂岩含量高的特点。块体流在顺坡搬运过程中,往往可以逐渐转换为浊流[47]。块体流向浊流转换是加水稀释的过程,体积浓度降低,碎屑含量减少,动能不断减小,故此时形成的MTCs侵蚀能力大于浊积岩。而对于浊流和块体流不具备成因转换的情况,两者都具有一定的侵蚀能力,以底部残留的剪切槽或侵蚀擦痕等结构形态规模来判断两者的侵蚀能力大小,剪切槽或侵蚀擦痕规模越大或下切越深对应的MTCs或浊积岩侵蚀能力越强。

浊积岩与MTCs的沉积研究先后经历了2个发展阶段[48]:第一阶段是基于浊流理论提出的鲍马序列和深水扇沉积模式;第二阶段是基于砂质碎屑流理论提出的MTCs斜坡沉积模式。Bouma[49]认为完整沉积的一套浊积岩可划分出Ta~Te 5个小层,并且粒度向上逐渐变细。其中最底部的Ta层发育正粒序或块状构造。Shanmugam等[50,51,52]认为Ta层出现的块状构造是塑性流或层流的产物,应属于MTCs而不是浊积岩。

6.2 MTCs的石油地质意义

深水油气勘探实践表明,MTCs可以构成油气的盖层和储层,甚至在特殊条件下能够作为油气的烃源岩。例如李磊等[53]通过对南海北部白云凹陷21 Ma重力流沉积物的研究,发现该区域的MTCs对下伏地层起着顶部封闭和侧向封堵的作用,MTCs扮演着盖层的角色(图6a)[54]。但MTCs作为储层的情况更为常见[55]。比如,北海维京地堑北部Statfijord油田[56]就是以MTCs为储层,储层物性较好地保留了供源区中侏罗统河流相砂岩的孔隙特征(图6b)[57]。以上2个实例体现了MTCs既可以作为盖层,又可以作为储层?这似乎是一个矛盾的结论。事实上,MTCs物性的好坏完全取决于母源区的初始沉积物结构,即MTCs是泥质岩滑塌的结果,此时MTCs的孔渗性差,多为盖层。反之,MTCs是砂质岩滑塌的结果,MTCs的孔渗性通常较高。当然,后期的成岩作用也将影响MTCs的物性条件。成岩作用也是沉积后期重要的影响因素之一。至于MTCs可否作为烃源岩,目前尚无有关报道。仅Crevello等[58]通过对婆罗洲的露头研究得出当地MTCs的有机质含量很高,但层内的烃类是否能够生成和运移还有待证明。

图6

图6   MTCs与周围地层的储盖关系模式图

(a)富含泥质MTCs的深水地层圈闭模式图[54];(b)富含砂质MTCs的深水构造圈闭模式图[57]

Fig.6   The modeles of reservoir cap relationship between MTCs and surrounding stratas

(a)A deep-water stratigraphic trap model map of muddy MTCs[54]; (b)A deep-water structure trap model diagram of sandy MTCs[57]


7 结 论

典型的MTCs由3个结构单元组成,即头部拉张区域、体部滑移区域和趾部挤压区域。其中头部属于弹性应力释放区域,以滑动沉积为主,体部属于重力卸载区域,以滑塌沉积为主,趾部属于挤压应力汇聚区域,以碎屑流沉积为主。三者在顺坡搬运过程中,存在由头部向趾部依次演变的关系。

MTCs的分类方案众多,其中以搬运过程为划分依据的分类得到更加广泛的运用,即典型的MTCs包括滑动岩、滑塌岩和碎屑岩3类,分别对应搬运过程的弹性形变、弹性和塑性形变以及塑性形变阶段。

在三维地震平面图上,MTCs通常可被识别出头部陡崖、体部滑塌褶皱和趾部挤压脊等构造单元,剖面上以弱—中振幅反射和丘状—长条状外形为特征,具有不规则的顶底界面;测井上,MTCs具有顶底界面突变的曲线响应特征;地层倾角测井上,MTCs具有较大的地形坡度值;露头上,MTCs通常具有一种或多种沉积构造,比如块状构造、滑塌褶皱和泥质撕裂屑等。

MTCs与浊积岩的主要区别是:MTCs整体以“冻结式”搬运沉积,平面上具长条形—似扇形的斜坡组合形态,发育不规则顶底界面,垂向上沉积厚度大,地层倾角相对后者较大;浊积岩以紊流的形式进行悬浮搬运沉积,具有水道—朵体的盆底组合形态,发育底部侵蚀面,垂向沉积厚度小,具有正粒序沉积特征。

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