Abstract:

It is difficult to divide deep water sequences in sequence stratigraphy study due to a lack of direct connection between the sequence and the shelf margin deposition.The Oligocene gas-bearing sandstones in Ruvuma Basin,East Africa,were deposited by gravity flow in deep water environment.A reasonable sequence stratigraphic division is helpful to the interpretation of favorable targets and regional oil and gas exploration.Under the guidance of the mixed sequence stratigraphic model,we put 3D seismic data into application and comprehensively analyzed multiple data to study the sequence stratigraphy,disclosing the genetic relationship between sedimentary processes and sedimentary systems in deep water environment.Sequence boundaries were identified by the strata termination, condensation section and MTD.Consequently,the corresponding relationship between sedimentary architecture element and system tract was established by analyzing the nature and scale of sediments and density difference of turbidity currents leading to deep water deposition,the Oligocene isochronous stratigraphy framework was established and the rules of deep water sedimentary evolution was analyzed.The Oligocene in Ruvuma Basin can be divided into two third-order sequences,for both of which large-scale MTDs accumulated locally on continental slopes near shelf break in early forced regression.But,the sedimentary characteristics of the two sequences show differences.The large lobes in the Lower Oligocene third-order sequence that are dominated by coarse-medium-coarse-grained sandstones and have large sand/shale ratio and high density are products of late forced regressions.The sediments of the leveed channel system were located in the lowstand system tract(LST)and early transgression, with the condensation section overlying the leveed channel system, all of which contribute to the complete system tract of the Lower Oligocene sequence.Due to the limited data obtained within the study area,large lobes are absent in the Upper Oligocene third-order sequence,within which there is a lack of late deposition in falling stage system tract(FSST).Besides,the sedimentary architecture elements in other system tracts are quite similar to those of the Lower Oligocene sequences.

Key words: sedimentary architecture element, system tract, sequence, deep water deposit, Oligocene, Ruvuma Basin, East Africa