Sedimentary and evolutionary characteristics of dereliction deltas in Chang 6 – Chang 2 oily members, Zhidan region, northern Shaanxi

Abstract: The study using well logging data and the sedimentological method reveals that large deltas and lake systems developed time and again in the Chang 6 –Chang 2 depositional stage in the Zhidan region, and a good intraplot source-reservoir-cap assemblage and widespread oil-bearing series are formed. In sub-south-north cross-section, the Chang 6–Chang 2 sedimentary association shows that deltaic deposits progressively overlap lake deposits from the bottom to the top and from the north to the south, forming upward coarsening dereliction depositional sequences; sedimentary facies belts moved continually toward the lake center from north to south, deltafront subfacies spread extensively and overlapped in multiphases, forming the main intraplot depositional body. Deltafront subfacies could be subdivided into underwater distributaries plain and anterior margin river mouth subfacies, and the main sand bodies are river mouth sand bank and distributaries channel sand body. The Chang 6 stage's anterior margin river mouth subfacies in the region were well-developed. The Chang 2 stage's underwater distributaries plain subfacies extended and developed rapidly, and deltaic plain subfacies began to come into the region. Key words: Zhidan region, Mesozoic, Chang 6–Chang 2 oily layer members, dereliction type delta, sedimentary and evolutionary characteristic