Publications

The Walio Field, located in the Salawati Basin, has been producing oil since the early 1970s from its main reservoir of Miocene Kais carbonate. With more than three hundred wells drilled to date, the field has become the largest oil producer in the basin but is now experiencing continuous declines due to high water cut. The good quality rock type within the reservoir is believed to be the main oil contributor for over 43 years of production and the un-swept oil is believed to be retained within thinner good rock type positioned within thick baffle rock type. This enlightens a new development concept to map the thin good quality rock type within thick baffle rock type. The un-swept oil is within thick baffle with porosity that is relatively lower than the production zone. Therefore, acoustic impedance inversion analysis is carried out to support the current geological model. This paper discusses how this concept works in this field by stressing acoustic impedance mapping. The acoustic impedance inversion study was performed on the 3D seismic pre stack data. This inversion is applying a model-based method with full bandwidth of the frequency contained in the seismic data. The acoustic impedance is correlate-able with the rock property, in this case, porosity. This is indicated by the moderate-to-good correlation between log acoustic impedance and porosity. Acoustic impedance volume interpretation was performed by conducting horizon picking within the reservoir. The result is the seismic zonation, which is subsequently compared with the well zonation to compute the correlation. The findings of this study are validated using the results of the latest workover job which provide a positive correlation between acoustic impedance, areas of thick baffle rock type, un-swept oil and the work-over result. The high value of acoustic impedance is interpreted as a good indication of un-swept oil since the un-swept oil is situated in the thin layer of high permeable reservoir within the baffle interval. This model can be used as guidance for the next workover job in the field.