By MIKE FORREST
Bright Idea Still Needed Persistence
The direct detection of hydrocarbons using seismic data owed more to individual persistence than to pure research.
As a Shell geophysicist in the late 1960s, I called management's attention to a curious, strong seismic reflector on the crest of a shallow structure being mapped for the 1967 offshore Louisiana lease sale. The reflector seemed to have a consistent parallel relationship with structural contours.
It was first thought that the reflection was caused by a "hard streak," a calcareous zone.
In the next year, drilling matched the strong reflector with a shallow horizon containing a 30-foot gas sand.
Russian research papers were found that mentioned direct detection of hydrocarbons by seismic means. In 1968, seismic was being used almost exclusively to map subsurface structural form. Even Shell's research lab was not pursuing the direct detection of hydrocarbons by seismic methods.
During late 1968, the author observed a strong reflection that appeared to tie with a thick pay sand on the south flank of the Bay Marchand salt dome. Strong seismic reflections were also observed in crestal positions on Plio-Pleistocene age structures, offshore Texas and Louisiana.
These were curious reflectors, appearing and disappearing, and were christened "bright spots" in discussions around the office coffee tables.
Most of the technical staff were skeptical that the "bright spot" observations were meaningful.
In the spring of 1969, Glenn Robertson, region manager of geophysics, asked me to document "bright spots" with well data and field studies.
Six fields were studied relating the presence of gas sands to apparent "bright spots." Typical gas-filled sands encountered on well logs had sonic velocities of only 3,000 to 4,000 feet per second, as compared to the overlaying shale with velocities of about 7,000 feet per second and water-filled sands at about 6,000 feet per second.
When this documentation was shown in June 1969 to Shell's VP for exploration, R.E. McAdams, "Mac" instantly recognized the importance to exploration of the "bright spot" phenomena.
An operations and research team was immediately organized to buttress the observations with fundamental physics, and to determine the exact circumstances under which this recognition of hydrocarbon technique would or would not work.
Shell found they needed much tighter teamwork between petrophysicists, geologists and geophysicists. All sonic and density observations in the borehole needed to tie the seismic, and vice versa.
Seismic data became vastly improved because of digital data processing, which helped preserve seismic relative amplitudes. An important validating criterion for a "bright spot" was the coincidence between the "bright spot" downdip limit and structural closure.
Shell maintained extreme confidentiality about the "bright spot" concept, but when the company went to an early 1970 sale, and representatives bid on a low relief structure with a "bright spot" located east of the Mississippi delta, they were out-bid by Mobil.
Ray Thomasson, Shell's offshore exploration manager, felt that Mobil's very high bid indicated that they understood "bright spots," too, and perhaps the competition knew more than Shell realized.
Later in 1970, Shell changed its technical focus toward quantifying the seismic amplitude changes and calculating pay sand thickness using a program developed by Aubrey Bassett. Of special interest were Eugene Island blocks 330 and 331, which were included in the 1970 offshore Louisiana lease sale.
The author mapped six bright spots on the prospect, named Posy by Shell. Only two of the "bright spots" were used in the bid evaluation, because of the stringent technical criteria being employed.
Chuck Roripaugh's pay thickness map from seismic, thought to be the first of its kind, indicated a thick sand on the flank of the structure in block 331, and the sand thinned toward the crest of the structure in block 330. Leighton Steward, lease sale project leader, calculated the prospect's oil and gas reserve potential.
Shell bid $13 million on both blocks, and won block 331. A partnership led by Pennzoil won block 330 with a $28 million bid. The Pennzoil block is the best block in the field with production over 260 million BOE, while Shell's block has about 150 million BOE.
Shell bid and lost two other Posy blocks that have an additional 150 million BOE. All six of the original "bright spot" horizons over prospect Posy were shown by drilling to be oil and gas reservoirs.
Shell was disappointed by the incompleteness of their success in the 1970 sale, but the sale review demonstrated that oil could sometimes be differentiated from gas by "bright spot" analysis.
Shell's research and operations team, under the leadership of Billy Flowers, chief geophysicist, used the data from the 1970 sale to improve the application of the technology.
Subsequently, Shell gained confidence and became much more aggressive in its use of "bright spots" in future sales in the Gulf of Mexico -- and the confidence in the technology was much in evidence with Shell's domination of the deep water sales of the 1980s. Quantitative analysis of seismic amplitudes became a major technical focus of Shell's exploration in all areas of the world.
What can we learn from the history of "bright spot" exploration within Shell Oil Company?
Well, perhaps that every great new idea looks a little silly -- at the start.
That every new idea is faced with skeptics.
That every great new idea needs to be coupled with persistence.
And that an operations/research multidisciplinary team can yield innovative technology -- and that can lead to economic success.
Is this exactly the way it happened?
Maybe not ... but that's the way I remember it.