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The Search for Oil & Gas:
“Bright Spot” Seismic Interpretation

The digital recording and processing of seismic data in the oil industry took quantum leaps in the mid-1960s, making possible a new method of interpretation by allowing geophysicists to measure the “relative wave amplitudes” between seismic traces for the first time.  Up to that point, seismic techniques only helped map subsurface structures and identify possible oil traps.  Operators still had to take the risk of drilling to find oil and gas.  But the new digital seismic data offered the enticing possibility of directly detecting hydrocarbons on the seismic record.  “Direct detection” was based on the principle that the acoustic impedance of a loosely cemented rock filled with hydrocarbons was different from that of a similar water-filled rock, and with advanced digital methods, this difference often could often be detected as an “amplitude anomaly” or “high-amplitude reflection” on the seismic record.  Shell Oil and Mobil Oil were the first companies to identify and quantify such anomalies, and factor them into their bids for offshore leases.  Mobil referred to them as “hydrocarbon indicators.”  Shell called them “bright spots,” a term widely adopted in the industry.

The bright spot story at Shell Oil began with  the 1967 offshore Louisiana and 1968 offshore Texas lease sales.  In mapping the subsurface structure on Prospect 370 using some of the best seismic data available at that time, Mike Forrest, a staff geophysicist in New Orleans, observed what he called a “strong seismic reflector” on the top of the structures, where production would most likely be found.  He also observed strong seismic reflectors on the crests of structures in the Plio-Pleistocene trend in both offshore Texas and Louisiana.  In the spring of 1969, he produced a study that correlated the seismic data on six to eight fields with well logs.  Shell’s exploration vice president R.E. McAdams ordered the Exploration and Production Research Center (EPRC) on Bellaire Blvd. in Houston to establish an Amplitude Analysis Project team in the Basic Measurements and Theory Section at the Bellaire lab to test and verify Forrest’s empirical observations.  Dr. David DeMartini, a EPRC researcher assigned to the New Orleans office to help understand the physics, did early calibration studies using wireline log measurements of velocity and density and wrote computer programs to calculate the expected effects of fluid substitution on porous rock densities, wave velocities and reflection coefficients using Gassmann’s equation and also to interpret the probabilities of various fluid saturants from calibrated seismic amplitude measurements and these calculated values using Bayes’ rule in late 1969 and 1970.  Gene McMahan also did calibration studies in 1969 and 1970 and was the first person to casually use the term “bright spot.”  During this same period, management spread the technique of bright spot interpretation to other exploration divisions with mixed success.

In the December 1970 federal lease sale offshore Louisiana, Shell Oil exploration managers applied bright spot evaluation for the first time, in a significant way, in its offshore bidding.  Under the leadership of Geophysics Manager Dick Grolla, Aubrey Bassett, a geophysicist in the offshore division, wrote a program to quantify the amplitude changes and gas sand thickness so potential oil and gas reserve estimates could be made.  Leighton Steward, the 1970 Geologic Lease Sale Coordinator, used bright spot interpretation to estimate oil and gas reserves on the Eugene Island 330 prospect, which would become the largest field discovered in federal waters on the Louisiana-Texas shelf.  In preparation for the  next major sales, in September and December 1972, researchers at the Bellaire laboratory improved deconvolution and made correlations with velocity and density logs from wells and also demonstrated the quantitive predictions of Gassmann’s equation with ultrasonic frequency velocity measurements made on cores.   

Over the next several years, bright spot technology improved considerably and became a very reliable tool for exploration in all geologic provinces and at greater depths than initially thought.  Shell Oil scanned almost every prospect in the Gulf of Mexico for bright spots and made bids based on quantitative measurements of seismic amplitude and sand thickness.  By reducing the potential for drilling a dry hole and modifying the weighting of risk, bright spot interpretation allowed Shell to put more money into its lease bids and more than make up for it in decreased drilling costs.

The Mobil story began in 1964, when a production geologist working in Mobil’s New Orleans Division wrote a letter regarding the correlation between high-amplitude reflections and a major producing zone in the South Marsh Island Block 23 field.  Mobil geophysicists later noted other high-amplitude reflections associated with structural culminations on prospects offered in several state and federal lease sales between 1965 and 1970.

In early 1970, Bob Hirsch, Mobil’s New Orleans Division Exploration Manager, organized a meeting in New Orleans to review evidence for the direct detection of hydrocarbons on seismic data.  Attendees included Sandy Blattner, Chief Geophysicist New York; Dr. Al Musgrave with the Exploration Services Center (ESC); and Dr. Bob Watson with the Field Research Laboratory (FRL).  A task force was formed for the purpose of developing specific criteria for recognizing and grading what were to be called, at Blattner’s suggestion, Hydrocarbon Indicators or HCI’s.  ESC and FRL agreed to the development of special date processing programs for amplitude preservation and polarity identification. 

A follow-up meeting held in Dallas during the summer of 1970 included attendees from most of Mobil’s larger exploration offices.  Examples of HCI’s from the Gulf of Mexico and offshore Nigeria were shown.  Later meetings included data from other locales with examples of phase-shift and dim-out anomalies.  Mobil’s bids at the December 1970 and subsequent Gulf of Mexico lease sales were almost exclusively based on probabilistic reserve estimates derived from HCI’s.

A number of geoscientists and managers contributed to Mobil’s HCI program.  Notable were Sandy Blattner, Al Musgrave, Bob Watson, Bob Hirsch, Offshore Texas Division Exploration Manager (1972-1975) and Corporate Exploration Manager (1975-1976), Graves Noble, Division Geophysicist in New Orleans (1966-1972) and Regional Geophysicist after 1972, and Lou Kihneman, who followed Hirsch in both New Orleans and Houston exploration managerial positions.  As at Shell, other geologists, geophysicists, engineers, data processors, and researchers also were involved in HCI related activities.  Especially important were the multidisciplinary task forces responsible for various modeling programs including an industry bid model.

Once the technology of bright spots or HCI’s was embraced, it had a giant impact on offshore exploration in the Gulf.  Shell and Mobil first to put money behind the technology in lease sales held in late 1970.  Other companies eventually caught on, helping the industry discover and economically develop fields in water depths extending out to 1,000 feet.  Decreased overall exploration costs afforded by the technology allowed companies to spend more on innovative production technologies, building ever larger steel-jacket fixed platforms in deeper water.      

Recognizing the pioneering efforts of the following individuals and companies who contributed to the development of this technology:

Aubrey Bassett, David DeMartini, Mike Forrest, Gene McMahan, H. Leighton Steward, Shell Oil Company; Sanford “Sandy” Blattner, H. Robert Hirsch, Lou Kihneman, Albert “Al” Musgrave, Graves Noble, Bob Watson, Mobil Oil (now ExxonMobil)


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