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3-D Seismic Sweeps Through Time*
By
R. Randy Ray1
Search and Discovery Article #40115 (2004)
*Adapted for online presentation from the Geophysical Corner column in AAPG Explorer, February 2003, entitled “3-D Seismic Maps Tell Tales,” and prepared by the author. Appreciation is expressed to him and to Larry Nation, AAPG Communications Director, for their support of this online version.
1R-3 Exploration Corp, Lakewood, CO 80228-4358 ([email protected])
Introduction
Three-dimensional seismic has become the tool of preference for petroleum exploration and development. Over the last 20 years, advancing technology in seismic recording systems has allowed the
acquisition
of large areas of 3-D seismic data, and high drilling success rates have fueled the use of 3-D seismic in basins around the world, in both onshore and offshore arenas.
As 3-D seismic has swept across the U.S. onshore, it leaves evidence of past activities where explorers saw potential in various basins. Early surveys were targeted for evaluating infill drilling around existing fields. Over time, surveys spread along productive trends looking for new fields. The current pattern of 3-D seismic coverage is a good indicator of exploration maturity.
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Patterns of ExplorationLocation information
about where 3-D seismic surveys have been shot is available from various
sources. The maps shown here were compiled from public records in
counties where survey locations must be reported as part of the
Looking at 3-D
seismic coverage maps in Texas and Oklahoma (Figure
1) and over the Rocky Mountain basins (Figure
2), one sees a pattern of decreasing seismic coverage as you move
from offshore to onshore areas. Although one might think this pattern
shows the overall cost increase of The exploration of Gulf of Mexico offshore areas is mostly dependent on seismic (no outcrops to map!), so the coverage is usually 100 percent of the area. In fact, many offshore areas have more than 100 percent coverage since there may be multiple surveys acquired by different companies over the same area. Moving onshore in Texas, the coverage stays high at 60 to 70 percent. Early surveys were acquired over fields and then spread along coastline parallel faults looking for new traps. "Bright Spots" and Light SpotsOne reason why seismic is so widespread in the Gulf Coast is because of the enhanced capability of seismic in "soft rock" settings. In Tertiary-and Cretaceous-age clastic rocks, seismic contrasts often allow the direct detection of hydrocarbons, or "bright spot" prospecting. New AVO techniques have extended that capability by allowing more detailed analysis of amplitude anomalies in pre-stack gathers. In the Permian Basin,
seismic coverage spreads over about 50 percent of the basin. Easy In the deep Anadarko
Basin, seismic coverage continues to drop to about 20 percent. And in
the Rockies there is a noticeable lack of 3-D seismic, with overall
coverage down to only 5 to 10 percent of the basin areas. Why? Seismic
Under-Utilized 3-D Seismic in RockiesBased on area coverage, 3-D seismic has been under-utilized in the Rocky Mountain basins. New "Basin Centered Gas" plays, where widespread, low-permeability, gas-saturated sandstones and shales hold large gas reserves, are new targets for seismic evaluation. Seismic techniques for mapping reservoir-enhancing characteristics, like fractures and over-pressured compartments, are developing rapidly. These "sweetspot" properties can be derived by analyzing vertical and horizontal velocity variations in 3-D data volumes. The promise of new discoveries will fuel the continued expansion of 3-D surveys in the Rockies and other areas. |