<p><strong>Fluid (or 'pore') pressure does not depend only on hydrostatic pressure — it also depends on stratigraphy, fluid content, and the geological history of the rock.</strong></p>

<p>The fluid 'stack' can be thought of as a (mostly) connected body of fluid — dominantly brine and hydrocarbons. The rock 'stack' is a similarly interconnected body. The fluid stack defines a typical soft lower limit of pore pressures; the rock stack defines the hard upper bound.</p>

<p>In practice, we don't experience lithostatic pressure in the pores because at some level of stress below lithostatic pressure, we find the <em>fracture pressure</em>. If the pore pressure reaches this level, the rock breaks and pressure equilibrates. This is the mechanism behind <a href="https://en.wikipedia.org/wiki/Hydraulic_fracturing" rel="nofollow noreferrer">induced hydraulic fracturing, or fracking</a>.</p>

<p><strong>But pore pressure does not have to be hydrostatic.</strong> <a href="https://en.wikipedia.org/wiki/Geologic_overpressure" rel="nofollow noreferrer">'Geopressure' or 'overpressure'</a> or <a href="http://www.glossary.oilfield.slb.com/Terms/u/underpressure.aspx" rel="nofollow noreferrer">underpressure</a> can occur if a stratigraphic layer is unusually tight, for example salt, and a porous formation experiences rapid uplift or burial. We might also depart from hydrostatic pressure as the density of the pore fluid changes, with salinity or gas saturation for example.</p>

<p><a href="https://i.sstatic.net/DNEh4.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/DNEh4.png" alt="enter image description here"></a></p>

<p>This cartoon shows pressure measurements (green) in a gas well. The other trends can be computed from models or data (for example the bulk density log). The shallow part of the well is hydrostatic; the deeper section shows overpressure. Natural gas is low density, so the gas zone shows a different trend. Clearly there will be a potentially dangerous pressure kick as the drill penetrates the top of the reservoir. If the gas trend were to hit fracture pressure, the trap would leak and the hydrocarbons would escape — a common natural phenomenon.</p>

<hr />

<p>You might also like to read these related questions:</p>

<ul>
<li><a href="https://earthscience.stackexchange.com/questions/2992/why-is-the-lithostatic-pressure-gradient-1-psi-ft-how-is-it-derived-from-densit">Why is the lithostatic pressure gradient 1 psi/ft? How is it derived from density?</a></li>
<li><a href="https://earthscience.stackexchange.com/questions/2106/how-do-i-derive-the-formula-for-lithostatic-overburden-pressure">How do I derive the formula for lithostatic (overburden) pressure?</a></li>
<li><a href="https://earthscience.stackexchange.com/questions/2283/what-is-the-reasoning-behind-the-statement-faulting-will-occur-along-the-plane">What is the reasoning behind the statement &quot;Faulting will occur along the plane where the shear stress is the highest&quot;?</a></li>
</ul>

CC BY-SA 3.0
2016-04-08T12:55:12.673Z
2016-04-08T12:55:12.673Z
2017-04-13T12:50:30.540Z
Matt Hall
<p>Why does lithostatic pressure not play a role? (or does it?)</p>

CC BY-SA 3.0
2016-06-05T01:30:02.567Z
Because rock as intrinsic mechanical strength. In absence of fluid, the pore-space/voids would just be empty (for a very long time).
stali
<geophysics><geothermal-heat><groundwater><hydrogeology>
Why is fluid pressure in a fractured rock reservoir only dependent on hydrostatic pressure?
ye-ti-800
# Why is fluid pressure in a fractured rock reservoir only dependent on hydrostatic pressure?

(Asked by: username_0 on 2016-04-06)

Why does lithostatic pressure not play a role? (or does it?)

**Tags:** geophysics, geothermal-heat, groundwater, hydrogeology

> **Comments:**
> **username_1**: Because rock as intrinsic mechanical strength. In absence of fluid, the pore-space/voids would just be empty (for a very long time).  

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## 1 answers

### Answer by username_2 on 2016-04-08. Score: 2 | Accepted answer

**Fluid (or 'pore') pressure does not depend only on hydrostatic pressure — it also depends on stratigraphy, fluid content, and the geological history of the rock.**

The fluid 'stack' can be thought of as a (mostly) connected body of fluid — dominantly brine and hydrocarbons. The rock 'stack' is a similarly interconnected body. The fluid stack defines a typical soft lower limit of pore pressures; the rock stack defines the hard upper bound.

In practice, we don't experience lithostatic pressure in the pores because at some level of stress below lithostatic pressure, we find the *fracture pressure*. If the pore pressure reaches this level, the rock breaks and pressure equilibrates. This is the mechanism behind [induced hydraulic fracturing, or fracking](https://en.wikipedia.org/wiki/Hydraulic_fracturing).

**But pore pressure does not have to be hydrostatic.** ['Geopressure' or 'overpressure'](https://en.wikipedia.org/wiki/Geologic_overpressure) or [underpressure](http://www.glossary.oilfield.slb.com/Terms/u/underpressure.aspx) can occur if a stratigraphic layer is unusually tight, for example salt, and a porous formation experiences rapid uplift or burial. We might also depart from hydrostatic pressure as the density of the pore fluid changes, with salinity or gas saturation for example.

[![enter image description here](https://i.sstatic.net/DNEh4.png)](https://i.sstatic.net/DNEh4.png)

This cartoon shows pressure measurements (green) in a gas well. The other trends can be computed from models or data (for example the bulk density log). The shallow part of the well is hydrostatic; the deeper section shows overpressure. Natural gas is low density, so the gas zone shows a different trend. Clearly there will be a potentially dangerous pressure kick as the drill penetrates the top of the reservoir. If the gas trend were to hit fracture pressure, the trap would leak and the hydrocarbons would escape — a common natural phenomenon.

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You might also like to read these related questions:

* [Why is the lithostatic pressure gradient 1 psi/ft? How is it derived from density?](https://earthscience.stackexchange.com/questions/2992/why-is-the-lithostatic-pressure-gradient-1-psi-ft-how-is-it-derived-from-densit)
* [How do I derive the formula for lithostatic (overburden) pressure?](https://earthscience.stackexchange.com/questions/2106/how-do-i-derive-the-formula-for-lithostatic-overburden-pressure)
* [What is the reasoning behind the statement "Faulting will occur along the plane where the shear stress is the highest"?](https://earthscience.stackexchange.com/questions/2283/what-is-the-reasoning-behind-the-statement-faulting-will-occur-along-the-plane)


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