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.