The overburden pressure is defined as the pressure exerted by the total weight of overlying formations above the point of interest. The total weight is the combined weight of both the formation solids (rock matrix) and formation fluids in the pore space. The density of the combined weight is referred to as the bulk density (ρb).
The overburden pressure can therefore be expressed as the hydrostatic pressure exerted by all materials overlying the depth of interest:
σov = 0.052 x ρb x D (1.4)
where
σov = overburden pressure (psi)
ρb = formation bulk density (ppg)
D = true vertical depth (ft)
And similarly as a gradient (EMW) in ppg:
(1.5)σovg = overburden gradient, ppg
ρb = formation bulk density (gm/cc)
(the factor 0.433 converts bulk density from gm/cc to psi/ft)In a given area, the overburden gradient is not constant with depth due to variations in formation density. This results from variations in lithology and pore fluid densities. In addition the degree of compaction and thus formation density, increases with depth due to increasing overburden.
A useful equation for calculating the overburden gradient under field conditions of varying lithological and pore fluid density is given by:
Note the densities in Equation (1.6) are expressed in gm /cc, instead of the usual units of ppg. With the exception of the oil industry, all other industries use the Metric system of units where density is usually expressed in gm/cc. The oil industry borrows many of its measurements from other industries.
A list of typical matrix and fluid densities is included in Table 1.1 below:
To convert densities from gm/cc to gradients in psi/ft simply use:
Gradient (psi/ft) = 0.433 x (gm /cc) (1.7)
To convert from psi/ft to ppg, use:
Density (ppg) = gradient (psi/ft) / 0.052 (1.8)
