Reconstruction of Late Oligocene depositional systems in the Beikang Basin by seismic facies analysis

Traditional seismic facies analysis is usually carried out based on the features of topography (shelf, continental margin and slope), external morphology (dome-like, wedge-like, plate-like）、internal structure（parallel, oblique and radiate）and reflection waves (amplitude, frequency, and continuity). Lithofacies may be defined if there are enough drilling wells. However, in the areas only having few wells and complicated geological conditions, it is difficult to reach an appropriate conclusion of lithofacies without additional data support. In this paper, we will introduce three new marks to help making interpretation of seismic facies into lithofacies, namely, the smoothness of seismic reflection waves, the cleanliness of seismic reflection waves and special wave forms. Thus a new method for seismic interpretation from seismic facies to lithofacies to sedimentary facies is proposed, which is suitable for the reconstruction of sedimentary systems under any geological and data conditions and can effectively enhance the quality of lithofacies identification and sedimentary facies conversion, and it is of great significance to oil and gas exploration. Combining this method with the method for traditional seismic facies analysis, six typical seismic lithofacies are identified for the Late Oligocene of the Beikang Basin, namely, sandy conglomerate facies, sand-mud-encrusted facies, interbedded sand-mud facies, mud-encrusted facies, pure mud-shale facies and volcanic facies. Four kinds of depositional systems are recognized, namely, the fluvial delta, shelf shallow sea, slope turbidite fan and deep-sea basin depositional systems; Two large sediment sources from the Borneo region are recognized; The spatial distributions of shelf delta-continental slope turbidite, deep-sea basin depositional system and igneous body are clarified. The results have provided basic support for further exploration of oil and gas in the Beikang Basin.