Geostatic and lithostatic pressure

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

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:

For example, in the Cenozoic of Louisiana, the geostatic gradient is 0.85 psi/ft at 1000 ft304.8 m and 0.95 psi/ft at 14,000 ft4,267.2 m.

See also

External links

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Geostatic and lithostatic pressure
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