seismic has become the tool of preference for petroleum exploration
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.
Patterns of Exploration
Location 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 acquisition
procedure and from the many companies who place their proprietary
surveys with seismic brokers for sale or trade.
Although these maps may not contain
all of the surveys shot, they do represent the general density of
coverage in various areas — and they reveal an interesting and
thought-provoking view of seismic activity and thus exploration
activity since the early 1980s, when 3-D seismic became widely available.
Looking at 3-D seismic coverage
maps in Texas, 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 acquisition, it primarily reflects
the economic success of 3-D seismic in finding hydrocarbon entrapments.
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
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 Spots
One 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 land access
and large ranches encouraged widespread surveys.
In the early 1990s we heard stories
about the Permian of "shoot the 3-D and the prospects will come,"
because of the multi-pay potential in stacked carbonate reservoirs.
Older, harder rocks meant geophysicists could not rely on bright
spots, and there was a push for higher frequency data to resolve
thin porosity zones in high velocity carbonate rocks.
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 acquisition is more
difficult in rough terrain, and federal lands restrict access to
a narrow time window of July 15 to November 15 in many areas.
These onshore areas also are
dominated by independent operators who have limited seismic budgets,
and many proposed large surveys were not shot due to lack of underwriting.
Rockies Under-Utilize 3-D Seismic
Based 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 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.