# Geostatic and lithostatic pressure

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Critical elements of the petroleum system Formation fluid pressure and its application Edward A. Beaumont, Forrest Fiedler Web page AAPG Store

The geostatic pressure at a given depth is the vertical pressure due to the weight of a column of rock and the fluids contained in the rock above that depth. Lithostatic pressure is the vertical pressure due to the weight of the rock only.

## Geostatic variables

Three variables determine geostatic pressure:

## Calculating geostatic pressure

We can calculate geostatic pressure using the formula below: ${\mbox{P}}_{{{\rm {G}}}}=[\rho _{{{\rm {m}}}}\times (1-\phi )\times {\mbox{d}}]+[\rho _{{{\rm {w}}}}\times \phi \times {\mbox{d}}]$

where:

• PG = geostatic pressure (psi)
• ρm = weighted average of grain (mineral) density (sandstone and shale = 2.65 g/cm3, limestone = 2.71 g/cm3)
• ρw = weighted average of pore-water density (g/cm3)
• φ = weighted average of rock porosity
• d = depth (ft)

To calculate weighted averages, use 1000 ft304.8 m
12,000.006 in
or 300 m984.252 ft
11,811.03 in
increments.