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
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
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
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
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
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
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
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.