Geostatic and lithostatic pressure
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:
- Densities of formation waters as related to salinities
- Net thickness of different lithologies, e.g., sandstone, shale, limestone
- Porosities of different lithologies
Calculating geostatic pressure
We can calculate geostatic pressure using the formula below:
![{\displaystyle {\mbox{P}}_{\rm {G}}=[\rho _{\rm {m}}\times (1-\phi )\times {\mbox{d}}]+[\rho _{\rm {w}}\times \phi \times {\mbox{d}}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/209bc02937838ec329c802fe7a07ac244422e528)
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 length::1000 ft or length::300 m increments.
Geostatic gradient
Geostatic gradient is the rate of change of geostatic pressure with depth. A geostatic gradient of 1 psi/ft results from an average density of 2.3 g/cm3.
How geostatic gradient varies
Geostatic gradients vary with depth and location. The gradient increases with depth for two reasons:
- Rock bulk density increases with increasing compaction.
- Formation water density increases because the amount of total dissolved solids (TDS) in the water increases with depth.
For example, in the Cenozoic of Louisiana, the geostatic gradient is 0.85 psi/ft at depth::1000 ft and 0.95 psi/ft at depth::14,000 ft.
See also
External links
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