Editing Tar pit (section)

{{short description|Asphalt pit or asphalt lake}}
{{Use mdy dates|date=October 2022}}
{{About|the geological formation|other uses|Tar pit (disambiguation)}}
[[File:Mother-of-the-Lake.jpg|thumb|Tar pit at [[La Brea, Trinidad and Tobago|Tierra La Brea]], [[Trinidad]]]]
[[File:Tar Pit Formation Image.png|thumb|upright=1.3|An anticlinal trap is feeding the tar pit on the surface through the vertical fracture in the strata (indicated by the red arrow). Once the crude oil reaches the surface, evaporation takes place and lighter hydrocarbons are vaporized, leaving behind sticky asphalt.]]
'''Tar pits''', sometimes referred to as '''asphalt pits''', are large [[Bitumen|asphalt]] deposits. They form in the presence of [[petroleum]], which is created when decayed [[organic matter]] is subjected to pressure underground. If this crude oil seeps upward via fractures, conduits, or porous sedimentary rock layers, it may pool up at the surface.<ref name=":15">{{Cite web |title=La Brea Tar Pits |url=https://ucmp.berkeley.edu/quaternary/labrea.html |access-date=2021-04-26 |publisher=[[University of California Museum of Paleontology]]}}</ref> The lighter components of the crude oil evaporate into the atmosphere, leaving behind a black, sticky asphalt.<ref name=":15"/> Tar pits are often excavated because they contain large fossil collections.<ref name=":15"/>

Tar pits form above oil reserves, and these deposits are often found in [[structural trap|anticlinal trap]]s. In fact, about 80 percent of petroleum found on Earth has been found in anticlinal traps.<ref name=":1">{{Cite web|title=Earth's physical resources: Petroleum: View as single page|url=https://www.open.edu/openlearn/ocw/mod/oucontent/view.php?id=4763&printable=1|access-date=2021-04-26|website=www.open.edu}}</ref> [[Anticline]]s are folds in [[stratigraphic layer]]s in which each half of the fold dips away from the crest. Such structures are usually developed above [[thrust fault]]s or in tectonic regions where the land is bending and folding. If the structure above the concave-down fold (arch) is a non-porous rock or [[aquitard]], such as [[shale]], it is considered an anticlinal trap.<ref name=":1" /> The figure in this section is a cartoon cross-section diagram that shows oil stuck in an anticlinal trap. If there is a [[Fault (geology)|fault]] or [[Fracture (geology)|fracture]] in the overlying [[strata]] above the oil reserve, the oil may migrate to the surface. This is possible by [[capillary fringe]] and because oil is less dense than water.<ref>{{Cite journal|date=2004-01-01|title=Coupled Thermo-Mechano-Chemical Processes in Shales: The Petroleum Borehole|url=https://www.sciencedirect.com/science/article/pii/S1571996004801010|journal=Elsevier Geo-Engineering Book Series|language=en|volume=2|pages=573–580|doi=10.1016/S1571-9960(04)80101-0|issn=1571-9960|last1=Dusseault|first1=Maurice B.|isbn=9780080445250|url-access=subscription}}</ref>