Changes

[[file:paleontology_fig2.png|thumb|{{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 <ref name=pt05r108>Magoon, L. B., Claypool, G. E., 1981, Petroleum geology of Cook Inlet Basin—an exploration model: AAPG Bulletin, v. 65, p. 1043–1061.</ref>.)]]

[[file:paleontology_fig2.png|thumb|{{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 <ref name=pt05r108>Magoon, L. B., Claypool, G. E., 1981, 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 systems, series, and 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 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 or isotopic ages from 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 systems, series, and 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 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 or isotopic ages from interbedded or cross-cutting rocks such as volcanic flows and 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 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.

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.

==Depositional environments and facies analysis==

==Depositional environments and facies analysis==

Most fossil species required specific conditions of temperature, 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: AAPG Memoir 31, 410 p.</ref><ref name=pt05r141>Scholle, P. A., Bebout, D. G., Moore, C. H., 1983, Carbonate depositional environments: AAPG Memoir 33, 708 p.</ref>.

Most fossil species required specific conditions of temperature, 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: AAPG Memoir 31, 410 p.</ref><ref name=pt05r141>Scholle, P. A., Bebout, D. G., Moore, C. H., 1983, Carbonate depositional environments: 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, siliceous, phosphatic, and 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, siliceous, phosphatic, and 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>

The major groups of marine fossils are nannoplankton (sometimes called coccoliths), foraminifera, radiolaria, diatoms, conodonts, ostracodes, palynomorphs (such as organic-walled dinoflagellates), spores and pollen, and various types of megafossils. Nonmarine facies may yield spores and pollen, and lacustrine facies may also contain ostracodes 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 nannoplankton (sometimes called coccoliths), foraminifera, radiolaria, diatoms, conodonts, ostracodes, palynomorphs (such as organic-walled dinoflagellates), spores and pollen, and various types of megafossils. Nonmarine facies may yield spores and pollen, and lacustrine facies may also contain ostracodes 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 distribution of fossils in a particular basin depends on the age of the strata, biogeographic setting, bathymetry, depositional environment, lithology, and 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, biogeographic setting, bathymetry, depositional environment, lithology, and 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, 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, 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 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, 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, 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.

==See also==

==See also==