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Figure 1. Index map of offshore Louisiana, showing location of Main Pass (MP) blocks 122/133 area, Bay Marchand Field (BM), Eugene Island (EI) Block 330 Field, and South Marsh Island (SMI) Block 130 Field.

Figure Captions (1-6)

Figure 2. The first of two seismic lines in Main Pass 122/133 area, offshore Louisiana, showing strong seismic reflections associated with low-relief structure.

Figure 3. The second of two seismic lines in Main Pass 122/133 area, offshore Louisiana, showing strong seismic reflections over crest of low-relief structure, with position of 6000-feet test.

Figure 4. Well log suite of one of several wells drilled in the area through interval with gas sands, corresponding to seismic-amplitude anomalies and with velocities of 5000 feet/second compared to 6700 feet/second in surrounding shales. NFG=net feet of gas (pay).

Figure 5. North-south seismic line across south flank of Bay Marchand Field (South Timbalier Block 26) shows strong seismic event at 3.0 seconds (on the north end of the line) with several hundred milliseconds of relief (about 2500 feet). The exploration well drilled in 1968 had 100 net feet oil that corresponded to the amplitude anomaly.

Figure 6. Seismic line in the Offshore Louisiana Plio-Pleistocene trend, showing several seismic amplitude anomalies on the crest of a Pleistocene structure. The strong amplitudes are at depths of 5000 and 10,000 feet.

Introduction

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 Block 331 (Figure 1) that has 150 million barrels of oil and gas equivalent (MMBOE) and South Marsh Island Block 130 Field (Figure 1) with 250 MMBOE.

“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 block South Timbalier 26 (Bay Marchand Field). North-south seismic line across south flank of that field shows strong seismic event at 3.0 seconds (on the north end of the line) with several hundred milliseconds of relief (about 2500 feet) (Figure 5). Structural mapping indicated the strong reflection conformed to fault closure. Urban Allen, Shell geologist, performed a “fault plane analysis” that indicated the amplitude anomaly (that was assumed to be associated with a sand) was juxtaposed with an oil sand across a small fault. After leasing the block, drilling found the amplitude anomaly corresponded with a 100- to 200-foot oil sand with approximately 100 MMBO.

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.”

Figure 7. Structure map, Prospect Posy, EI 330 Field, contoured on “J” sand, which, at approximately 1.7 seconds, has a good “Bright Spot” that conforms to structural closure. Interpretation by Mike Forrest. Primary crest of structure (in Block 330) and secondary crest (in Block 331) are part of a large closure extending through several blocks. Location of northeast-southwest seismic lines (Figures 8 and 9), as well as the locations of Shell 331 #3, Shell 331 #1, and Pennzoil 330 #1 wells and cross-section through them.

Figure Captions (7-18)

Figure 8. Northeast-southwest seismic line (388-8486) across the main structure that is located in Block 330, with “Bright Spot” associated with “J” sand. Location of line shown in Figure 7.

Figure 9. Another northeast-southeast seismic line (375-1127) in the same vicinity as line in Figure 8, showing “J” sand “Bright Spot.” This seismic line is the same as shown in Figure 6. Location of line shown in Figure 7.

Figure 10. Map of “J” sand “Bright Spot” outline, which conforms to structural closure, as shown in Figure 7. Outline is based on unmigrated seismic data; the Fresnel Zone correction and outline after structural migration are also shown.

Click here for sequence of Figures 7 and 10 (structure and “Bright Spot” outline, respectively).

Figure 11. Seismic line over crest of Prospect Posy, together with Amplitude/Background (A/B) measurements at “J” sand level. Line is through location of Shell#1 well. Measurements were made using program called Payzo, written by Aubrey Bassett, Shell Geophysicist.

Figure 12. Map of Amplitude/Background at “J” sand level, Prospect Posy. In much of the updip area, A/B is about four; there is consistent strong amplitude in the area around the location of Shell #3 well.

Click here for sequence of Figures 7, 10, and 12 (structure, “Bright Spot” outline, and B/A, respectively).

Figure 13. Portion of east-west seismic line 388-6411 through the location of Shell # 3 well. Note the good “Bright Spot” at “J” sand level with a “Flat Spot” indicating a thick pay. Location of line shown in Figure 7.

Figure 14. Map of “J” sand gross thickness, Prospect Posy, prepared by Chuck Roripaugh, Shell geophysicist, from seismic data. Gross thickness of more than 150 feet in northwest portion of Block 331 in area of Shell #3 well thins to less than 60 feet on crest of structure in Block 330.

Click here for sequence of Figures 7, 10, 12, and 14 (structure, “Bright Spot” outline, B/A, and gross sand thickness, respectively).

Figure 15. Portion of well log from Shell # 3 well, showing 160 feet of gross sand in the “J” sand interval, with 66 net feet gas (NFG). Location of well shown in Figure 7.

Figure 16. Geologic cross-section from Shell #3 well to Pennzoil #1 well. Note the multiple oil and gas pays and the thinning of the gross interval and of individual sands from Block 331 to crest of structure in Block 330. Line of section shown in Figure 7.

Figure 17. Seismic acoustic impedance log from data in Shell #1 well, with pays annotated, compared to the seismic data, across the well, in Runsum format. Runsum, developed after the 1970 sale, is a pseudo-acoustic impedance log derived by integrating the seismic trace (running sum = Runsum).

Figure 18. Reflection coefficient vs. depth for Plio-Pleistocene trend. These “trend curves” were derived from petrophysical data acquired in the Pleistocene trend during 1970/1971.

Prospect Posy

Eugene Island (EI) 330 Field, 1969-1970

Development

Posy was one of the first prospects where Shell made detail amplitude and thickness measurements. Shell leased EI Block 331 and discovered 150 MMBOE. Ultimate recovery of the entire EI 330 Field, in the Plio-Pleistocene trend, is 750 MMBOE.

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 Block 330, but a secondary crest is present in the north half of Block 331(Figure 7); both crests are part of a large closure that spans several offshore blocks. Northeast-southwest seismic lines across the main structure in Block 330 illustrate the “J” sand “Bright Spot” (Figures 8 and 9). These are good examples of a prospect with amplitude anomalies in the offshore Pleistocene, or Plio-Pleistocene, area.

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 Block 331 in the area of Shell #3 to less than 60 feet on the crest of the structure in Block 330 (Figure 14). Potential reserves in the “J” sand, along with those in the deeper “L” sand, were calculated by Leighton Steward, Shell geology project leader, and Shell bid and won Block 331 for $13 million in the 1970 lease sale.

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 Block 331 to Block 330 (Figure 16). All of the oil and gas pays correlate with amplitude anomalies of varying quality. The seismic data across the well (shown as a pseudo-acoustic impedance log [Runsum format, which is the mathematical integration of the seismic trace]), compared to seismic acoustic impedance derived from data in Shell #1 well, also show good correlation of seismic and well data (Figure 17).

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.

Figure 19. Structure map, Prospect Pine, South Marsh Island 130, near top of stacked seismic amplitude sequence. Interpretation by Jules Laine, Shell geophysicist.

Summary

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 Block 331 contributing 150 MMBOE.

Figure Captions (19-23)

Figure 20. East-southeast-west-northwest seismic line, Prospect Pine and West Pine. Stacked “Bright Spots” on west flank of the salt/shale diapir are oil and gas pays; the single “Bright Spot” across the syncline to the west at Prospect West Pine is caused by low-saturation gas.

Figure 21. East-west Runsum (pseudo-acoustic impedance log) cross-section, Prospect Pine, of seismic amplitude package.

Figure 22. East-west geologic cross-section through the first two exploration wells in Prospect Pine, a downdip and an updip test, both with oil and gas pays.

Figure 23. Synthetic seismogram from petrophysical data from Shell #1 and #2 wells, Prospect Pine, with oil/gas pay thicknesses.

Prospect Pine

South Marsh Island (SMI) 130 Field, 1972-1973

Development

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 block on the west flank where the best amplitude anomalies were observed (Figure 19). West Pine is shown as a southeast-dipping structure bounded by faults. Laine also made the “Bright Spot” interpretation.

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).

Summary

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

Recoverable Hydrocarbons

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

Good ideas come from operations people; these need to be followed-up with research team(s).
New ideas require persistence. “Beware of the skeptics.”
In hindsight, good ideas are very often simple and easy to understand.
If technical staff and management agree on application of new technology, “go for it.”

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.

References

David S. Holland, John B. Leedy, David R. Lammlein, 1990, Eugene Island Block 330 Field—U.S.A. Offshore Louisiana, in Structural Traps III: Tectonic Fold and Fault Traps: AAPG Treatise of Petroleum Geology Atlas of Oil and Gas Fields, p. 103-143.

David S. Holland, John B. Leedy, David R. Lammlein, 1990, Eugene Island Block 330 Field—U.S.A. Offshore Louisiana, Search and Discovery Article #20003 (http://www.searchanddiscovery.net/documents/97015/eugene.htm).