The formation bulk density prediction for intact and fractured siliciclastic rocks

Abstract

The formation bulk density is one of the most important rock properties required for reservoir evaluation and geomechanical analysis. In intervals where the formation bulk density logs are not acquired, the industry practice is to estimate the formation bulk density from the compressional-wave velocity using empirical relationships. The major problems with the existing empirical relationships are: (1) they were developed primarily for specific lithologies (in most cases clean formations) and have failed to produce reasonable estimates when applied over a lithological column that consists of several stratigraphic units; (2) they are not applicable to rocks that contain microcracks/fractures. In this paper, a new formation bulk density prediction method that can be applied to a wide range of intact and fractured siliciclastic rocks is being proposed based on experimental data. The model is then validated using wireline log data acquired from an onshore well in the tertiary deltaic system of the Niger Delta basin. In the new model, the formation bulk density is expressed as a function of sonic velocity difference and shale volume factor. In general, an excellent agreement exists between the predicted and measured formation bulk density using the new technique. The statistical analysis shows that the new formation bulk density prediction model outperforms the most widely used empirical relationships with the least-root-mean square errors and least residual values.