Depositional Model of Prograding Delta Confluences:A case from Es₃^m Members in the Paleogene Dongying Sag

Key words: 
• delta confluences area /
• sedimentary system /
• depositional model /
• Paleogene Shahejie Formation /
• Dongying Sag

Abstract: As one of the most important sedimentary facies for development of clastic rocks, prograding deltas are critical to petroleum exploration. Hence, it has important significances to reservoir prediction to strengthen researches on depositional model of prograding delta confluences area. Confluences of Dongying delta and Yong'an delta took place in the middle depositional stages of the third section in the Paleogene Shahejie Formation (Es₃^m) in Dongying Lake, Bohai Bay Basin. However, it has not received adequate attentions to issues of confluences patterns, and reservoir predication about the two delta systems. Based on core observations, loggings and seismic data, this presentation analyzed sedimentary characteristics of confluences deltas of Es₃^m in Dongying sag, discussed sedimentary stages and confluences process, and constructed a depositional model for prograding confluences deltas, for sake of providing reference for further exploration deployment and geological research. Results show that lake and prograding delta deposits were well developed and reservoir sandbodies were mainly deposited in underwater distributary channels, mouth bar secondary and scattered turbidites in the Es₃^m in Dongying Sag. It is considered that sedimentary scope of Dongying delta were possibly over estimated and extent of Yong'an Delta were probably underestimated. Depositions of the Es₃^m in Dongying Sag can be divided into 9 stages and their processes are differentiated to part-confluence and full-confluence stages. Subaqueous deltas confluences are suitable place for water flow convergence and loading of sediments. Superimposed sandbodies are favorable reservoirs with crucial significances for petroleum explorations in delta confluences area.