Biostratigraphic correlation and age determination
|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Author||Robert L. Fleisher, H. Richard Lane|
Correlation of “tops”
Species extinctions, often referred to as “tops,” are used as horizons of correlation. The first downhole occurrence (“+” in the illustration below) in a well section is the datum most commonly used. The inception (“*” in the illustration below), or lowest occurrence of a species or lineage, is a reliable datum only in core or outcrop samples because caving is virtually unavoidable in cutting samples; however, it can help refine the stratigraphy. The overlap of species extinctions and inceptions allows the development of range zones (see figure below), which can be correlated from site to site.
A biostratigraphic zone is a body of rock defined or characterized by its fossil content. The clustering of fossil extinctions often represents missing or condensed sections. Correlation of tops is the most rapid and economical biostratigraphic technique and is the one most commonly used.
Figure 1 shows how the overlap of species' ranges (between inception and extinction) is used to define zones.
Planktonic vs. benthic “tops”
Planktonic (floating) and nektonic (swimming) organisms are generally less affected by local environmental factors such as water depth, physical obstacles, or changes in substrate than are benthic species (bottom dwellers). This characteristic makes the fossils of planktonic forms—particularly calcareous nannofossils, planktonic foraminifera, dinoflagellates, and graptolites—and nektonic organisms such as conodonts excellent regional and even worldwide time markers in marine strata. Summaries of zonations based on the ranges of planktonic microorganisms include Blow Kennett and Srinivasan Bolli et al. Berggren and Miller and Berggren et al..
Benthic taxa are most useful for detailed local correlations and paleoenvironments. Many are too environmentally sensitive, however, to be good regional markers. Distribution of benthic forms is frequently restricted by basin configuration or other barriers to migration.
Changes in abundance or composition
Changes in the abundance or species composition of fossil assemblages within a biostratigraphic zone are useful in refining correlations.
Figure 2 shows how variations in the relative proportion of individual species within assemblages can be used to characterize correlatable fossil “populations.”
Ratios of in situ vs. reworked species
Ratios or percentages of in situ vs. reworked calcareous nannofossil or palynomorph species may differentiate among distinct sediment packages.
Figure 3 shows how a comparison of the presence and abundance of different components of the assemblage (e.g., in situ vs. reworked faunas and floras) may enhance local correlations and help us identify sediment source. Generally, where in situ fossils are relatively abundant, reworked fossils are less common.
High-resolution methods are quantitative approaches that, in addition to species ranges, use subtle paleontological changes (e.g., fossil abundance and diversity peaks). These data can be generated from actual counts of species abundance or from estimates of relative abundance. This detailed, time-consuming approach provides closely spaced correlations that are particularly effective for sequence stratigraphy studies local or field correlation of reservoir units, or any problem where detailed resolution is vital. The use of multiple fossil groups in the same sections can greatly increase both the resolution and the level of confidence in the analysis.
Correlation of assemblages
In the absence of index fossils (e.g., within areas of high clastic dilution or when extending a chronostratigraphic horizon across environmental boundaries), correlation methods other than the traditional use of fossil extinctions play an important role. These frequently involve characteristics of the assemblage as a whole:
- Changes in the abundance or species composition of fossil assemblages within a biostratigraphic zone
- Ratios or percentages of in situ vs. reworked calcareous nannofossil or palynomorph species
An example of the first type is the thin interval in the Lower Pliocene characterized by high abundances of the planktonic foraminiferal genus Sphaeroidinellopsis spp., which represents a set of paleoceanographic conditions that can be correlated in many areas of the Mediterranean.
Assemblage data can also help us recognize diagnostic elements of stratigraphic sequences.
Determining absolute ages through physical or chemical techniques such as radioisotope analysis, magnetostratigraphy, or fission-track dating is not, by itself, a paleontologic application. Approximate absolute ages can be derived for fossil assemblages in strata; a number of time scales have been published relating absolute age to the established sequence of (primarily planktonic) fossil events. Three of the most commonly used scales are Berggren et al., Haq et al., and Harland et al.
Time scales are revised and updated in the literature as new data become available. These time scales differ somewhat in the absolute ages assigned to the various fossil events (inceptions and extinctions). In most applications, the consistent use of a single time scale is more important than the choice of scale. Although absolute ages are not necessarily critical for well correlations, they are vital in studies that rely on determinations of geologic rates.
- Applications of biostratigraphy
- Paleoenvironmental analysis
- Quantitative paleoenvironmental analysis
- Palynofacies and kerogen analysis
- Thermal maturation
- Biostratigraphy in sequence stratigraphy
- North American Commission on Stratigraphic Nomenclature, 1983, North American Stratigraphic Code: AAPG Bulletin, vol. 67, p. 841–875.
- Blow, W. H., 1979, The Cainozoic Globigerinida: Leiden, E., J. Brill, 1413 p.
- Kennett, J. P., and M. S. Srinivasan, 1983, Neogene Planktonic Foraminifera: A Phylogenetic Atlas: Stroudsburg, Pennsylvania, Hutchinson Ross, 265 p.
- Bolli, H. M., J. B. Saunders, and K. Perch-Nielsen, 1985, Plankton Stratigraphy: Cambridge, Cambridge University Press, 1032 p.
- Berggren, W. A., and K. G. Miller, 1988, Paleogene tropical planktonic foraminiferal biostratigraphy and magnetobiochronology: Micropaleontology, vol. 34, no. 4. p. 362–380., 10., 2307/1485604
- Berggren, W. A., D. V. Kent, C. Swisher, and M.-P. Aubry, 1995, A revised Cenozoic geochronology and chronostratigraphy, in W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol, eds., Geochronology, Time Scales, and Global Stratigraphic Correlation: Society for Sedimentary Geology (SEPM) Special Publication 54, p. 127–208.
- Armentrout, J. M., R. J. Echols, and T. Lee, 1990, Patterns of foraminiferal abundance and diversity: implications for sequence stratigraphic analysis, in J. M. Armentrout, and B. F. Perkins eds., Sequence Stratigraphy as an Exploration Tool: Concepts and Practices in the Gulf Coast: Program and Extended and Illustrated Abstracts of the Eleventh Annual Research conference of the Gulf Coast Section of the Society of Economic Paleontologists and Mineralogists Foundation, p. 53–59.
- Cita, M. B., 1975, The Miocene/Pliocene boundary: history and definition, in T. Saito, and L. H. Burckle, eds., Late Neogene Epoch Boundaries: New York, Micropaleontology Press Special Publication 1, p. 1–30.
- Iaccarino, S., 1985, Mediterranean Miocene and Pliocene planktic foraminifera, in H. M. Bolli, J. B. Saunders, and K. Perch-Nielsen, eds., Plankton Stratigraphy: Cambridge, Cambridge University Press, p. 283–314.
- Berggren, W. A., D. V. Kent, and J. J. Flynn, 1985a, Paleogene geochronology and chronostratigraphy, in N. J. Snelling, ed., The Chronology of the Geological Record: Geological Society of London Memoir 10, p. 141–195.
- Berggren, W. A., D. V. Kent, and J. A. van Couvering, 1985b, Neogene geochronology and chronostratigraphy, in N. J. Snelling, ed., The Chronology of the Geological Record: Geological Society of London Memoir 10, p. 211–260.
- Berggren, W. A., D. V. Kent, J. D. Obradovich, and C. Swisher, 1992, Towards a revised Paleogene geochronology, in D. R. Prothero, W. A. Berggren, eds., Eocene-Oligocene Climatic and Biotic Evolution: Princeton, Princeton University Press, p. 29–45.
- Haq, B., U., J. Hardenbol, and P. R. Vail, 1988, Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change, in C. K. Wilgus, B. S. Hastings, C. G. St. C. Kendall, H. W. Posamentier, C. A. Ross, and J. C. Van Wagoner, eds., Sea-level Change: An Integrated Approach: SEPM Special Publication 42, p. 71–108.
- Harland, W. B., R. L. Armstrong, A. V. Cox, L. E. Craig, A. G. Smith, and D. G. Smith, 1990, A Geologic Time Scale—Revised Edition: Cambridge, Cambridge University Press, 263 p.
- Gradstein, F. M., F. P. Agterberg, J. G. Ogg, J. Hardenbol, P. Van Even, J. Thierry, and Z. Huang, 1995, A Triassic, Jurassic, and Cretaceous time scale, in W. A. Berggren, D. V. Kent, M.-P. Aubry, and J. Hardenbol, eds., Geochronology, Time Scales, and Global Stratigraphic Correlation: Society for Sedimentary Geology (SEPM) Special Publication 54, p. 93–125.