Publications

We present a case study analysis of gross rock volume (GRV) sensitivity to depth conversion for undeveloped zones at Buntal Gas Field, Block B, Natuna Sea. Methods incorporated included: (1) a linear average velocity-time function, (2) a 2nd order polynomial time-depth function, (3) an instantaneous velocity-depth V0k method, and (4) calibrated seismic stacking velocities. We discuss and analyze best practices for each method, review a statistical depth error analysis, and highlight the benefits and limitations of each method. The linear average velocity function and polynomial function are simple and easy to apply. By using Dix corrected raw hand-picked seismic stacking velocities as input a gas slow down effect was detectable corresponding to the productive gas zones at Buntal. This provided confidence that seismic stacking velocities were viable for predicting velocity distribution away from well control and suggests the method is well suited for this project owing to the limited spatial control from wells. The V0K method requires good quality calibrated velocity log, however, in this study area the data were not ideal with spikes due to washout/borehole rugosity and gas slow down effects. Despite these limitations, where clear seismic amplitude fluid responses are evident, the gas-water contact showed good consistency with the amplitude conformance to structure, a positive qualitative indicator that the method accurately predicts away from well control. Comparison of GRV computations using each method showed range of number, mainly on stacking velocity method, emphasizing the importance of depth conversion in prospect characterization. The use of a variety of depth conversion techniques further enabled a means of determining a range of GRV uncertainty for volumetric analysis. For these reasons it is a recommended best practice to run multiple depth conversion scenarios to evaluate uncertainty ranges for GRV and well depth prognosis.