Changes

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, ostracods, 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 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 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.

===Geological age===

===Geological age===

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.

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}}). Ostracods 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===

===Lithology===

===Lithology===

Lithology is one of the most useful criteria in selecting the types of fossils used in solving exploration problems. Organic-walled fossils, such as spores, pollen, and dinoflagellates, are found in most argillaceous rocks, but they are rare in clean sandstone and limestone. Calcareous nannoplankton, foraminifers, conodonts, and ostracodes can be recovered from shales, sandstones, and many types of limestone. Siliceous radiolaria and diatoms occur in shale and deep water limestone.

Lithology is one of the most useful criteria in selecting the types of fossils used in solving exploration problems. Organic-walled fossils, such as spores, pollen, and dinoflagellates, are found in most argillaceous rocks, but they are rare in clean sandstone and limestone. Calcareous nannoplankton, foraminifers, conodonts, and ostracods can be recovered from shales, sandstones, and many types of limestone. Siliceous radiolaria and diatoms occur in shale and deep water limestone.

===Diagenesis===

===Diagenesis===