Buoyed by respectable commodities prices
and armed with a passel of innovative drilling technologies, oil
and gas finders increasingly are casting their collective eyes toward
the potential for big finds in the deep-water Gulf of Mexico (GOM).
Of course, finding the “big one” is only the beginning
in this tricky operations environment.
Once the find is confirmed, the operator has the
thorny problem - among other issues - of moving the product to market.
Pipeline and other infrastructure placement, however, is no easy
undertaking in thousands of feet of water, which is rife with unknowns.
Using 3-D seismic data, however, scientists can map
the seafloor at greater depths and higher resolution than ever before
to garner critical information to aid in deep-water development.
A recent mapping project using high-resolution 3-D,
for instance, has identified some “new” old features on the muddy
ocean bottoms. The data derived from the study can be a powerful
assist to determine the initial location of deep-water facilities
and pipeline paths, and to help define the parameters of additional
site-specific investigations.
The research effort was a joint undertaking between
BHP Petroleum and the Texas A&M Department of Oceanography.
The seismic data set used by the study team covers
the seafloor along the Sigsbee Escarpment on the continental rise
across an area of more than 165 GOM OCS blocks in the western Atwater
Valley, southern Green Canyon and eastern Walker Ridge protraction
areas.
Water depths there range from 4,000 feet to 9,000
feet.
While this is the first mapping exercise to use 3-D
data on a regional scale, the GOM seafloor has been mapped before
using a mélange of methods, with varying resolutions.
“Maps covering the whole of the northern Gulf of
Mexico and more have been generated from such data sources as gravity,
magnetics, satellite sensors and 2-D seismic surveys,” said BHP
geologist Erik Scott. “Until recently, however, the highest resolution
data sets with the most extensive coverage have been surface-towed,
multi-beam, side-scan sonar (NOAA Seabeam) and long-range sonar
(GLORIA).”
These data sets, he noted, have been used to generate
maps with approximately a 30-meter resolution on the seafloor, but
the resolution is lower at deep and ultra-deep water depths because
of the increased travel time from source to receiver.
“Two-D seismic data provide a higher resolution of
the seafloor,” Scott said, “but because of their nature of a lattice
of separate lines, they lack the spatial resolution to define subtle
features on the seafloor.”
An Excellent Reflector
Three-D seismic surveys, thanks to their close spacing
of data points, offer greater spatial resolution than does 2-D seismic
to map the seafloor and illuminate subtle features.
In the past, however, cost and technology limitations
restricted those surveys to limited areal extent of usually less
than 25 GOM OCS blocks - which meant maps generated from those data
provided a postage-stamp look in a regional context of the northern
GOM.
With the growing movement toward progressively deeper
waters to explore for hydrocarbons, 3-D seismic surveys are now
available on a regional scale. For instance, the former WesternGeco
acquired and processed a regional 3-D seismic data set in the Green
Knoll/Walker Ridge area.
The seafloor makes an excellent acoustic reflector,
producing a strong horizon to map, according to Scott. By mapping
the seafloor reflector on the WesternGeco data, he and his peers
constructed a higher resolution map than previously possible and
came up with an intriguing discovery.
“The data revealed a set of regionally extensive
furrows - a previously unknown Gulf of Mexico seafloor feature,”
Scott said.
“Early investigations show the erosive style of the
furrows change in a predictable pattern in conjunction with an increase
in current flow velocities,” he continued, “and the erosional features
of the furrow field indicate the presence of strong ocean bottom
currents.”
Seafloor maps derived from the seismic data depict
a field of regionally extensive bed forms comprised of furrows,
anti-dunes and other related features on the sea bottom on the abyssal
plain south of the Sigsbee Escarpment and around Green Knoll - a
prominent feature rising more than 2,000 feet from the surrounding
surface.
The bed forms also have been imaged with a deep-tow
survey over the Bryant Fan area further to the southwest.
Although the survey area shows only the partial extent
of these bed forms, Scott said, it is sufficient to illustrate the
regional nature of the features.
Seeing Proves Believing
The most prominent attribute on the regional dip azimuth
map created by the research team is a set of linear features that
extend from Green Knoll and off both the east and southwest edge
of the survey area. They are much like the bed forms imaged over
the Bryant Fan area that were identified as sedimentary furrows,
roughly 10 meters deep, 30 meters wide and spaced about 100 meters
apart in an area 10-25 kilometers wide, lying to the south of the
Sigsbee Escarpment.
Such sedimentary furrows are longitudinal bed forms
that are found in fine-grained sediments caused by bottom-current
erosion. They are found in myriad environments, including lakes,
rivers, shallow marine settings and on the continental rise.
The research team noted that initial investigations
of the data show that the furrows change in a predictable pattern
in conjunction with an increase in current flow velocities, according
to findings by J.R. Allen, who performed controlled flume experiments
on furrow formation in 1969.
Scott said the transition in furrow morphology from
increasing flow velocities is clearly shown in the deep-tow data
from Bryant Fan.
“The experimental work by Allen and the nature of
the furrows that have been identified from maps from the regional
3-D seismic data indicate bottom currents of up to two knots and
possibly stronger have occurred,” Scott said.
“In fact, a current meter south of the Sigsbee Escarpment
measured a two-knot current on the ocean bottom in a southwesterly
direction.”
Characteristics of the furrow field on both a regional
and a local scale are illuminated by the seafloor dip azimuth map.
The map indicates the current that formed the furrows and related
features is flowing along the Sigsbee Escarpment from the northeast
toward the southwest.
Besides the linear nature of the furrows, a host
of cross-cutting relationships can be observed. Whether these indicate
multiple current events or a change in a single event is currently
unknown, according to the research group.
The furrow field wraps around Green Knoll, providing
evidence of the topography’s influence on bottom currents along
the Sigsbee Escarpment.
Scott said direct observations of the sea floor at
two locations in the furrow field confirm the presence of the furrows.
Dives were made in the DSV Alvin in the Farnella Canyon area (Walker
Ridge 805) and off the southeast corner of Green Knoll (Walker Ridge
35). The general seafloor was flat to slightly undulating and interrupted
by erosional furrows scouring into the muddy surface.
Future Impact
The potential for strong bottom currents in the deep-water
GOM could impact energy industry activity.
For now, three wells have been drilled on the continental
rise, one is currently drilling and another is planned. The wells
haven’t encountered any significant currents, and although current
meters gave no indication of strong currents, they were in place
only long enough to drill the wells.
The regional extent of the furrow field will affect
pipeline placement. Regional seafloor maps can help direct the pipeline
path and also assist in the initial design of the pipeline, indicating
in general terms which parameters to engineer to.
It’s not known how often bottom currents might occur
over the life of a producing field - and neither the strength nor
the duration can be surmised. Scott said it appears the currents
occur over geologic time rather than over field production time,
and could occur on the order of every 10, 50, 100, 1,000 or 10,000
years.
And we’re not talking cataclysmic-style events.
“It doesn’t appear it’s a one-off event where all
of a sudden a huge current comes and forms the furrows,” Scott said.
“My personal opinion is that pulses of higher current flows come
through that form furrows, and there are constant currents that
keep them open.
“The data we have show there are currents today at
Green Knoll, where we found furrows with the Alvin, but 60 miles
away there are no currents at all,” he said. “But there are scours
and flute marks to provide evidence that currents were there, just
not for a while.
“It’s clear that seafloor and near-seafloor maps
from regionally extensive 3-D seismic data can aid in the development
of the infrastructure in the deepwater Gulf of Mexico.”