Abstract:
The shale volume factor is among the critical petrophysical parameters for
reservoir characterization and formation evaluation. Inaccurate estimates of the
shale volume factor can lead to poor reserves or resource estimates and wrong
business decisions. While the current industry standard is to estimate the shale
volume factor from the gamma ray logs using the concept of the gamma ray
index, a relationship between the shale volume factor and the gamma-ray index
needs to be established for any region/basin under consideration. For most
applications in the Niger Delta Basin, a linear relationship is often assumed.
However, there is no proven relationship between the shale volume factor and
the gamma-ray index for the formations in the Niger Delta Basin. This paper
proposes a new shale volume factor prediction correlation for the Niger Delta
Basin in Nigeria. The correlation development is based on establishing a
relationship between the shale volume factor obtained from cores and the
gamma ray index obtained from petrophysical logs for over thirty wells drilled
across the Niger Delta Basin. The results show that the relationship between the
shale volume factor and the gamma-ray index is not linear as often assumed but
a power law model. The new probabilistic correlation predicts lower shale
volume factors than the linear model for all ranges of the gamma-ray index. This
recent correlation will significantly impact how the hydrocarbon resources and
reserves are quantified in the Niger Delta Basin.
