Reservoir Prediction Utilising AVO Walkaway VSP Data to Calibrate Surface Seismic Gathers, a Case Study in Indonesia

In prediction of reservoir properties using seismic data, use is frequently made of AVO techniques. However, such techniques may not always be successful. Hence a new technique has been devised — based on recording a subsurface AVO response in the well. Recording is achieved from five triaxial geophone shuttles at 15 m intervals — ideally one wavelength (of the predicted dominant source frequency) above the target. The source (two 150 cubic-inch Sleeve Guns) was deployed from the marine vessel’s boom and moving in a line such that shot spacing is as close as possible to the surface seismic offsets. The line might extend 3 km to each side the well. Processing of the AVO walkaway data takes place in three steps: data orientation; separation of the down and up P-wavefields; and deconvolution (using the downgoing P-wavefield to deconvolve the upgoing P-wavefield) to remove propagation effects. The surface seismic data (in this case a 3-D data set) are processed (gathered) and put through a rigorous 12-stage signal processing sequence. The AVOWSP and the surface gathers are then carefully tied to the zero-offset VSP. After application of the Offset Dependent Gain Function the data are solved for P-wave reflection amplitude (at angle 0°) and the gradient. Those results can then be mapped and contoured — to identify anomalous areas, at known sites (wells) and elsewhere. Additional refinement is continuously being sought, and is welcome.