Rock Typing in Petroleum Geology: Past and Future

In the literature, numerous methods linking a characteristic pore length scale (lc) to permeability (k) and porosity (ϕ) have been used to identify different classes of rocks and petrofacies (known as rock typing). One of the most-widely applied techniques is based on the Winland equation, which was first proposed empirically using experimental measurements. However, due to its empiricism, the interpretation of its parameters is not very clear. In this study, a new theoretical approach is proposed for identifying rock types based on the permeability and the formation- resistivity factor (F). Such an approach provides theoretical insights into the parameters of the Winland equation. We also show that for precise rock typing, the knowledge of the formation factor (measured routinely through petrophysical evaluation) is required. Results demonstrate that, although some rock samples might belong to the same type on the k-vs.-1/F plot, they might appear scattered on the k-vs.-ϕ plot. That is because of the fact that both k and F are complex functions of ϕ, while the latter itself is simply an index of the total pore volume, which does not necessarily provide information on the pore connectivity that contributes to both k and F.