Acoustic wave velocity modelling in sedimentary sequences

Abstract

Layered-clay shale density increases systematically with increasing depth of burial by progressive expulsion of pore water: [See full text for equation] Using elastic wave propagation theory, this shale density function can be transformed into a shale velocity prediction. Providing shale and water chemistry remain relatively stable, the progressive reduction in shale porosity and contained water also results in systematic changes to shale resistivity. More general formation (mixed lithology) sonic slowness can be modelled using established log interpretation transforms (the Archie porosity - resistivity and Raymer-Hunt-Gardner porosity - slowness relationships). The difference between measured formation resistivity and predicted shale resistivity is then interpreted in terms of departure of the expected slowness from that of pure shale. When added to the expected shale slowness, the combined slowness shows good agreement with observed sonic log data. This provides a new technique for prediction of formation velocities. The technique can be used to improve the quality of sonic log data, and can be applied where no sonic data were acquired. Numerous technical and commercial applications result.