Changes

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 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 [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.