Conventional and Unconventional Resources
Oil and other fossil fuel resources cannot be replenished on a
timescale of interest to humans. Oil was formed during periods of extreme
global warming millions of years ago. At that time, algae and zooplankton
multiplied in warm, sunlit waters, converting solar energy via photosynthesis
into organic material composing their bodies. Upon their death, their organic
remains slowly settled in the stagnant (anoxic) ocean depths forming thick
layers of organic matter. Later, sediments washed in by rivers buried this
organic material preventing it from being oxidized by air, thereby preserving
it. Over time, this material was buried deeper and deeper and slowly “cooked”
by increased temperature and pressure, changing it first into a waxy material
called kerogen and then with more heat into liquid and gaseous hydrocarbons.
During the process, called catagenesis by petroleum geologists, long-chain
hydrocarbons are broken into shorter ones.
The temperatures and pressures needed for oil formation occur at
depths between 2286 (7500 feet) and 4572 m (15,000 feet) called the “oil
window”. If too cool, oil remains trapped in the rock as keragen (shale oil);
and, if too hot, oil is converted to natural gas (primarily methane) by thermal
cracking. So, why is oil often found closer to the surface? Surface erosion can
bring it closer, or oil, once liberated from the source rock, can migrate
upward and even appear on the surface as seeps. Oil can also be trapped in
porous reservoir rock beneath an impermeable cap rock, such as shale. These
porous rocks form oil reservoirs and consist of sandstone or ancient coral
reefs that hold oil like a sponge. Most oil “traps” are anticlines that form
when the overlying cap rocks are buckled by tectonic movement forming a dome
that traps the oil. If the reservoir is large enough, it forms an oil field
from which oil can be extracted by drilling and pumping through the overlying
rock layers.
Oil and gas are generated from organic-rich rocks by thermogenic or
biogenic processes. When such “source rocks” are exposed over periods of
geologic time to high temperatures the organic material breaks down releasing
oil and gas that then migrate toward the surface due to buoyancy. The
temperature required for thermal maturation of a source rock varies depending
on the type of organic material but the minimum temperature for oil generation
is approximately 50 C and for gas generation is 100 C. Most thermogenic oil and
gas are generated at depths of 2–6 km.
Methane may also be produced from source rocks by the biogenic
breakdown of organic material in source rocks. Such biogenic gas is produced at
lower temperatures and at shallower depths. Whether of thermogenic or biogenic
origin, oil and gas can be trapped by buoyancy in “reservoirs” in porous rocks
beneath an impermeable layer of rock typically at depths of over 1 km. The
permeable nature of the host rock, high pressures related to depth of burial,
and the concentration of oil and gas in discrete reservoirs allow relatively
easy extraction of oil and gas by drilling wells. Until the 1990s almost all
the world’s oil and gas production was produced from such “conventional”
fields. Natural gas in conventional fields may occur with (“associated gas”) or
without (“nonassociated gas”) oil.
In either case the gas may contain compounds that can be separated
at the surface as liquids. Natural gas liquids (NGLs) such as propane, butane,
and pentane are sometimes included in reporting oil production but are not
insignificant: they comprise, for example, 3.5% of the total energy production
of the United States.
Estimates of the world’s original endowment of
conventional oil and gas resources have tended to increase because of the
advances in technology over time. Advances in geologic concepts, drilling
technologies, seismic imaging, and computer modeling using large datasets have
progressively revolutionized the search for oil and gas over the last century.
Likewise, advances in production technologies, pipeline construction, and
tankers have all made resources available that at one time would have been
considered economically unviable. Advances in science and technology allow more
oil and gas to be found in old fields, within existing petroleum provinces, and
in new frontier regions.
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