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2dev>Jmdonev at 17:25, 17 December 2018

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<onlyinclude>Cap rocks are relatively impermeable [[rock]]s layers that seal the top of [[reservoir]]s and other geologic formations.</onlyinclude> Cap rocks are commonly referred to in the context of topping [[oil and gas reservoir]]s but can also top formations of softer or less resistant rock and salt domes. Cap rocks are often formed from shale, anhydrite, or salt as discussed below.<ref name= Hyne>Hyne, N., 2012. "Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production", Tulsa: PennWell Corporation.</ref>

==Reservoirs==
[[File:Anticline Reservoir.jpg|300px|thumb|Figure 1. An anticline oil and gas reservoir. The cap rock would be the impermeable rock layer.<ref>Source unknown. Please contact us if you know this image's source. The cap rock is the upper part of the formation.</ref>]]

Cap rocks form the top of reservoirs and create a layer that is impervious to [[fluid]] flow (Figure 1). By preventing fluids from flowing through it, the fluids are prevented from escaping the reservoir.<ref name= Hyne/>

This particularly applies to oil and gas reservoirs. Oil and gas reservoirs often form in "traps". There are a few types of traps: anticlinal, fault, stratigraphic, and reef and/or salt traps. In all cases, the cap rock overlies the reservoir rock(s) (where the [[hydrocarbon]]s are held), sealing the top of the trap and often the sides. [[Source rock]]s (where the hydrocarbons form) are usually at lower depths than the cap rock but not necessarily directly below it.<ref name= Hyne/>

==Less Resistant Rock==
[[File:Hoodoos, Drumheller, Alberta.jpg|thumb|Figure 2. Drumheller Hoodoos.<ref>Wikimedia Commons. (Nov.28, 2018) "File:Hoodoos, Drumheller, Alberta.jpg". [Online} Available from: https://commons.wikimedia.org/wiki/File:Hoodoos,_Drumheller,_Alberta.jpg</ref>]]
When a rock that is harder to break down (resists [[weathering]]) overlies an easier to break down rock (less resistant to weathering), the rock on top is referred to as cap rock. The cap rock, in this case, can protect the lower more delicate layers from weathering and [[erosion]]. Alternatively, the cap rock can remain while the more delicate lower layers are weather and eroded from under it. Both cases result in unique natural features. Buttes, mesas, scarps all form in this fashion but the most distinctive feature that forms this way are the hoodoos (Figure 2). <ref>Lindquist, R.C., 1977. The geology of Bryce Canyon National Park, Bryce Canyon, Utah: Bryce Canyon Natural History Association.</ref>

==Salt Domes==
[[File:Salt dome.jpg|thumb|right|Figure 3. Salt dome. The dark yellow-orange layer is a sulphur-rich cap rock layer.<ref>General Kinematics. (Nov.28, 2018) "Sulfur Mining & Processing: What to Know". [Online} Available from: https://www.generalkinematics.com/blog/sulfur-mining-processing-know/</ref>]]
Salt domes are large masses of underground salt with a small amount of anhydrite (Figure 3). Salt usually has a lower [[density]] than the surrounding rock and will attempt to flow upwards when it can. This often results in salt domes forming plug or bulge shapes as it tries to fit through weak spots in the overlying rock layer. As the salt forces its way upwards and begins to dissolve, a cap rock forms on top of the dome. Anhydrite is an evaporite mineral that is not as easily dissolved as salt. It reacts with bacteria and other substances in the ground to change into sulphur, dolomite, limestone, and/or gypsum, all of which build up to make the cap rock.<ref name= Hyne/>

==For Further Reading==
*[[Gas]]
*[[Oil]]
*[[Source rock]]
*[[Oil and gas reservoir]]
*[[Reservoir]]
*Or explore a [[Special:Random|random page]]

==References==
{{reflist}}
[[Category:Uploaded]]

Cap rock - Revision history

Jmdonev: 1 revision imported

2dev>Jmdonev at 17:25, 17 December 2018