Geology drives changes in geophysics
Salt Stymied GOM Progress
In the early 1990s, exploration success began to dry up in the deeper waters of the Gulf of Mexico.
“We knew there was a world-class working source system and well-developed reservoir. We knew the basin was structured. But we weren’t finding new fields,” recalled Cindy Yeilding.
Yeilding, an AAPG member and former Distinguished Lecturer, is exploration performance unit leader for BP in Houston. She describes her job as exploration manager in the Gulf of Mexico.
At that time, the oil industry considered the Gulf a promising province for deepwater fields, so the lack of progress was even more disappointing.
A series of discoveries in the 1980s, as drilling stepped further offshore, had raised explorers’ hopes.
“The first really big, exciting deepwater discovery was Ram-Powell in 1985. Then Shell and BP made the discovery at Mars (in 1989). Next Ursa followed, along with some other discoveries.
“Then they tapered off right after that,” Yeilding said.
In brief, the prospect evaluation problem could be summed up in one word:
Exploration had progressed far enough offshore to be stymied by the extensive salt bodies that dominate the Texas-Louisiana Outer Continental Shelf (OCS) and slope.
Present-day Gulf salt bodies were once part of the deeper, Jurassic Louann salt, or mother salt.
According to the U.S. Minerals Management Service, allochthonous tabular salt in the Gulf of Mexico characterizes subsalt plays there. The salt can appear in sheets, tongues, nappes or canopies.
Exxon hit on a Gulf subsalt play in 1990, when it found oil and gas on its Mississippi Canyon Mica prospect. But seismic work for the industry began to stumble.
“Going back to the late ‘80s and early ‘90s, it was very difficult to see anything under the salt,” said AAPG member Jerry Kapoor, WesternGeco imaging group manager in Houston.
“It was pretty much like a blurry, snowy TV picture.”
Subsalt in the Spotlight
As we all know now, this story has the happiest of endings.
BP was about to begin the work leading directly to its discoveries at Atlantis, Mad Dog and Thunder Horse.
Along the way, the industry would find Gulf hydrocarbons at Mahogany, Fuji, Enchilada and Gemini.
Those would presage other, major deepwater and ultra-deepwater discoveries in the new century’s first decade, including names like Marco Polo and Tahiti and Great White and Tobago and Jack.
Developing the ability to image, evaluate and explore subsalt prospects offshore is one of the truly great success stories of the modern oil and gas industry.
AAPG’s Annual Convention in San Antonio in April will offer abundant opportunities for insight into the industry’s current understanding of salt and subsalt exploration.
Yeilding and Holly Harrison of BP will serve as co-chairs for a poster session on Subsalt Plays of the World on Tuesday afternoon, April 22.
Harrison is a veteran Gulf of Mexico geologist who like Yeilding has been on the cutting edge of salt and subsalt exploration – including a crucial role in the important Mahogany discovery.
Overviews and updates, ranging from the Gulf of Mexico to Australia to Morocco, will include the current and highly publicized subsalt exploration taking place in the Santos Basin offshore Brazil.
Presentations on Monday, April 21, include the sessions New Insights into Allochthonous Salt Tectonics in the morning and Salt Tectonics at Active and Passive Margins in the afternoon.
Salt tectonics also will be the subject of a Tuesday afternoon poster session, and several other papers and posters throughout the meeting will touch on salt-influenced exploration and development.
Challenges to Overcome
In fact, most presentations will deal with subsalt success stories. But 15 years ago, the industry was losing headway in the Gulf of Mexico’s waters.
BP ruminated on to make sense of its Gulf prospects, “then the logic moved to thinking the updip part of those structures was below salt,” Yeilding said.
In regard to seismic, “most of the water was regional 2-D and subregional 3-D, but it was all time migration,” she noted.
Time-migration imaging conventionally assumes that seismic waves are propagated in straight rays. For the complex subsalt targets in the Gulf, a better method was needed.
In particular, datapoints had to be migrated to or near their actual coordinate positions before traces were gathered into each seismic stack.
The concept of prestack depth migration already existed and had been used for subsalt imaging. However, it had never been applied to 3-D imaging of the complexity and scale necessary for offshore Gulf subsalt.
Yeilding said “a lot of innovation” was needed to meet the challenges of depth migration and velocity modeling for subsalt prospects. Also, acquisition, processing and interpretation proved time-consuming, complicated and very expensive.
“Initially we did work with Western over Mica,” she recalled. “We did a 10 OCS block prestack depth migration. That was really cutting edge.”
Fortunately, advances in computer technology enhanced the handling of very large datasets. Improved processing algorithms developed, and geophysicists continually refined their techniques for subsalt imaging.
“It’s a very leaky industry,” Yeilding observed. “One person will figure out how to do something and then the next person will learn to do it better.”
The 21st Century Breakthrough
While the new approach led to a number of significant discoveries in the Gulf, by 2000 the industry had just about exhausted the possibilities of what began to look like limited data.
“We’d gotten to the point where we’d pretty much processed everything we could get out of narrow-azimuth seismic,” Yeilding said.
“At BP we ran kind of parallel thought processes. First we looked at seafloor acquisition. Then we eventually got to a place where doing wide-azimuth streamer (see page 16) looked reasonable,” she added.
As it turned out, both were feasible. Ocean-bottom cable or seafloor nodes could be used for broader seismic surveys, especially in shallow water.
But for deeper waters and extensive areas, BP turned to acquisition of wide-azimuth seismic using multiple vessels.
“Initially, the biggest challenge we had was logistical. Technically the concepts had always been there. The comment was that you needed a flotilla of vessels to run a wide-azimuth survey,” Yeilding said.
Key to designing a wide-azimuth survey was a solid geological understanding of salt history, subsalt deposition, prospect environment, likely structure and potential geometries – not to mention the nature of the play itself.
“This is one of the places where the geological concepts drove the changes in geophysics,” Yeilding said.
In only a few years, wide-azimuth and multi-azimuth seismic surveys have become the leading edge of geophysical work in the Gulf of Mexico.
“The uptake and the application has been incredibly fast,” Yeilding noted.
“It’s a step-change, but it’s not perfect. We’re still struggling with reservoir-scale imaging with subsalt,” she said.
Anyone attending the AAPG Annual Convention can draw lessons and concepts from the descriptions of subsalt imaging and exploration, according to Yeilding.
It’s a matter of visualizing a process developed to see what had never before been visualized.
“How do I take the concepts that have evolved in the subsalt and apply that to my area? What lies in the unimageable?” she said.
The development of offshore wide-azimuth seismic has taken subsalt imaging to a new level in the Gulf. New breakthroughs could bring further progress.
Yeilding knows that story isn’t over.
“Every month, every week almost,” she said, “there’s a big surprise.”