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3-D Reveals Furrows on Gulf Floor

Current Technology

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).”

Image Caption

Figure 2 - A northern Gulf of Mexico seafloor map based on NOAA Seabeam data. The location of the study area is outlined, along with the deep-tow survey and DSV Alvin dive locations. Images courtesy of Erik Scott.

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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.”

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