COMPARING AND CONTRASTING A MEANDERING POINT BAR SEQUENCE AND BARRIER ISLAND SYSTEM WITHIN THE UPPER ARANG FORMATION, BELANAK FIELD, WEST NATUNA BASIN

摘要

Two distinct depositional systems are interpreted from the middle Miocene Upper Arang depositional systems in gas bearing reservoirs from the Belanak Field, Block B, Natuna Sea. The first is a meandering fluvial point bar system in the locally named Arang-7 sands, and the second is interpreted barrier island system in locally named Arang-8 sands. The geomorphic features of these depositional systems can be distinguished from high quality 3D seismic data and stratigraphic correlations from well data. The reservoir depositional models are used to construct a more robust predictive model for OGIP and infill well optimization. The Arang-7 seismic amplitude response displays a geomorphological character consistent with a point bar depositional system. A relationship is observed between net pay thickness and the magnitude of the full stack amplitude response providing a spatial trend for predicting net pay distribution outside of well control. The point bar depositional element exhibits a stack of fining upward log character cycles that correspond to an overall decrease in porosity from the basal units to the top of the depositional body related to grain-size controls from basal channel lags to overlying finer-grained bar fill. In contrast, the Arang-8 amplitude highlights geomorphological features interpreted as a barrier island system. It is noted that barrier island systems are rarely preserved in the geologic record hence attention is paid to presenting evidence to support the model. Seismic amplitudes in this depositional element demonstrate positive correlations with net pay and porosity thereby providing trend data to constrain predictive net pay and porosity distribution maps. As a consequence of these studies, the two reservoirs have been characterized as viable economic targets and drilling plans are in now place to produce these reservoirs.