WALTER, OTIS H. , Consulting Geologist, Mandeville, LA
The identification of higher seismic reflection amplitudes as hydrocarbon indicators to support drilling prospects has a long and successful history. However, due to the fact that a wide range of subsurface conditions do produce higher amplitude reflections, the perception that the amplitude is indeed related to hydrocarbons can go unfulfilled.
Several recent drilling examples from the Eastern Gulf of Mexico shelf illustrate a cause other than hydrocarbon presence for the higher amplitude, bright-spot upon which the prospect was based. Post drilling evaluations of these cases show a range of lithologic causes for the impedance contrasts. Specific examples illustrate a wet reservoir with high contrast at the base, a low impedance nonreservoir facies, and high impedance hard zones within the reservoir intervals.
For each case examined, a uniform template is applied showing the well logs, synthetic seismogram, and the seismic traces through the zone of interest. This process facilitates the identification of the actual cause of the higher amplitude and comparison between wells. A larger database, derived from this type of evaluation completed for an area or trend would prove useful as a source of analogs for modeling as well as cataloging potential pitfalls and improving comparative risk assessment.
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas