Sedimentary characteristics and favorable oilbearing facies belts analysis of nearshore subaqueous fan of Nantun formation in Tanan depression

Key words:  Tanna depression  Nantun formation  nearshore subaqueous fan  sedimentary characteristics  characteristics of distribution and evolution  favorable oilbearing facies belts

Abstract:

The sedimentary characteristics, distribution, evolution and favorable oilbearing facies belts of nearshore subaqueous fan of Nantun formation in Tanan depression were studied through the integrated utilization of data of seismology, welllogging and core. The results show that three kinds of subfacies and four kinds of microfacies can be identified in nearshore subaqueous fan, and the different facies belts show obvious distinctions in sedimentary characteristics. The development of nearshore subaqueous fan concentrated at the downthrown side of the basin controlling fault in the east part of the depression. The overlaying of multistage nearshore subaqueous fan sediments formed two fan aprons which are characterized by "southeast and northeast each contiguous, flourish at first while decline at last" at the southeast and northeast part of the area. Owning to episodic activities of the basin controlling fault and the distinctions between the sources from southeast and northeast respectively, the southeast fan aprons are a little earlier than the northeast ones in the time of the formation and the decline of the fan aprons. The physical property of braided channels of middle fan subfacies of nearshore subaqueous fan is the best, and the reservoirs with lowmiddle permeability developed. The reservoir porosity of different places and burial depths shows a great discrepancy, therefore it is hard that the reservoirs formed a large joint area and highquality ones. The study area mainly developed lithologic hydrocarbon reservoirs and structurallithologic composite hydrocarbon reservoirs whose individual scale is small but quantities are numerous.