异重流沉积研究进展与展望

Advances and Perspectives in the Study of Hyperpycnal Flow Deposition

[意义]异重流(hyperpycnal flow)是一种源自洪水期河口、受重力驱动而沿沉积盆地底部流动、以准稳态存在的高密度沉积物重力流。异重流沉积研究对于地质科学进步、地质灾害防治和油气勘探开发等具有重要的理论和现实意义,已成为地球科学与工程领域的研究热点之一。[进展与结论]通过对国内外相关研究进展的综合分析,结合国内湖相异重流沉积典型实例研究,认为:(1)异重流的形成需要两种流体之间有足够的密度差、蓄水水体有足够的深度和突发的洪水事件3个基本条件,就输入流体密度条件而言,海相(或咸水)环境要远大于陆相(或淡水)环境;(2)异重流的形成主要受控于构造、气候、地形、物源供给等因素的综合作用,活跃的构造、湿润的气候、陡峭的地形和充足的碎屑物质供给有利于异重流的发生;(3)在不同沉积位置和沉积背景条件下,异重岩(hyperpycnite)的沉积特征不尽相同,故难以用单一的标志来判识不同条件下沉积的异重岩;(4)在靠近(分流)河口近端(扇根),异重流沟道可与水下分流河道相连,以主水道的侵蚀、填充作用为主,形成下切的主水道砂砾岩体,以侵蚀面+滞留沉积+正粒序砂岩为主要鉴别特征,可含动物及植物茎干化石;(5)在中部沟—扇过渡地区以及扇缘地区,可依次形成分支沟道、水下天然堤、沟道侧缘、朵叶体等沉积单元,成对出现的逆粒序—正粒序组合、层内微侵蚀界面和成层分布的植物碎片(炭屑)可以作为扇中至扇缘地区异重岩的重要判识标志;(6)异重流对沉积物的运载方式有悬浮载荷和底床载荷,沟道充填沉积物可由悬浮载荷和底床载荷共同搬运,水下天然堤、朵叶体沉积物则以悬浮载荷为主要搬运方式。[展望]展望未来,异重流沉积研究可在以下方面取得长足发展:(1)异重流流体性质转换、流体对沉积物的搬运—沉淀机理等理论基础方面不断完善;(2)古代异重流沉积环境分析、沉积特征识别、沉积微相(要素)分析、沉积构型解剖等方面趋向深入;(3)陆相湖盆环境异重流沉积特色研究更加清晰;(4)有关异重岩的成岩作用研究成为储层沉积学的新方向;(5)异重岩储层预测与非常规油气地质意义等研究不断与生产实际紧密结合,异重岩储层将不断被发现;(6)针对异重流沉积的水槽实验、水下观测、数值模拟等研究方法的进步将促使相关研究不断取得突破。

[Significance]Hyperpycnal flow is a type of high-density sediment gravity flow originating from an estu-ary during a flood period,flowing along the bottom of the sedimentary basin in a quasi-steady state.The study of hy-perpycnal flow deposition has theoretical and practical significance for the progress of geological science,disaster prevention,and oil and gas exploration and development,and it is currently one focus of Earth science research and engineering.[Progress and Conclusions]A comprehensive analysis of progress in research in China and globally,combined with typical examples of hyperpycnal flow deposition in China,has revealed several aspects.(1)The for-mation of hyperpycnal flow requires three basic conditions:sufficient density difference between the two fluids,suffi-cient water depth,and sudden flood events.In terms of input fluid density conditions,a marine(or brackish)envi-ronment far exceeds that of a terrestrial(or fresh water)environment;(2)Hyperpycnal flow is mainly results from the combined effect of structure,climate,terrain and material supply.Active structure,humid climate,steep terrain and sufficient debris for material supply are conducive to hyperpycnal flow;(3)The sedimentary characteristics of hyperpycnal flow deposits(hyperpycnites)differ with sedimentary position and sedimentary background condition,and the numerous conditions present a problem of identifying hyperpycnites by a single symbol;(4)At the proximal end(fan root)near the(distributary)estuary,the hyperpycnal flow channel is connected to the underwater distribu-tary channel.The main identification features are the erosion and filling of the main channel.The erosion surface+lag deposition+normally graded sandstone may also contain animal and plant-stem fossils;(5)Branch channels,un-derwater natural dikes,channel lateral edges,leaf deposition units and/or pairs of inverse-to-normal-grading combi-nations may be present in the central ditch-fan transition and fan edge areas.Intralayer erosion interfaces and layered distribution of plant debris(carbon chips)are important identifiers in middle-fan to fan-edge areas;(6)The sedi-ment may contain the bed load in addition to the suspended load.The channel-filling sediment is carried jointly by the suspended load and the bed load;the suspended load is the main transport mode for the formation of underwater natu-ral dikes and leaf body sediments.[Prospects]Future studies of hyperpycnal flow deposition concentrating on the fol-lowing aspects will be beneficial.(1)Constant improvement in the theoretical basis of fluid property conversion,flu-id-sediment transport and the precipitation mechanism;(2)Detailed analysis of ancient hyperpycnal flow environ-ments,sedimentary feature identification,detailed sedimentary facies(elements)analysis and sedimentary architec-ture analysis,among other aspects;(3)The sedimentary characteristics of hyperpycnites in continental lake basins will be clarified;(4)The study of the diagenesis of hyperpycnites has become a new direction of reservoir sedimentol-ogy;(5)Predicting the presence of hyperpycnite reservoirs.The geological significance of unconventional oil and gas is closely combined with production practice,and more hyperpycnite reservoirs will be discovered;(6)Progress in the development of methods of researching hyperpycnal flow deposition(e.g.,tank experimentation,underwater ob-servation and numerical simulation)will continue to promote breakthroughs.