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[[file:paleontology_fig2.png|thumb|300px|{{figure number|2}}Example of geological time using the stratigraphic column of Cook Inlet basin, Alaska. Geological time table modified after van Eysinga<ref name=pt05r158>van Eysinga, F. W. B., 1975, Geologic Time Table: Amsterdam, Elsevier (chart).</ref>. Tertiary stages are from Wolfe<ref name=pt05r172>Wolfe, J. A., 1977, Paleogene floras from the Gulf of Alaska region: U., S. Geological Survey Professional Paper 997, p. 108.</ref>. (From Magoon & Claypool.<ref name=pt05r108>Magoon, L. B., Claypool, G. E., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1000/1043.htm Petroleum geology of Cook Inlet Basin—an exploration model]: AAPG Bulletin, v. 65, p. 1043–1061.</ref>)]]

[[file:paleontology_fig2.png|thumb|300px|{{figure number|2}}Example of geological time using the stratigraphic column of Cook Inlet basin, Alaska. Geological time table modified after van Eysinga<ref name=pt05r158>van Eysinga, F. W. B., 1975, Geologic Time Table: Amsterdam, Elsevier (chart).</ref>. Tertiary stages are from Wolfe<ref name=pt05r172>Wolfe, J. A., 1977, Paleogene floras from the Gulf of Alaska region: U., S. Geological Survey Professional Paper 997, p. 108.</ref>. (From Magoon & Claypool.<ref name=pt05r108>Magoon, L. B., Claypool, G. E., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1000/1043.htm Petroleum geology of Cook Inlet Basin—an exploration model]: AAPG Bulletin, v. 65, p. 1043–1061.</ref>)]]

Key fossil datums in reference sections provide the basic division of the geological section into [[stratigraphic system|system]]s, [[stratigraphic series|series]], and [[stratigraphic stage|stages]].<ref name=pt05r72>Hancock, J. M., 1977, The historic development of biostratigraphic correlation, in Kauffman, E. G., Hazel, J. E., eds., Concepts and Methods of Biostratigraphy: Stroudsburg, PA, Dowden, Hutchinson and Ross, p. 3–22.</ref> However, reference sections may not contain a complete record of sedimentation because the contacts between many strata are [[Unconformity|unconformable]]. Therefore, the continuous span of time is divided into [[eras]], [[epochs]], and [[ages]] ([[:file:paleontology_fig2.png|Figure 2]]). The intervals of geological time are calibrated to absolute time by means of [[radiometric ages|radiometric]] or [[isotopic ages]] from [[interbedded rocks|interbedded]] or [[cross-cutting rocks]] such as [[volcanic flows]] and [[ash beds]].

Key fossil datums in reference sections provide the basic division of the geological section into [[stratigraphic system|system]]s, [[stratigraphic series|series]], and [[stratigraphic stage|stages]].<ref name=pt05r72>Hancock, J. M., 1977, The historic development of biostratigraphic correlation, in Kauffman, E. G., Hazel, J. E., eds., Concepts and Methods of Biostratigraphy: Stroudsburg, PA, Dowden, Hutchinson and Ross, p. 3–22.</ref> However, reference sections may not contain a complete record of sedimentation because the contacts between many strata are [[Unconformity|unconformable]]. Therefore, the continuous span of time is divided into [[Wikipedia:Era (geology)|eras]], [[Wikipedia:Epoch (geology)|epochs]], and [[Wikipedia:Age (geology)|ages]] ([[:file:paleontology_fig2.png|Figure 2]]). The intervals of geological time are calibrated to absolute time by means of [http://dictionary.reference.com/browse/radiometric+dating radiometric] or [http://geology.about.com/od/geotime_dating/a/timeyardstick.htm isotopic ages] from interbedded or cross-cutting rocks such as [http://geology.campus.ad.csulb.edu/people/bperry/IgneousRocksTour/VolcanoesAndLavaFlows.html volcanic flows] and [[Wikibooks:Historical_Geology/Volcanic_ash|ash beds]].

In most wells, the LADs of fossils are the most useful datum planes for subdividing, dating, and correlating the [[lithostratigraphic section]] ([[:file:paleontology_fig1.png|Figure 1]]) because the drilling procedure may extend the FADs of fossils by caving of 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 Kauffman, E. G., Hazel, J. E., 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 Kauffman, E. G., Hazel, J. E., 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 [[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 [[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.