深海沉积理论发展及其在油气勘探中的意义

A Review on Deep-Sea Sedimentation Theory:Significances to Oil-gas Exploration

深海沉积理论的进展主要涉及"鲍玛序列"、浊流、砂质碎屑流和深海层序地层四方面。以Shanmugam为代表认为:"鲍玛序列"并非唯一浊流产物,可含有砂质碎屑流、浊流、内潮汐、内波、等深流多种流体作用的结果;过去识别的"鲍玛序列"A段有浊流也有相当部分是砂质碎屑流成因,B—D段交错层理则是底层牵引流沉积产生的;只有浊流的沉积物才能称为浊积岩;"高密度浊流"是指砂质碎屑流而不是浊流;浊流是一种有牛顿流和紊乱状态的沉积物重力流;浊积岩没有复杂的颗粒悬浮层和碎石浮层,不发育逆粒序。Shanmugam等关于"鲍玛序列"这一新解是深海沉积学理论的一个进步。深海牵引流过去数十年取得了较大进步,但理论与实践脱节。深海层序地层是深海沉积理论进展的另一个方面,层序界面类型、体系域沉积有自身的独特性:层序界面至少存在斜坡侵蚀面、低水位下超面和水下沉积间断面三种;当沉积背景以陆源碎屑为主时,LST主要为盆底扇,TST和早期HST表现为非钙质远洋沉积,晚期HST一般不发育;当碳酸盐沉积为沉积背景时,LST主体为碎屑流和跨塌蹦落沉积或淡水透镜体,TST和早期HST为钙质细粒沉积,晚期HST可能存在有较小规模的钙屑海底扇。建议慎重解释"鲍玛序列",审视"浊积扇"理论,废弃"浊积扇"概念,加强深海牵引流沉积机理方面的研究;LST浊积砂体、砂质碎屑流形成的块状不规则砂体、深海牵引流砂体可在深海储层预测方面具有巨大潜势,深海沉积作用及其过程的精细研究在指导深海油气勘探方面将会发挥越来越为重要的作用。

It has been challenged for the sedimentological theories, such as ＂Bouma Sequence＂ and ＂turbidite fan＂, in the direction of deep-sea oil-gas exploration. This is because, as Shanmugam et al. say, the ＂Bouma Sequence＂ is not a unique product of turbidity sedimentation, but results from cooperation of several sedimentation, i. e. , sandy debris flow, turbidity current, internal tide and wave, contour current. Following the interpretation by Shanmugam et al. , it is summarized as ： sediments of the ＂Bouma Sequence＂ section A could be caused by sandy debris flow besides of turbidity current, those with cross-beddings within the sections B-D be traction current; only those by turbidity current could be called turbidite; so-called ＂high density turbidity＂ is not real turbidity but sandy debris flow; turbidity current is Newton current and turbulent sediment gravity flow; there are no suspended layer and reverse grading within turbidite. It lacks linkage for deep-sea traction current between theory and practice although advances have been made. And as an aspect of sedimentation, deep-sea sequence stratigraphy is to some degree different from that in shallow sea： there are at least three types of sequence boundaries slope erosional surface, downlap surface of lowstand systems tracts （LST）, and submarine sediment hiatus surface; when it is dominated by a terrigenous depositional setting, LSTs would be composed of submarine fan sediments, transgressive systems tracts （TSTs） and early highstand systems tracts （HSTs） are probably of non-calcareous pelagic sediments, and few HSTs are found ; when it becomes carbonate setting, LSTs are dominant of slump and debris sediments, TSTs and early HSTs could spread on submarine fan, and late HSTs could be consisted of calciturbidites. Support remarks and suggestions are proposed by review and experienced works of deep-sea sedimentation ： it would be better to invert comma ＂Bouma Sequence＂ to make a mark showing a note of not-unique causal turbidite result ; it is necessary to use the term ＂submarine fan＂ and/or ″deep-sea fan instead of ＂turbidite fan＂ ; study of both theoretic deep-sea traction currents （e. g. internal tide and wave, contourite） and practical field needs to promote in future. It is great potential and prosperous for oil-gas exploration in deep-sea sediment, especially for reservoir prediction of LST turbidite, sandy debris and bottom traction sandy bodies. However, to get a new burst in deep-sea oil-gas exploration, a key is that more precise and detailed study on theory of deep-sea sedimentation and depositional procession would be engaged in future.