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[[file:BrokenConcretion22.jpg|thumb|400px|A broken concretion with fossils inside; Late Cretaceous Pierre shale, near Ekalaka, Montana. Photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster).]]

[[file:BrokenConcretion22.jpg|thumb|400px|A broken concretion with fossils inside; Late Cretaceous Pierre shale, near Ekalaka, Montana. Photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster).]]

Paleontology, the study of fossil organisms and their traces, has been used in the exploration for and exploitation of hydrocarbons since the later half of the nineteenth century. Because  [[fossil assemblage]]s change through time, they aid in the prediction of depth to reservoirs, to casing points, and to overpressured zones. [http://www.stratigraphy.org/upload/bak/chron.htm Chronostratigraphic] [[Fossil assemblage|assemblages]] are the basis for correlating [[stratum|strata]] among wells, across basins, and between basins. Correlation is the method for predicting the lateral continuity and physical and chronostratigraphic equivalency of strata. Fossil assemblages are also evidence of the [[depositional environments]] of the drilled |strata and thus play a role in predicting the location of [[reservoir]]s, [[source rock]]s, and [[pinch out|pinch-out]] of porous strata.

Paleontology, the study of fossil organisms and their traces, has been used in the exploration for and exploitation of hydrocarbons since the later half of the nineteenth century. Because  [[fossil assemblage]]s change through time, they aid in the prediction of depth to reservoirs, to casing points, and to overpressured zones. [http://www.stratigraphy.org/upload/bak/chron.htm Chronostratigraphic] [[Fossil assemblage|assemblages]] are the basis for correlating [[stratum|strata]] among wells, across basins, and between basins. Correlation is the method for predicting the [[lateral]] continuity and physical and chronostratigraphic equivalency of strata. Fossil assemblages are also evidence of the [[depositional environments]] of the drilled |strata and thus play a role in predicting the location of [[reservoir]]s, [[source rock]]s, and [[pinch out|pinch-out]] of porous strata.

==Age and correlation of strata==

==Age and correlation of strata==

In most wells, the LADs of fossils are the most useful datum planes for subdividing, dating, and correlating the [http://www.stratigraphy.org/upload/bak/litho.htm lithostratigraphic] section ([[:file:paleontology_fig1.png|Figure 1]]) because the drilling procedure may extend the FADs of fossils by caving of [[Mudlogging: drill cuttings analysis|cuttings]]. However, in certain conditions, the LAD may be overextended by reworking of the specimens above an unconformity, and the FAD may be in older rocks due to contamination from the drilling mud.<ref name=pt05r129>Poag, C. W., 1977, Biostratigraphy in Gulf Coast petroleum exploration, in E. G. Kauffman and J. E. Hazel, eds., Concepts and Methods of Biostratigraphy: Stroudsburg, PA, Dowden, Hutchinson and Ross, p. 213–234.</ref> The fossil top may also be depressed (or older) in a given well for a number of reasons: the strata with the uppermost part of the range may be eroded, environmental conditions prevented the species from living there, or the specimens may have dissolved. If a species is not abundant at the top of its range, it may be missed in drilling and sampling.

In most wells, the LADs of fossils are the most useful datum planes for subdividing, dating, and correlating the [http://www.stratigraphy.org/upload/bak/litho.htm lithostratigraphic] section ([[:file:paleontology_fig1.png|Figure 1]]) because the drilling procedure may extend the FADs of fossils by caving of [[Mudlogging: drill cuttings analysis|cuttings]]. However, in certain conditions, the LAD may be overextended by reworking of the specimens above an unconformity, and the FAD may be in older rocks due to contamination from the drilling mud.<ref name=pt05r129>Poag, C. W., 1977, Biostratigraphy in Gulf Coast petroleum exploration, in E. G. Kauffman and J. E. Hazel, eds., Concepts and Methods of Biostratigraphy: Stroudsburg, PA, Dowden, Hutchinson and Ross, p. 213–234.</ref> The fossil top may also be depressed (or older) in a given well for a number of reasons: the strata with the uppermost part of the range may be eroded, environmental conditions prevented the species from living there, or the specimens may have dissolved. If a species is not abundant at the top of its range, it may be missed in drilling and sampling.

Knowing the age and thickness of the strata enables prediction of depth to reservoir or [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=casing%20point casing points] and depth to maturation of source rocks. For example, casing points are important for engineering decisions when drilling unconsolidated Plio-Pleistocene ({{Ma|Pliocene|Pleistocene}}) muds in the [[Gulf of Mexico]] and offshore Trinidad. Drilling stops when key fossils are encountered, and casing is set to prevent the hole from collapsing or to control high pressure zones that lie deeper.

Knowing the age and thickness of the strata enables prediction of depth to reservoir or [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=casing%20point casing points] and depth to [[maturation]] of source rocks. For example, casing points are important for engineering decisions when drilling unconsolidated Plio-Pleistocene ({{Ma|Pliocene|Pleistocene}}) muds in the [[Gulf of Mexico]] and offshore Trinidad. Drilling stops when key fossils are encountered, and casing is set to prevent the hole from collapsing or to control high pressure zones that lie deeper.

Fossil assemblages also define the position of [[Unconformity|unconformities]] and the duration of hiatuses and may aid in the recognition of faults and the correlation of strata across faults.

Fossil assemblages also define the position of [[Unconformity|unconformities]] and the duration of hiatuses and may aid in the recognition of faults and the correlation of strata across faults.