Oil & Gas Exploration
Oil & gas traps
The high pressures under which hydrocarbons are formed also force or “squeeze” oil and gas out of their original source shale deposit. Oil and natural gas will migrate vertically and laterally through porous rock formations, such as sandstone and limestone, as well as through faults and natural fractures until an impermeable barrier stops their movement. These barriers to hydrocarbon movement are commonly called traps. Oil and/or natural gas accumulate in these traps to form large hydrocarbon deposits. The rock layer in which the hydrocarbons are trapped is called the reservoir.
There are two main categories of conventional oil/gas traps: structural and stratigraphic. Structural traps form when rock layers are deformed and the resulting geometry prohibits the hydrocarbons from migrating any further. Common structural traps include anticlinal folds and faults. Salt Creek, Wyoming’s most productive (and nearly oldest) oil field is an anticlinal trap.
Nowood anticline, Bighorn Basin. Photo by Wayne Sutherland.
Stratigraphic traps are due to depositional variation in rock type (facies changes) or truncation of the original reservoir rock. The most common stratigraphic traps are pinch-outs, facies changes, unconformities, and carbonate reefs. Cottonwood Creek, east of Worland in the Bighorn Basin, is an excellent example of a large stratigraphic trap with a facies change from the permeable Phosphoria Formation to the impermeable Goose Egg Formation.
Exploration geologists and engineers use a variety of techniques to determine where oil and natural gas are located underground. Data from nearby wells, regional geology, computer models, satellite imagery, and mapped surface oil seeps are some of the tools that can help predict where a productive oil well might be located.
Because it enables geologists to map underground formations and structures, geophysical (seismic) surveying is one of the best methods of finding oil and gas. A noise source – typically either a vibrating/thumping truck or an explosive discharge – directs sound waves through the subsurface. The waves reflect off the different rock layers and structures. Geophones record the time it takes for the waves to return to the surface.
The raw seismic data is then processed and interpreted to determine the possibility of oil and/or gas.
Three-dimensional (3D) seismic surveys now allow geologists to make even more accurate interpretations and predictions of oil and gas deposits.
Ultimately, drilling test wells confirms whether oil and/or gas exists in a suspected area or formation. While some test wells lead to the successful discovery of hydrocarbons, others turn out to be “dry holes.” On occasion, test wells help discover hydrocarbons in rock layers other than the original target formation.