A COMPARISON OF DEPTH CONVERSION METHODS IN BUNTAL GAS FIELD, BLOCK B, NATUNA SEA, INDONESIA

摘要

We present a case study analysis of gross rockrnvolume (GRV) sensitivity to depth conversion forrnundeveloped zones at Buntal Gas Field, Block B,rnNatuna Sea. Methods incorporated included: (1) arnlinear average velocity-time function; (2) a 2ndrnorder polynomial time-depth function; (3) anrninstantaneous velocity-depth V0k method; and (4)rncalibrated seismic stacking velocities. We discussrnand analyze best practices for each method, reviewrna statistical depth error analysis, and highlight thernbenefits and limitations of each method.rnThe linear average velocity function and polynomialrnfunction are simple and easy to apply. By using Dixrncorrected raw hand-picked seismic stackingrnvelocities as input a gas slow down effect wasrndetectable corresponding to the productive gasrnzones at Buntal. This provided confidence thatrnseismic stacking velocities were viable forrnpredicting velocity distribution away from wellrncontrol and suggests the method is well suited forrnthis project owing to the limited spatial control fromrnwells.rnThe V0K method requires good quality calibratedrnvelocity log, however, in this study area the datarnwere not ideal with spikes due to washout/boreholernrugosity and gas slow down effects. Despite thesernlimitations, where clear seismic amplitude fluidrnresponses are evident, the gas-water contact showedrngood consistency with the amplitude conformancernto structure, a positive qualitative indicator that thernmethod accurately predicts away from well control.rnComparison of GRV computations using eachrnmethod showed a range of numbers, on the mainrnoutlier being the stacking velocity method,rnemphasizing the importance of depth conversion inrnprospect characterization. The use of a variety of depth conversion techniquesrnfurther enabled a means of determining a range ofrnGRV uncertainty for volumetric analysis. For thesernreasons it is a recommended best practice to runrnmultiple depth conversion scenarios to evaluaternuncertainty ranges for GRV and well depthrnprognosis.