Depositional Features and Controls on Gravity Flow Channel of Dongying Formation of Paleogene System, Zhanhua Sag, Jiyang Depression

Key words: 
• Zhuangxi Oilfield

Abstract: Sandstones of the lower member of Dongying formation of paleogene system in Zhuangxi oilfield, Zhanhua sag, Jiyang depression are of stringbelt shaped extension, which are extremely similar to the geometry of sandstones deposited by meandering fluvial. Depositional genesis of the sandstone is one of the key basis for high resolution stratigraphic framework construction, reservoir properties distribution prediction and remainingoil redevelopment. Combined with regional geological setting, stratigraphic succession, petrological features, depositional features association, and sand body geometries of this member, it is considered that the sandstone of lower member of Dongying formation, which extends along the long axis of faulted basin, was deposited by slumpingtyped gravity flowturbidity current channel of long distance sediments source in the semideep to deep lacustrine environment with the confine of lacustrine bottom configuration. The sediment sources were from the fandeltas, sublacustrine fans in Chengdao area in the northern part. In terms of the depositonal regime of gravity flow channel, the turbidity channel was subdivided into three types of microfacies: central channel, channel margin, and subaqueous overflow. Sedimentary succession of individual central channel equals to members association of AB, ABD and AC of Bouma sequence. Reservoir rocks in the channel center are dominated by gravelly sandstone with normal graded bedding and hybrid sandstone with massive bedding, which are medium porosity and permeability. The external controls on the slumpingtyped turbidity current channel in study area is of the long term base level fall caused by volcanic eruption, episodic activity of faults controlled basin, the internal controls of the abundant sediments source, stepped faults perpendicular to the long axis of faulted basin, and negative lacustrine bottom configurations. A depositional model of tubidity current channel is established based on the depositional features and conditions of sediments.