GEOL 115: The Earth’s Energy Resources

Today we also started to look at exploration techniques in earnest. Using geologists in the field is cheap. In the early days of the oil business geologists could do surface geology to find oil fields directly. This ranged from simply drilling on oil seeps (the Jed Clampett technique) (here’s an old paper on seeps that you need to be on a campus computer to access) to mapping the surface exposures of anticlines. This image
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shows the surface expression of an anticline in the Zagros fold belt of Iran. The NW-SE trending oval structure near “A” is an anticline. It looks a little like a hot dog that has been sliced in half (along its long dimension) and placed on a flat surface.   These are easy to find, as you can imagine, and virtually all of them have been found. Although there are almost no locations today where a geologist can map a structure on the surface and use it to find an oil field at depth, geologists can learn a great deal about potential reservoir and source rocks from surface work. Clever geologists will also be able to predict the occurrence of rocks in the subsurface from exposures at the surface.

We noted that surface geochemistry is sometimes used in exploration. Cynics would argue that surface geochemistry is best equipped to find known oil fields. The basic principle is sound: all petroleum-producing basins contain seeps. Surface geochemistry attempts to find the very small seeps that must issue from almost all petroleum and gas accumulations.

In the days when there are few wells drilled in the US OCS, oil companies would group into consortia to drill COST wells (Continental Offshore Stratigraphic Test). This technique could still be used in any area where geological data were unavailable. The companies select a location and drill a well for the purpose of getting geological information.

The most common exploration method; however, is seismic exploration. A source of seismic energy is used to cause seismic waves to travel through the Earth. These waves are reflected by layers of rock and observed using geophones or hydrophones. The data can be processed to produce a crude image of the shallow crust.

Geophysicists realized that the differences in seismic velocities between rocks saturated with salt water and those saturated with gas could be considerable. This pronounced change in acoustic impedance would produce a response seen on seismic reflection profiles called a bright spot (see also here). After this phenomenon became known and understood it increased success in petroleum and gas exploration. Another direct detection technique is known as a flat spot. Flat spots are seismic reflections produced by the horizontal interface between gas and water in reservoir rocks. As geophysical processing has become easier and more sophisticated more techniques have evolved. The fundamental problem when trying to explore for oil is that the change in acoustic impedance between oil saturated and water saturated rocks is not great. The second is that geologists must always endeavor to understand why particular phenomena appear in geophysical data and not merely rely on empirical interpretation. You can check this out if you are really interested in some of the science behind “bright spots” and using seismic data to identify hydrocarbon-charged rocks.

None of these techniques requires that the exploring company acquire anything other than the right to be on the surface of the land. This is usually easy to get and may be free if the lands are public and the activity will have no impact (e.g., doing surface geology). When it comes time to drill everything changes.