V.

Exploration

In order to find crude oil underground, geologists must search for a sedimentary basin in which shales rich in organic material have been buried for a sufficiently long time for petroleum to have formed. The petroleum must also have had an opportunity to migrate into porous traps that are capable of holding large amounts of fluid. The occurrence of crude oil in Earth’s crust is limited both by these conditions, which must be met simultaneously, and by the time span of tens of millions to a hundred million years required for the oil’s formation. See also Geology.

Petroleum geologists and geophysicists have many tools at their disposal to assist in identifying potential areas for drilling. Thus, surface mapping of outcrops of sedimentary beds makes possible the interpretation of subsurface features, which can then be supplemented with information obtained by drilling into the crust and retrieving cores or samples of the rock layers encountered. In addition, increasingly sophisticated seismic techniques—the reflection and refraction of sound waves propagated through Earth—reveal details of the structure and interrelationship of various layers in the subsurface. Ultimately, however, the only way to prove that oil is present in the subsurface is to drill a well. In fact, most of the oil provinces in the world have initially been identified by the presence of surface seeps, and most of the actual reservoirs have been discovered by so-called wildcatters who relied perhaps as much on intuition as on science. (The term wildcatter comes from West Texas, where in the early 1920s drilling crews encountered many wildcats as they cleared locations for exploratory wells. Shot wildcats were hung on the oil derricks, and the wells became known as wildcat wells.)

An oil field, once found, may comprise more than one reservoir—that is, more than one single, continuous, bounded accumulation of oil. Several reservoirs may be stacked one above the other, isolated by intervening shales and impervious rock strata. Such reservoirs may vary in size from a few tens of hectares to tens of square kilometers, and from a few meters in thickness to several hundred or more. Most of the oil that has been discovered and exploited in the world has been found in a relatively few large reservoirs. In the United States, for example, 60 of approximately 10,000 oil fields have accounted for half of the productive capacity and reserves.