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Gulf of Mexico "Bright Spots" - Early Shell Discoveries*
By
Michael C. Forrest1
*Adapted as online version from oral presentation made at AAPG Convention, Houston, Texas, March, 2002.
1Consultant (Shell Oil Company, retired), Dallas, Texas ([email protected])
Acknowledgment: Thanks to Shell Offshore in New Orleans for permission to make this presentation and allowing the author to review old files so that data “from the time” could be included in this article.
Abstract
The author observed a strong seismic reflection, with attenuation below the event, at a depth of 3000 feet on the crest of a low-relief structure in Main Pass area, offshore Louisiana during 1967. The most likely interpretation was that a calcareous zone, a “hard streak,” caused the strong reflection. Later, two exploration wells penetrated the shallow reflection and found a 25-foot gas pay with very low sonic log velocity, a “soft” reflection. Also, an amplitude anomaly with about 2500 feet of relief at a depth of 12,000 feet was observed to be associated with a thick oil sand.
During 1968 and early 1969, strong seismic reflections were observed on exploration prospects in the offshore Texas and Louisiana Pleistocene trend. Digital acquisition and processing preserved the relative amplitudes of seismic data in contrast to automatic gain control. The term “bright spot” was coined during informal discussions. Seismic was primarily used to map structure at that time, and most geoscientists doubted the relationship of “bright spots” to gas/oil pays. During mid 1969, six oil and gas fields were studied in the offshore Louisiana Pliocene trend, and observed “bright spots” were correlated with gas sands with a very low sonic log velocity. Shell management formed an operations/research team to study seismic amplitude changes related to gas and oil pays.
The first significant application of “bright
spot” technology was in 1970 when Shell technical staff predicted the thickness
of a gas sand and mapped other pays on Eugene Island Block
331 (150 MMBOE).
During 1972, Shell predicted oil pays in the discovery of South Marsh Island 130
Field (250 MMBOE). Many other discoveries followed, especially Cognac in deep
water.
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This article describes oil exploration
studies that were performed in Shell Oil Company (USA) during 1967 to
1972. Strong seismic reflections were observed over crests of structures
in the Gulf of Mexico. After a calibration study was completed where
petrophysical data were available, an operations/research team was
formed, and “Bright Spot” technology was applied in acquiring leases in
the 1970 and 1972 federal offshore lease sales. Shell early discoveries
include Eugene Island “Bright Spot” History
In 1967, Mike Forrest, Shell geophysicist, observed high amplitude reflections that appear to conform to the shallow crest of a low relief closure in the Main Pass (MP) 122/133 area, offshore Louisiana (Figures 2 and 3).
In early 1968, as illustrated by one of several wells drilled in the area (Figure 4), gas sands at 2000 feet to 3000 feet correspond to seismic amplitude anomalies. Sonic log shows that velocity of gas sands is less than 5000 feet/second compared to 6700 feet/second in surrounding shales, thus causing the strong reflections. The author read a Russian geophysical abstract that discussed the theory and an example of direct detection of oil and gas pays using seismic data. (Today, MP 133 area is a 300 BCF shallow gas field that was developed starting in the mid 1970’s.)
In late
1968, Shell was preparing to bid at a lease sale on offshore
During the late 1960’s, seismic acquisition and processing changed from analog to digital format, thus allowing the preservation of “relative seismic amplitudes” along a seismic trace and between traces. Seismic amplitude anomalies were observed on the crests of many structures in the Offshore Louisiana and Texas Plio-Pleistocene trend. One prospect (Figure 6) showed strong amplitudes at depths of 5000 and 10,000 feet, thus providing an economic incentive to study the meaning of the strong reflections. Because the Pleistocene trend was essentially an unexplored province at the time, no well data were available to help determine the cause of the strong reflections.
In 1969, the term “Bright Spot” was coined in Shell New Orleans office. Gene McMahan. Shell geophysicist, looked over Mike Forrest’s shoulder one day and stated, “Those strong reflections look like bright spots.” The term became a fixture after discussions in coffee shop and meetings.
During the spring of 1969, Mike Forrest reviewed several gas and oil fields in the Pliocene/Miocene trend in the shallow waters of the Louisiana shelf. A data package was prepared that showed gas/oil pays related to “Bright Spots” on seismic data and calibrated to low impedance (velocity multiplied by density) intervals on well logs. These field studies were shown to R.E. “Mac” McAdams, VP Exploration, who immediately formed an operations research team to study and calibrate seismic “Bright Spots.”
During the next year, there were several drilling successes and failures based on “Bright Spots” observed on then current Shell leases; this time was referred to as “Peak and Valley Days.”
Click here for sequence of Figures 7 and 10 (structure and “Bright Spot” outline, respectively).
Prospect PosyEugene Island (EI) 330 Field, 1969-1970
Posy was one of the first prospects
where Shell made detail amplitude and thickness measurements. Shell
leased EI
Prospect Posy at “J” sand level, at
approximately 1.7 seconds, has a good “Bright Spot” that conforms to
structural closure, as interpreted by the author. The primary crest of
the structure is located in the middle of
The outline of “J” sand “Bright Spot,” based on unmigrated seismic data, is shown in Figure 10. Amplitude/Background (A/B) measurements at “J” sand level (Figure 11) were made, using the program Payzo written by Shell Geophysicist Aubrey Bassett. The map of Amplitude/Background measurements at the “J” sand level shows a consistently strong amplitude in the area around Shell #3 well, and A/B in much of the updip area is approximately four (Figure 12).
Figure 13
is part of an east-west seismic line through the location of Shell # 3
well. The good “Bright Spot” at “J” sand level, with a “Flat Spot,”
indicates a thick pay. A map of “J” sand gross thickness, derived from
seismic data by Chuck Roripaugh, Shell geophysicist, shows a maximum
thickness of more than 150 feet in the northwestern part of
There was a good match between seismic
interpretation and well data (Figure 15). In Shell #3 well, the “J” sand
interval has 160 feet of gross sand, with 66 net feet gas. Further,
additional well data confirmed the thinning of the gross interval and of
individual sands from
Crossplots called “trend curves” show reflection coefficient vs. depth for gas, oil, and wet sands (Figure 18), derived from petrophysical data acquired in 1970/1971 from wells in the Pleistocene trend, were used to help interpret amplitude anomalies in the 1972 Federal lease sale. Petrophysical “trend curves” are more complex than shown, as sand quality plays a major role in petrophysical measurements and interpretation.
The first mapping of
Amplitude/Background and thickness was performed by Shell at Prospect
Posy. The “J” sand was characterized as thick gas pay, and in hindsight
the “L” sand was the first oil “Bright Spot.” In addition, several other
qualitative “Bright Spots” defined pays. As noted above, Eugene Island
330 Field has ultimate recoverable hydrocarbons of 750 MMBOE (Holland et
al., 1990;
1999), with Shell
Prospect PineSouth Marsh Island (SMI) 130 Field, 1972-1973
First detail application of Runsum seismic processing was made at Prospect Pine, and there was calibration to petrophysical trend curves. “Bright Spots” were used successfully to predict oil pays; this was very important at the time as oil was a much more valuable resource than gas.
In 1972,
the Prospect Pine structure was depicted by Jules Laine, Shell
geophysicist, as a small salt/shale diapir with a radial fault pattern
and a downthrown fault
Stacked “Bright Spots” are present on west flank of Prospect Pine, and a single “Bright Spot” is present across the syncline to the west at Prospect West Pine (Figure 20). The Pine amplitudes are oil and gas pays, whereas the West Pine amplitude, which has the same measured amplitude as an oil pay across the syncline at Pine, is caused by low-saturation gas in a sand. Shell tested low quantities of gas, and the sonic log showed cycle skipping, suggesting that the sand had about 10% gas saturation. The theory of low-saturation gas sands having seismic amplitudes as high-saturation gas sands was developed at about the same time as the West Pine observation.
Figure 21 shows the Runsum of the Prospect Pine seismic amplitude package in an east-west cross-section, with potential pays being represented by this format of integrating the seismic trace. For comparison, a geologic cross-section through the first two exploration wells in Prospect Pine shows oil and gas pays in both a downdip and an updip test (Figure 22). The synthetic seismogram derived from petrophysical data from two exploration wells at Prospect Pine shows features correlative with oil/gas pay thicknesses (Figure 23).
The first application of Runsum processing and petrophysical “trend curves” was at Prospect Pine/West Pine. The original estimate of 100MMBO was based on high probability “oil calls.” The current estimate of total recoverable hydrocarbons is more than 250 MMBOE. First recognition of low-saturation gas problem (LSG) was at West Pine.
Concluding Remarks
Shell discoveries using “Bright Spots” on the shelf of the Gulf of Mexico (GOM) are estimated to found 1.5 to 2 BBOE. In GOM deep water, the estimate of recoverable hydrocarbons is approximately 4 BBOE. The present of “Bright Spots” was a key factor in entering GOM Deep Water during 1983 to 1986.
Lessons from Shell initial “Bright Spot” studies and Prospect Posy and Pine successes
Shell Team Members - 1967 to 1972 Successful exploration is always a team effort. Each of the persons listed below played a significant role in “Bright Spot” technology studies.
Billy Flowers, Glenn Robertson, Mike Forrest, Urban Allen, Dick Grolla, Aubrey Bassett, Chuck Roripaugh, Ray Thomasson, Leighton Steward, “Mac” McAdams, Miner Long, Manny Baskir, Dave DiMartini, Gene McMahan, Lee Backsen, Bill Scaife, Harlan Ritch, Sam Mitchell, J.T. Smith, Jules Laine, and others.
David S.
Holland, John B. Leedy, David R. Lammlein, 1990, Eugene Island
David S.
Holland, John B. Leedy, David R. Lammlein, 1990, Eugene Island |