By MIKE FORREST
'Toast' Was On Breakfast Menu
Editor's note: This is the second of three articles by Mike Forrest, working in collaboration with Marlan Downey, on the application of "bright spot" technology, which helped Shell Oil Company discover many large oil and gas fields in the Gulf of Mexico during the late 1960s to the late 1980s. He also wants to thank Aubrey Bassett for his contributions to the article.
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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:
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.