Quantifying erosion in sedimentary Basins from sonic velocities in Shales and Sandstones

Abstract

If it is assumed that sonic velocity in a sedimentary rock decreases with burial-depth according to a known velocity/depth relationship, and that velocity is not reduced by erosion from maximum burial-depth, any erosion of that rock from maximum burial-depth may be quantified using velocity data. The displacement, on the depth axis, of sonic velocity in a given unit from the normally compacted velocity/depth relationship yields apparent erosion (i.e. amount of missing section or height above maximum burial-depth). Shales have been considered to be the only lithology to follow a sufficiently predictable velocity/depth relationship with burial-depth to be used in the estimation of erosion from sonic velocity data. However, erosion estimates based on sonic velocities in the Lower Cretaceous Allaru/Oodnadatta Mudstone of the Eromanga Basin are statistically similar to those derived from velocities in the Middle Jurassic Hutton Sandstone (an important hydrocarbon reservoir sandstone in the basin). Similarly, erosion estimates based on velocities in the Lower Triassic Bunter Sandstone (an important hydrocarbon reservoir sandstone) of the United Kingdom Southern North Sea are statistically similar to those based on velocities from the Lower Triassic Bunter Shale. The consistency of results from the shaly and sandy units analysed suggests that overcompaction (i.e. anomalously fast sonic velocity) of the sandstones is controlled by erosion from previously greater burial-depth, rather than by burial-depth independent sedimentological and/or diagenetic processes. The results validate the use of sandstones in maximum burial-depth studies, and perhaps more importantly suggest that, even in reservoir sandstones, burial-depth is the primary control on compaction and hence porosity.