Thanks to digital data
acquisition and processing, the so-called "bright spot" technology rapidly
evolved during the early 1970s.
New computer programs were designed
by Shell Oil during that decade to measure seismic amplitude changes and
pay thickness, and -- most importantly -- seismic data was being calibrated
with petrophysical data.
Oil and gas were being predicted
in Gulf of Mexico reservoirs at depths up to 10,000 feet. Unfortunately,
several troublesome technical issues were soon recognized, including that
low saturation gas accumulations in a sand (sometimes called "fizz gas")
can cause a seismic amplitude change that could be misinterpreted as oil
pay in a good sand or gas in a poor quality sand.
Shell aggressively applied its
"bright spot" technology on seismic data over all offshore Gulf of Mexico
prospects. The result was Shell had major economic successes at South
Marsh Island 130 Field (Prospect Pine) and Mississippi Canyon 194 Field
(Prospect Cognac).
Prospect Pine was up
for sale at the 1972 Federal Offshore Lease Sale. Several "bright spots"
were identified on the west flank of a shale diapir at depths of 4,000
to 7,000 feet.
Jules Laine used a tight grid
of 2-D seismic data to map structure at several levels and did a quantitative
analysis of the "bright spots." Shell's processing sequence included a
so-called "RUNSUM" section, which was a seismic inversion composed of
the running sum of the amplitudes on each seismic trace. The resulting
integrated traces closely imitated band limited acoustic impedance logs.
The new presentation provides
a means to measure the combined effect of velocity and density changes
at each reflecting interface.
Another program called "ROVER"
was applied to "RUNSUM" sections to quantify the amplitude measurements
and calculate pay sand thickness on selected seismic events.
Laine's quantitative "bright spot"
analysis indicated probable oil pays -- one of the "bright spots" even
appeared to show a two-step anomaly, suggesting gas over oil over water
in the reservoir. A nearby dry hole provided a seismic calibration to
a series of water-bearing sands.
Technology manager Bill Scaife
and petrophysicist Harlan Ritch were assembling petrophysical trend curve
data across the entire Louisiana shelf area. Acoustic impedance data vs.
depth were plotted:
- For wet reservoirs.
- For oil.
- For gas pays in clean and shaly
sands.
- For the adjacent hard (high
impedance) shales and soft (low impedance) shales.
Regional differences were recognized,
and local areas with similar characteristics were grouped into trends
for later quantitative analysis of amplitude variations.
Using probability analysis, the
Shell geological and geophysical team rated each "bright spot" with the
likelihood of oil, gas or water. Using the seismically derived sand thicknesses
and oil/gas recovery factors, prospect volumes were calculated from the
sum of the independent probability estimates. Under the leadership of
lease sale manager Dick Grolla, and geophysics manager Ed Maunder, the
geoscience team used these numbers along with their knowledge of prospect
geology to make a histogram that displayed the reserve estimate distribution
plotted against the probability of success for each prospective sale block.
These data were used to guide
the Shell bids on prospect Pine, as well as other prospects in the 1972
lease sale.
Prospect Pine was estimated to
be over 100 million barrels, and Shell bid and won the two blocks, South
Marsh Island 130 and 129, for approximately $30 million dollars each.
The first wildcat was proposed
at a far downdip location to test the "bright spot" hypothesis that an
oil rim was present. Management was concerned about the near-synclinal
location, but approved the location, and found their confidence in their
staff's interpretation confirmed by discovery of several off-the-crest-of-the-structure
oil and gas pays matching the seismic interpretation.
Pine has a total ultimate of over
225 million barrels of oil and gas equivalent.
In the 1974 Federal Lease Sale,
Shell used the Prospect Pine methodology -- with improvements -- to bid
and win three of the four blocks on prospect Cognac, located in 1,000
feet of water just south of the Mississippi River delta.
Billy Frank mapped the structure,
and Lynn Chenault provided the "bright spot" interpretation -- and gave
a high probability of finding several oil pays on the structure.
Shell bids on three of the blocks
were each over $50 million and one bid exceeded $100 million. These very
high bids were justified on the basis of the high confidence of significant
oil pays being present.
The discovery well was logged
in the middle of a Friday night, and Leighton Steward, division exploration
manager, supplied the champagne and led a 6 a.m. toast.
Shell formed a production unit
with Amoco, who had won the fourth block, and Cognac started production
in 1978. Reserve estimate at the time of the lease sale was about 150
million barrels; Cognac is now expected to have ultimate production of
about 275 million barrels of oil and gas equivalent.
These successes on the shelf and
in the deeper water prepared Shell for its aggressive actions in taking
hundreds of deep water blocks in the series of lease sales in the mid-
to late 1980s … but that's another story.
And what did I learn from these
discoveries?
I think I learned a company should
aggressively make investments on its new technology if the technical experts
and management have mutual confidence that the application of the technology
will lead to economic success.
Is this exactly the way it happened?
Maybe ... at least,
that's the way I remember it.