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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==

Because fossil species evolved through time in genetically related lineages and because extinction events were followed by new [[Fossil assemblage|assemblages]] of [http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml species], fossils provide the best criteria for dividing the geological record into time intervals characterized by the first-appearance datums (FADs) and last-appearance datums (LADs) of key species ([[:file:paleontology_fig1.png|Figure 1]]). Fossil tops and bases may be [http://en.wiktionary.org/wiki/isochronous synchronous] in sections where the strata record continuous deposition and where suitable environmental conditions persisted.

Because fossil species evolved through time in genetically related lineages and because extinction events were followed by new [[Fossil assemblage|assemblages]] of [http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml species], fossils provide the best criteria for dividing the geological record into time intervals characterized by the first-appearance datums (FADs) and last-appearance datums (LADs) of key species ([[:file:paleontology_fig1.png|Figure 1]]). Fossil tops and bases may be [http://en.wiktionary.org/wiki/isochronous synchronous] in sections where the strata record continuous deposition and where suitable environmental conditions persisted.

[[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., and G. E. Claypool, 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 [[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]].

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 E. G. Kauffman, and J. E. Hazel, 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 [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.

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.

==Depositional environments and facies analysis==

==Depositional environments and facies analysis==

Most fossil species required specific conditions of temperature, [[Wikipedia:Substrate (biology)|substrate]], and surrounding medium to carry on their normal activities. When conditions differed from ideal, the species is not present or is not preserved. The types of sedimentary rocks deposited under specific environmental conditions are ''biofacies'' and are identified by their specific rock properties including fossils.<ref name=pt05r140>Scholle, P. A., Spearing, D., 1982, Sandstone depositional environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], 410 p.</ref><ref name=pt05r141>Scholle, P. A., Bebout, D. G., Moore, C. H., 1983, Carbonate depositional environments: [http://store.aapg.org/detail.aspx?id=656 AAPG Memoir 33], 708 p.</ref>

Most fossil species required specific conditions of temperature, [[Wikipedia:Substrate (biology)|substrate]], and surrounding medium to carry on their normal activities. When conditions differed from ideal, the species is not present or is not preserved. The types of sedimentary rocks deposited under specific environmental conditions are ''biofacies'' and are identified by their specific rock properties including fossils.<ref name=pt05r140>Scholle, P. A., and D. Spearing, 1982, Sandstone depositional environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], 410 p.</ref><ref name=pt05r141>Scholle, P. A., D. G. Bebout, and C. H. Moore, 1983, Carbonate depositional environments: [http://store.aapg.org/detail.aspx?id=656 AAPG Memoir 33], 708 p.</ref>

Fossils most commonly used in hydrocarbon exploration are microfossils (generally smaller than about 2 mm) because they can be recovered from drill cuttings without much damage to the fossils. Different processing techniques separate [[Calcareous microfossils|calcareous]], [[Siliceous microfossils|siliceous]], [[Phosphatic microfossils|phosphatic]], and [[Palynomorphs (organic-walled microfossils)|organic-walled fossils]].<ref name=pt05r100>Kummel, B., David, R., eds., 1965, Handbook of paleontological techniques: San Francisco, CA, W. H. Freeman, 852 p.</ref><ref name=pt05r55>Feldman, R. M., Chapman, R. E., Hannibal, J. T., eds., 1989, Paleotechniques: The Paleontological Society Special Publication No. 4, 358 p.</ref>

Fossils most commonly used in hydrocarbon exploration are microfossils (generally smaller than about 2 mm) because they can be recovered from drill cuttings without much damage to the fossils. Different processing techniques separate [[Calcareous microfossils|calcareous]], [[Siliceous microfossils|siliceous]], [[Phosphatic microfossils|phosphatic]], and [[Palynomorphs (organic-walled microfossils)|organic-walled fossils]].<ref name=pt05r100>Kummel, B., and R. David, eds., 1965, Handbook of paleontological techniques: San Francisco, CA, W. H. Freeman, 852 p.</ref><ref name=pt05r55>Feldman, R. M., R. E. Chapman, and J. T. Hannibal, eds., 1989, Paleotechniques: The Paleontological Society Special Publication No. 4, 358 p.</ref>

The major groups of marine fossils are [[Calcareous_microfossils#Calcareous_nannofossils|nannoplankton]] (sometimes called [http://dictionary.reference.com/browse/coccolith coccoliths]), [http://www.ucmp.berkeley.edu/foram/foramintro.html foraminifera], [[Siliceous_microfossils#Radiolarians|radiolarians]], [[Siliceous_microfossils#Diatoms|diatoms]], [[Phosphatic_microfossils#Conodonts|conodonts]], [[Calcareous_microfossils#Ostracods|ostracods]], [[Palynomorphs_(organic-walled_microfossils)|palynomorphs]] (such as organic-walled [[Palynomorphs_(organic-walled_microfossils)#Dinoflagellates|dinoflagellates]]), [[Palynomorphs_(organic-walled_microfossils)#Spores_and_pollen|spores and pollen]], and various types of megafossils. Nonmarine facies may yield spores and pollen, and [http://www.merriam-webster.com/dictionary/lacustrinelacustrine] facies may also contain ostracods and diatoms. These and other fossils are described in the ''Encyclopedia of Paleontology''.<ref name=pt05r54>Fairbridge, R. W., Jablonski, D., eds., 1979, The Encyclopedia of Paleontology: Stroudsburg, PA, Dowden, Hutchinson and Ross, Inc., 886 p.</ref>

The major groups of marine fossils are [[Calcareous_microfossils#Calcareous_nannofossils|nannoplankton]] (sometimes called [http://dictionary.reference.com/browse/coccolith coccoliths]), [http://www.ucmp.berkeley.edu/foram/foramintro.html foraminifera], [[Siliceous_microfossils#Radiolarians|radiolarians]], [[Siliceous_microfossils#Diatoms|diatoms]], [[Phosphatic_microfossils#Conodonts|conodonts]], [[Calcareous_microfossils#Ostracods|ostracods]], [[Palynomorphs_(organic-walled_microfossils)|palynomorphs]] (such as organic-walled [[Palynomorphs_(organic-walled_microfossils)#Dinoflagellates|dinoflagellates]]), [[Palynomorphs_(organic-walled_microfossils)#Spores_and_pollen|spores and pollen]], and various types of megafossils. Nonmarine facies may yield spores and pollen, and [http://www.merriam-webster.com/dictionary/lacustrinelacustrine] facies may also contain ostracods and diatoms. These and other fossils are described in the ''Encyclopedia of Paleontology''.<ref name=pt05r54>Fairbridge, R. W., and D. Jablonski, eds., 1979, The Encyclopedia of Paleontology: Stroudsburg, PA, Dowden, Hutchinson and Ross, Inc., 886 p.</ref>

The distribution of fossils in a particular basin depends on the age of the strata, [[Biogeography|biogeographic setting]], [[Paleobathymetry|bathymetry]], [[Depositional environments|depositional environment]], [http://www.thefreedictionary.com/lithology lithology], and [[Diagenesis|diagenetic events]]. Knowledge of these factors influences the decisions of whether to search for fossils in the drill cuttings or core and what fossil group to use.

The distribution of fossils in a particular basin depends on the age of the strata, [[Biogeography|biogeographic setting]], [[Paleobathymetry|bathymetry]], [[Depositional environments|depositional environment]], [http://www.thefreedictionary.com/lithology lithology], and [[Diagenesis|diagenetic events]]. Knowledge of these factors influences the decisions of whether to search for fossils in the drill cuttings or core and what fossil group to use.

===Source rocks===

===Source rocks===

Deposition, concentration, and preservation of organic matter to form source rocks require special environmental conditions. Fossils aid in the recognition of these unique settings, which are primarily defined by [[geochemical]] and [[petrological]] parameters. The Cretaceous [[Mowry Shale]] in Wyoming, for example, is an important source rock and contains from 1 to 5.2% organic carbon.<ref name=pt05r45>Davis, H. G., Byers, C. W., Pratt, L. M., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0009/1100/1103.htm Depositional mechanisms and organic matter in Mowry Shale (Cretaceous), Wyoming]: AAPG Bulletin, v. 73, p. 1103–1110.</ref> The richest source rock is [[homogeneous pelagic mudstone]] that contains layers of [[siliceous radiolaria]], [[Kerogen types|kerogen]], |and fish debris. This mudstone was deposited in a restricted basin, where the water column was [[stratified]] and bottom waters were depleted in oxygen, allowing the preservation of organic matter. [[Anaerobic]] conditions are indicated by the presence of shallow water [[pelagic]] species and by the virtual absence of bottom-dwelling species or traces of animal activity. Lateral changes in the kinds and abundances of fossils and sedimentary structures also give clues to the oxygen gradient in the basin. Nearshore bottom-dwelling [[biota]] are found only at the margins of the Mowry sea.

Deposition, concentration, and preservation of organic matter to form [[source rock]]s require special environmental conditions. Fossils aid in the recognition of these unique settings, which are primarily defined by [[Geochemistry|geochemical]] and [[petrology|petrological]] parameters. The Cretaceous [[Mowry Shale]] in Wyoming, for example, is an important source rock and contains from 1 to 5.2% organic carbon.<ref name=pt05r45>Davis, H. G., C. W. Byers, and L. M. Pratt, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0009/1100/1103.htm Depositional mechanisms and organic matter in Mowry Shale (Cretaceous), Wyoming]: AAPG Bulletin, v. 73, p. 1103–1110.</ref> The richest source rock is homogeneous [http://www.merriam-webster.com/dictionary/pelagic pelagic] [[mudstone]] that contains layers of [[Siliceous_microfossils#Radiolarians|siliceous radiolaria]], [[kerogen]], and fish debris. This mudstone was deposited in a restricted basin, where the water column was stratified and bottom waters were depleted in oxygen, allowing the preservation of organic matter. [http://dictionary.reference.com/browse/anaerobic Anaerobic] conditions are indicated by the presence of shallow water pelagic species and by the virtual absence of bottom-dwelling species or traces of animal activity. Lateral changes in the kinds and abundances of fossils and sedimentary structures also give clues to the oxygen gradient in the basin. Nearshore bottom-dwelling biota are found only at the margins of the Mowry sea.

==See also==

==See also==