Application of normalized velocities for depth conversion of the bookabourdie field, Cooper Basin, S.A.

Abstract

The interval velocity of a layer can vary laterally with changes in layer depth, composition and compaction history. As these can have varying trends across a study area, it is desirable to separate their influences on the layer's interval velocity. A velocity-depth relation based on the one-sixth power law after Acheson (1963), provides a method of separating and quantifying the influences of these variables on interval velocity. Carter (1989) applied Acheson's relation to depth conversion of seismic data, equating lateral changes in a layer's depth-normalized interval velocity with changes in layer composition. Such changes, as may be due to lateral changes in sand/shale ratio, porosity, etc., follow geologically predictable trends and thus become easier to map across an area. A test of the normalized velocity approach to depth conversion, carried out on the geologically complex Bookabourdie Field, gives encouraging results. Using only the velocity control of wells available at the time of the field's discovery, the method successfully predicts the depths to the objective reservoir to within 1%, for seven of the eight wells in the field.