Downey: Thinking Like Oil

American Association of Petroleum Geologists (AAPG)

I would like to suggest that far too much of the technical work purporting to guide exploration for petroleum is trivial, redundant and has little of use to offer toward finding new oil and gas accumulations.

All geology is interesting; some geologic work is novel; damn little of the work we see is useful in finding new oil and gas fields!

All sorts of “interesting” work is being published in sedimentation, stratigraphy, structural analysis, geochemistry, petrophysics and geophysics. Try to remember that only academics are being paid for their advancement of science; you are being paid to find oil and gas.

In exploration, you will be looking at data many already have seen; you need to think what no one else has thought!

As Oklahoma’s famous humorist Will Rogers said, “It ain’t what we know that gives us trouble, it’s what we know that ain’t so!”

Recently, I attended an international conference, and in my summary comments I said, “In 35 presentations, I saw ONE illustration, in one talk, that gave useful guidance to finding oil.”

It is very, very hard to find buried treasure (new oil and gas fields); the search needs our best talents, our brightest and most innovative people, and the search needs to be focused, like a laser, on locating underground oil and gas.

AAPG Honorary member, past president and Sidney Powers medalist Marlan Downey presented these remarks at last year's AAPG Mid-Continent annual meeting in Wichita, Kan.
AAPG Honorary member, past president and Sidney Powers medalist Marlan Downey presented these remarks at last year's AAPG Mid-Continent annual meeting in Wichita, Kan.

There are a few simple laws that govern where oil forms, how it moves, and where it chooses to reside.

Do you know the rules?

Let’s pretend oil can talk – and let’s listen.

In oil’s origin, migration and current residency, oil always obeys rather simple chemical and physical laws. Since oil always obeys these laws, shouldn’t you know them?

Oil knows that we need to start with abundant, naturally accumulating organic matter, which has been buried to depths sufficient for the earth’s temperature to transform the solid organic matter to oil and gas.

As the solid organic matter is cooked to oil, it expands, creates high pressure, pushing oil into very fine pores and into adjacent rocks.

After moving into connected, largely water-filled pore space, the oil moves upward by buoyancy, with buoyancy force provided by the difference in density of oil versus water, multiplied by the height of the connected oil column. The larger the connected oil column, (the oil buoyancy force), the smaller the void openings that can be entered by the migrating oil.

If you are searching for new oil fields, do you know where the oil came from? The direction? The depth? The source horizon? Oil might say (if it could talk) that it is important to know where and when it was created – if you’re looking for where oil has gone.

Have you identified the rock layers that transmitted the oil? The seal that confines the migrating slugs of oil? The structural form of that horizon that is “roofing” migration? Have you noted any structural “noses” that may be perturbing the migrating oil? Have you noted any down-dip fields that are filled-to-spill?

You know to look up-dip from fields filled-to-spill, since any field, filled-to-spill has … spilled.

Do you have any idea how hard it is to recognize an oil field when you have drilled through it with a wildcat well?

Electric log interpretations can be very accurate after calibration, but well logs in wildcats often give equivocal data. Sidney Powers Medalist Bob Sneider made a study of field histories, and came to the astounding conclusion that it took, on average, three wildcat penetrations before a 500 million barrel field would be recognized!

How many fields have YOU drilled through, without recognizing them?

Oil (if it could speak) might remind us that traces of oil remain in any rocks that it moved through, while migrating to its resting place. Do you understand that recognizable traces of oil always remain in migration zones? Do you understand the concept of “snap-off?” Have you looked for migration “shows?”

If we were listening, oil might “whisper” to petrophysicists and geophysicists that its presence in pore space alters rock physics and chemistry in a predictable manner. Do you know the magnitude and character of those modifications? Can you model the effect of fluid substitutions on acoustical and resistivity properties?

A conversation with oil might reveal that part of the created oil always remains in its source rock. It has taken us a hundred years to recognize that about as much oil remains in the source rock – as has ever left it!

(If you had “listened to oil” earlier, YOU might have been the first person to produce oil from the Bakken!)

When you find oil in the very tiny pores of source rocks, what does such presence of oil in very tiny pores mean? The oil within those very, very fine pores provides an actual measurement of the expulsion pressures created by the transformation of solid kerogen to liquid oil.

It takes thousands of pounds of pressure to force oil into such tiny pore throats.

As oil moves away from its source rock birthplace and migrates in permeable rocks to its resting place in a reservoir, oil always obeys the law of gravity. When oil is immersed in water, gravity produces the force of buoyancy. In the most general description, the difference in density between oil and water, multiplied by the height of the connected oil column, gives the buoyancy force.

When oil moves through pores, what laws govern which pores it can push its way into – and whether oil stops, or continues to move? Do you know those laws? Do you understand why pore entry pressure measurements are important to understand?

It’s why oil goes here … and not there!

Let’s ask oil: Does it care whether it resides in carbonate rocks or sandstones? In barrier bars or river channel sands? In Ordovician or Tertiary-aged rocks? In reef or back reef environments?


Oil cares about the size of HOLES and the portals connecting them – not the name of the rock.

Oil doesn’t care about environments of rock deposition, or how old the rock is or what name geologists use to anoint the rock framework.

Oil will fill ANY pore or crevice presented to it, as long as the buoyancy pressure of the oil column is greater than the entry pressure of the pore throat. What IS the buoyancy pressure exerted by the migrating oil slugs typically seen in your exploration province? Do the oil reservoirs in your area of interest reflect oil buoyancy pressures of five psi? Fifty psi? Five hundred psi?

Pore entry pressure measurements of reservoirs are a thousand times more relevant to finding oil than, say, descriptions of ancient environments of deposition! Indeed, much of the environment-of-deposition work done has relevance ONLY because it may have a second-order connection to rock pore entry pressure.

Understand what the migrating oil considers to be an “acceptable” pore throat size for entry in your area of interest. Perhaps you may have observed that adequate pore throat size is only found in alluvial channel sandstones? Then, (and only then) you may usefully concentrate efforts on mapping alluvial sandstone environments to locate oil reservoirs.

Oil knows where it can and cannot go. Shouldn’t YOU?

Since oil always follows the rules, shouldn’t you know the rules?

Perhaps it’s time to concentrate our professional attention on the factors that actually influence where oil will be, and save our studies of peripheral phenomena for our personal entertainment?

Herbert Hunt shared with me the response he received after announcing to his legendary father that he wanted to be a geologist:

“You’ll cost us millions; you’ll be wanting to test ideas instead of finding oil!”

Herbert always has remembered that his task was to find oil, not to test ideas or prove concepts.

Pay less attention to geologic peripherals. Although all geology is interesting, damn little is useful. Focus your efforts on following oil from source to trap.

Geology is a science; geophysics is a science; successful exploration is a business, requiring a ruthless focus on essentials.

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