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The integration of all aspects of geology with geophysics is necessary in any stratigraphic exploration program. The Niagara (Middle Silurian) pinnacle reef play in the Michigan basin provides interesting insights into this multifaceted problem.
The Michigan basin is an intracratonic structural basin which has been in existence at least 500 million years. During Niagaran time, tall (up to 700 ft, 213 m) hydrocarbon-bearing pinnacle reefs grew on a ramp or shelf between a deeper basinal area and a nonproductive basin-rimming barrier reef bank. These pinnacle reefs are encased in Salina (Upper Silurian) evaporites and carbonates that effectively act as both seal and source for the hydrocarbons.
Geological studies of Niagara and Cayugan paleogeography and regional stratigraphy combined with detailed lithofacies have aided in the mapping of the pinnacle reef trends and the identification of local reef proximity indicators.
Today, seismic data is the major exploration method for actually locating the pinnacle reefs, but major problems had to be overcome before it was an effective tool. Surface topographic features (such as glacial moraines and sand dunes) plus buried preglacial valleys caused severe statics, velocity, and "noise" problems. Judicious application of good basic data processing procedures with particular attention paid to statics corrections, velocity analyses tied to well control, and f-k filtering, commonly solve these data processing problems.
Even though shooting geometry has been optimized to reduce "noise," reflections are often the third or fourth strongest mode on a field record with mode converted shear waves, refractions, multiples, and wave-guided phenomena being stronger. F-k filtering, following the principle of reflection mode amplification (introduced herein), can substantially enhance the signal to noise ratio (reflection to nonreflection ratio).
The variable distribution of the Salina evaporites and carbonates and their irregular solution margins can, however, in places, produce reeflike seismic anomalies. By careful geologic mapping with close attention to facies and lithologic detail and by using simple seismic modeling, the differences between pseudo-reef anomalies and actual reef anomalies can often be distinguished, and the presence of seismic reef proximity indicators can be confirmed with a high level of confidence. The 70% and greater wildcat success ratios maintained consistently by several companies which has allowed them to lead in the exploration of the trend are ample testimony to this.
The final results of careful attention to geologic detail during the entire geophysical analysis, from field acquisition through interpretation, are not only accurate and detailed seismic stratigraphic interpretations, but also cost-effective exploration programs.
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