COMBINED GEOSTATISTICAL INVERSION AND SIMULTANEOUS AVA INVERSION: EXTENDING THE LIFE OF A MATURE AREA, KOTABATAK FIELD, CENTRAL SUMATRA BASIN, INDONESIA

摘要

The Kotabatak Field is one of several major oil fields within the prolific Central Sumatra Basin, Indonesia. Extensive production since 1971 has resulted in over 250 MMBO being produced, and has provided dense well control, which has enabled many features of the reservoirs to be mapped. However, surprises with respect to the exact position and orientation of these thin reservoirs are still regularly encountered when drilling new wells or during water injection. In particular, reservoir characterization using a 2002 vintage 3D seismic has been complicated due to limited acoustic impedance (AI) contrast between the productive sands within the Bekasap Formation and the intra-formation shales. Rock physics analysis, however, revealed an increased separation in the combined P- and S-velocity domains. This analysis justified the use of Simultaneous Angle Dependent Inversion (SADI), which allows for a deterministic inversion to P-and S-impedance using P-wave angle gathers and well control. These rock properties helped to map the outline and orientation of the reservoirs. Although the simultaneous inversion process has increased resolution at the gross reservoir scalee, it had only limited resolution for very thin beds. Therefore, an extra Geostatistical Inversion step was added to the inversion workflow. Geostatistical Inversion is a simulation method that produces reservoir models at a higher resolution than deterministic inversion, while still being constrained to match to the seismic data. With the higher vertical resolution comes increased uncertainty. Multiple, equi-probable simulations of lithology and properties are produced by the Geostatistical Inversion and allow the interpreter to assess the uncertainty of the reservoir distribution and properties. All simulations honor the seismic data, the SADI results, the well log data and other geological information. The high-resolution geostatistical simulation results of Kotabatak have provided an effective interpretation framework that assists detailed sequence stratigraphic mapping and information on reservoir quality. Recent new well log information confirms the results.