Conventional Oil & Gas Traps
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.
Stratigraphic traps are due to depositional variation in rock type (facies changes) or truncation of the original reservoir rock. Cottonwood Creek field in the Bighorn Basin is an excellent example of a large stratigraphic trap formed by a change from the permeable Phosphoria Formation to the impermeable Goose Egg Formation.