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The Offshore Industry Anual
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fossils. Using this principle, geoscientists can determine the "relative age" of the fossil. nce fossils are identified, the group of species found together at a given layer within the sediment or rock can be compared with other well-known successions of rocks. This analysis is known as biostratigraphy, the separa- tion of rock units on the basis of the description and analysis of the fossil species they contain.
Relating the fossils at one strati- graphic level in one area to those in another area at the same level is known as correla- tion. The ideal fossil for bios- tratigraphic correlation, known as the marker or index fossil species, is one that is easily identifiable, abundant, prevails over a wide geographic area, and evolves rapidly. For biostrati- graphic analysis, samples are collected from wells. The samples are labeled by well depth and location. While most fossils are destroyed during the drilling process, microfossils remain intact and are identified. A paleontological report is then made, listing the important foraminifera and coccoliths and the depth at which they were found. The paleontologist transfers this fossil informa- tion into a database where the paleo information from a particular well is correlated with adjacent wells, determining the timing of events. Looking at groups or the assemblage of fossils in the well, a paleontologist can determine what the approximate water depth, or paleobathymetry, was at the time of deposition. This critical information reveals much about the depositional environment, its organic productivity, and the potential for forming hydrocarbons.
Using certain fossil species that indicate the age of sedimentary strata, and other species that reveal the water depth, the geoscien- tist reconstructs time slices and cross-sections to tell the geologic history of the area being studied. The paleontologist can generate various paleo maps that often coincide with sedimentation and structural trends. Integration of microfossil data with seismic and well log data leads to a more complete assessment of hydrocarbon development and reservoir delineation. The paleontologist plays detective with beautiful microfossils, searching for critical clues left untouched or unseen for millions of years. With geologists and geophysicists applying the fossil evidence as age controls onto seismic sections, well logs, and structure maps, those tiny micro- fossils provide an essential framework for regional geologic his- tory...and petroleum fields in particular. (Reproduced, with permission, from the January/February 2005 edition of Ocean Science, Volume 2, Issue 1, a publication of the Minerals
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LEFT: An example of the rapid evolutionary changes in foraminifera over 6.5 million years, from the early Paleocene (64.5 million years ago), at the bottom, to the late Paleocene (58 million years ato), at the top.
RIGHT: The foraminiferan microfossil Globorotalia.
ABOVE: Reconstruction of a Middle Cambrian (505 million years ago sea bot- tom based on fossils found in the Burgess Shale in British Columbia.
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