Historical Geology Understanding Our Planet\'s Past

or the oxygen level in the bottom water must be low so the material does not

oxidize before burial under thick sedimentary layers.This is because oxidation

causes decay, which destroys organic material.Therefore, areas with high rates

of accumulation of sediments rich in organic material are the most favorable

sites for the formation of oil-bearing rock.

After deep burial in a sedimentary basin, high temperatures and pressures

generated in Earth’s interior chemically alter the organic material into hydro-

carbons.If the hydrocarbons are overcooked, however, natural gas results. Oil is

often associated with thick beds of salt. Because salt is lighter than the overlying

sediments,it rises toward the surface, creating salt domes that help trap the oil.

Hydrocarbon volatiles (fluids and gases) along with seawater locked up

in the sediments migrated upward through permeable rock layers and accu-

mulated in traps formed by sedimentary structures that provide a barrier to

further migration. In the absence of a cap rock, the volatiles continue rising

to the surface and escape. Also, much petroleum has been lost by the destruc-

tion of the reservoir by uplift and erosion of the confining structure. Several

Figure 112Open pit
coal mining at the West
Decker mine, Montana.
(Photo by P. F. Narten,
courtesy USGS)

Historical Geology