三角洲沉积模式存在的问题与讨论

三角洲是一类非常重要的沉积相,是油气聚集的有利场所。准确可靠的三角洲沉积模式,对指导油气的勘探和开发有重要意义。关于教科书中经典的三角洲沉积模式和一些相关概念,很少有人质疑过。但研究发现,该模式及一些相关概念存在许多问题,诸如对于三角洲平原、三角洲前缘和前三角洲等亚相的定义含糊不清,各亚相之间缺乏明确的、具可操作性的界限。由于海(湖)平面是波动的,模式中未明确三角洲平原与三角洲前缘究竟以哪种海(湖)平面为界。三角洲平原与正常河流冲积平原的边界、三角洲前缘与前三角洲的边界、前三角洲与正常海(湖)的边界均未明确,也没有系统总结它们各自的识别、区分标志,致使不同的研究者根据自己的理解对古代同一地层可能得出不同的结论。三角洲有多种类型,且不同类型的三角洲沉积特征不同,应有各自的沉积模式,但许多研究者将曲流河三角洲沉积模式用于所有三角洲亚相、微相划分。经典沉积模式中,三角洲前缘发育水下分支河道和水下天然堤,但笔者等研究表明这2个微相不存在。而三角洲平原上普遍发育的砂体——汊口滩与并口滩,在经典三角洲沉积模式中未提及。作者针对三角洲沉积模式存在的各种问题,进行了深入讨论,并结合现代沉积,系统总结了能用于古代沉积不同亚相和微相的识别、区分标志。

Problems and discussions about delta depositional models

Deltas are an important type of sedimentary facies,and are also good reservoirs for oil and gas. A reliable depositional model of deltas is an essential guide for oil and gas exploration and development. However,few people have ever queried the classical delta model and relevant concepts in textbooks. But through our study,it is found that there exist many problems about the depositional model and relevant concepts. For example,definitions of delta plain,delta front and prodelta are vague,while the boundaries between them are not well defined and hard to determine. The boundary between delta plain and delta front is sea(or lake)level,but the level fluctuates frequently due to tide(or alternating raining and drying seasons),so which level should be the boundary？Similarly,the boundaries between delta plain and fluvial plain,delta front and prodelta,as well as prodelta and normal sea(lake)are also inexplicit. Their characteristics and distinctions have not been systematically summarized. Thus different researchers may give different interpretations as to sedimentary environments for the same strata. There are various types of deltas,and each type should have its own depositional model. However,many researchers have tried to apply the meandering delta model to all deltas. In the classical model,delta front develops subaqueous distributary channels and subaqueous levees,but this is proved to be wrong by the authors work in this paper. On delta plain, some new types of sandbodies,branch-mouth bars and branch-coalesce bars are found,but they are not mentioned in the classical delta model. In this paper,the problems within delta models are discussed in depth,and especially the distinction characteristics for subfacies and microfacies of deltas are summarized on the basis of modern deltas study.