Geologic overpressure in
stratigraphic
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy has three related subfields: lithostra ...
layers is caused by the inability of
connate pore fluids to escape as the surrounding mineral matrix compacts under the
lithostatic pressure
Pressure is force magnitude applied over an area. Overburden pressure is a geology term that denotes the pressure caused by the weight of the overlying layers of material at a specific depth under the earth's surface. Overburden pressure is also ca ...
caused by overlying layers. Fluid escape may be impeded by sealing of the compacting rock by surrounding impermeable layers (such as
evaporite
An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
s,
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...
and cemented
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates ...
s). Alternatively, the rate of burial of the stratigraphic layer may be so great that the efflux of fluid is not sufficiently rapid to maintain
hydrostatic
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
pressure.
Common situations where overpressure may occur: in a buried river channel filled with coarse sand that is sealed on all sides by impermeable shales, or when there is an explosion within a confined space.
It is extremely important to be able to diagnose overpressured units when drilling through them, as the
drilling mud weight (density) must be adjusted to compensate. If it is not, there is a risk that the pressure difference down-well will cause a dramatic decompression of the overpressured layer and result in a
blowout at the well-head with possibly disastrous consequences.
Because overpressured sediments tend to exhibit better
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
than would be predicted from their depth, they often make attractive
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
reservoirs and are therefore of important economic interest.
Petroleum geology
Oil wells
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