Changes

In most wells, the LADs of fossils are the most useful datum planes for subdividing, dating, and correlating the lithostratigraphic section (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 (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 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.

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

===Geological age===

===Geological age===

The geological age of strata determines which fossils may be present. Nannoplankton first evolved in the Late Triassic. Planktic foraminifers evolved during the Late Jurassic and were very diverse during the Cretaceous and Tertiary. Diatoms are found in rocks as old as Early Cretaceous, and radiolaria have a spotty occurrence back to the Ordovician. Ostracodes and benthic foraminifers can be found in marine rocks as old as Ordovician. Phosphatic conodonts are locally abundant from Ordovician to Triassic strata. Of the organic-walled fossils, a variety of marine groups and spores are found throughout the Paleozoic. Marine dinoflagellates were common during the Jurassic. Fossil pollen is no older than Early Cretaceous. Megafossils, large benthic foraminifers, and calcareous algae can be found in most marine strata of the Phanerozoic.

The geological age of strata determines which fossils may be present. Nannoplankton first evolved in the Late Triassic ({{Ma|Late Triassic}}). Planktic foraminifers evolved during the Late Jurassic (({{Ma|Late Jurassic}})) and were very diverse during the Cretaceous and Tertiary ({{Ma|Cretaceous|Quaternary}}). Diatoms are found in rocks as old as Early Cretaceous ({{Ma|Early Cretaceous}}), and radiolaria have a spotty occurrence back to the Ordovician ({{Ma|Ordovician}}). Ostracodes and benthic foraminifers can be found in marine rocks as old as Ordovician. Phosphatic conodonts are locally abundant from Ordovician to Triassic strata. Of the organic-walled fossils, a variety of marine groups and spores are found throughout the Paleozoic. Marine dinoflagellates were common during the Jurassic. Fossil pollen is no older than Early Cretaceous. Megafossils, large benthic foraminifers, and calcareous algae can be found in most marine strata of the Phanerozoic.

===Paleogeographic setting===

===Paleogeographic setting===

==Exploitation applications==

==Exploitation applications==

Fossil assemblages may provide evidence for local environmental conditions that also influenced deposition of potential reservoirs and source rocks. For example, reefs are formed by a community of interacting species that live together under the same conditions. Communities of terrestrial organisms normally are transported and concentrated into fluvial or lacustrine deposits and are mixed with aquatic organisms. Marine organisms may also be transported, and species from different communities may be mixed by storms and other currents. (For more information on depositional environments, see the chapters on “Lithofacies and Environmental Analysis of Clastic Depositional Systems” and “Carbonate Reservoir Models: Facies, Diagenesis, and Flow Characteristics” in Part 6.)

Fossil assemblages may provide evidence for local environmental conditions that also influenced deposition of potential reservoirs and source rocks. For example, reefs are formed by a community of interacting species that live together under the same conditions. Communities of terrestrial organisms normally are transported and concentrated into fluvial or lacustrine deposits and are mixed with aquatic organisms. Marine organisms may also be transported, and species from different communities may be mixed by storms and other currents. (For more information on depositional environments, see [[Lithofacies and environmental analysis of clastic depositional systems]] and [[Carbonate reservoir models: facies, diagenesis, and flow characteristics]]).

===Reservoirs===

===Reservoirs===

===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==

* [[Core description]]

* [[Core description]]

* [[Overview of routine core analysis]]

* [[Overview of routine core analysis]]

==References==

==References==