Geostatic and lithostatic pressure
|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|
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.
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:
- 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 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 1000 ft304.8 m and 0.95 psi/ft at 14,000 ft4,267.2 m.
- Normal hydrostatic pressure
- Normal hydrostatic pressure gradients
- Abnormal hydrostatic pressure
- Causes of overpressure
- Causes of underpressure