Reefs Hid 'Between the Lines'

Like many major discoveries, the Michigan pinnacle play wasn't obvious -- at the start.

For Shell, the concept of a Silurian shelf margin carbonate bank, with associated pinnacle reefs, arose out of a regional study made by Peter Lucas in 1962 and 1963. Pete's concept drew on analogies with Ontario pinnacle reefs, and on work on recent carbonate sedimentation models.

Reed Peterson, Shell district manager, persistently championed the technical concept and convinced Bert Bally, Shell chief geologist, of its reasonableness.

Gerry Pirsig, Shell's Denver area exploration manager, was intrigued by the hope for seismic detection of the carbonate bank and the hypothesized reefs. Pirsig established a geologic-geophysical team to create an exploration plan. Fabiano Lobato designed an experimental seismic acquisition and processing approach to overcome the obstacle of shooting through a layer of glacial drift with highly variable thicknesses and lithologic properties.

The first experimental seismic, in 1963, used squiggle traces for interpretation. Careful review of the field records indicated much better data when the charges were placed in the water table.

In 1964, the experimental effort resulted in variable area recording (VAR), and six-fold stack. Each year, the experimental team provided a little better data.

The Michigan play concept had two elements:

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Like many major discoveries, the Michigan pinnacle play wasn't obvious -- at the start.

For Shell, the concept of a Silurian shelf margin carbonate bank, with associated pinnacle reefs, arose out of a regional study made by Peter Lucas in 1962 and 1963. Pete's concept drew on analogies with Ontario pinnacle reefs, and on work on recent carbonate sedimentation models.

Reed Peterson, Shell district manager, persistently championed the technical concept and convinced Bert Bally, Shell chief geologist, of its reasonableness.

Gerry Pirsig, Shell's Denver area exploration manager, was intrigued by the hope for seismic detection of the carbonate bank and the hypothesized reefs. Pirsig established a geologic-geophysical team to create an exploration plan. Fabiano Lobato designed an experimental seismic acquisition and processing approach to overcome the obstacle of shooting through a layer of glacial drift with highly variable thicknesses and lithologic properties.

The first experimental seismic, in 1963, used squiggle traces for interpretation. Careful review of the field records indicated much better data when the charges were placed in the water table.

In 1964, the experimental effort resulted in variable area recording (VAR), and six-fold stack. Each year, the experimental team provided a little better data.

The Michigan play concept had two elements:

  • A shelf margin.
  • The hypothesized basinal reefs.

The Silurian shelf margin play was much larger and easier to map. Reservoir quality in the margin was questionable, and sealing configurations were doubtful.

The reef play in northern Michigan was only a concept, but if Silurian reefs were present, they were likely to be porous and would provide well-sealed traps. Historical data from southern Michigan and Ontario, however, indicated that known Silurian reefs were generally very small and contained only trivial quantities of hydrocarbons.


The cost of acquiring the experimental seismic in Michigan was astronomical and set new records for cost per mile of data.

Results of the experimental seismic acquisition were brought into the Denver office for analysis. Geologists Russ Baughman and Sam McCulloch partnered with geophysicists Leo Buonasera and Ed Johnson to create seismic-geologic models describing the play concept in acoustical terms. Leo Buonasera was given the difficult task of interpreting the data.

Reflectors appeared, and disappeared; no drape or structural reversals were seen. It was very disappointing to most of us. I remember looking at a seismic line in Buonasera's office, and being astonished to hear Leo say, "I think there's something there!"

"There?" I asked, "There? Where there is no data?"

Shell Exploration always made a ranking of all its new exploration plays, as units competed for funding. In 1967, when I brought Gerry Pirsig the ranking of potential new plays, I had the Michigan play ranked in fifth place for exploration opportunity, with an economic benefit to Shell of 80 million barrels.

Gerry smiled and said, "Marlan, let's increase the potential to 100 million barrels and move it up two places. It may not look worth doing, otherwise!"

Pirsig agreed to test a concept that interruptions in seismic data continuity might mean that reef growth altered lateral seismic continuity. Shell laid out a three-well drilling program on its sparse experimental seismic grid; one well in a non-anomalous area for calibration, one in the shelf margin, and one in a data-absent zone where a reef was suspected.

Shell and Amoco were pursuing astoundingly similar concepts, neck-and-neck, and the first wildcat discovering a Silurian reef in 1970 touched off a fierce land play for acreage in northern Michigan.

Seismic acquisition was extremely expensive, and fears were expressed in Shell that it would spend more money on seismic than it might make on oil from Silurian reefs.

To gain an idea of the real scope of the pinnacle play, a count was made of the size and frequency of "acoustic-anomalies-that-might-be reefs" as seen on the Shell grid of experimental seismic. After counting the anomalies encountered on the few widely spaced seismic lines, the numbers were extrapolated by sampling theory to the entire prospective area.

(This mathematical approach, by the way, came from work done during World War II on optimum search theory to find submarines).

When the results were presented to management, a comment was made, "A billion-barrel field in a thousand places!"

Seismic techniques for mapping the reefs improved rapidly; unit costs dropped, reefs became "obvious" on the seismic data and wildcat success ratios soared. Shell geologists and geophysicists had a "field day" as exploration operations discovered hundreds of new reef fields. It looks easy to recognize a Silurian pinnacle reef -- now.

Shell's technical emphasis in the Michigan reef play switched from exploration-as-guided-by-seismic to design of efficient development practices that would allow profitable production from hundreds of very small fields, each only one or two hundred acres in area.

The Michigan reef play was the most profitable single onshore play for Shell over a 30-year period, and yielded gross recovery of over 350 million barrels of oil and 2 TCFG.


And what did I learn from watching the Michigan Basin pinnacle reef play?

I learned a lot.

I think I learned:

  • That major discoveries require sound technical and business ideas and an unbroken chain of management supporters.
  • That appropriate technology can provide an incredible advantage to see fields where others see nothing.
  • That great seismic interpreters are worth their weight in gold.
  • That actual success is often greater than expectation.
  • That exploration for difficult targets requires persistence and a sound concept evaluation plan.
  • That a hundred profitable little fields can be as good as a single large field.

Of course, this may not be the way it all happened -- but that's the way I remember it.

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