Page 62 - Introduction to Petroleum Engineering

46 PROPERTIES OF RESERVOIR FLUIDS
According to the “biogenic” theory, hydrocarbon gases and liquids found in forma-
tions today are the product of short‐ and long‐term processes that acted on remnants
of organisms such as algae and plankton that lived millions of years ago in aqueous
environments. Upon death, the organisms formed organic‐rich sediments. Oil and gas
were formed by bacterial action in the anaerobic conditions of the sediment and
subsequent thermal processes that occurred after the sediment was buried under many
additional layers of sediment. These thermal processes break larger organic molecules
into smaller molecules. Although some oil and gas may remain in the original sedi-
ment, which is also known as source rock, much of it has migrated upward as a result
of buoyancy effects. Some of the upward migrating fluid was trapped by impermeable
formations; the remainder continued migrating to the surface where it dissipated.
Accumulation of oil and gas in a trap forms a reservoir, which is the target of modern
exploration activities.
Much of the oil and gas that is produced from shales today is thought to be in the
original source rock. If the temperature of a shale formation is at least 200°F, the thermal
process of producing oil and gas, or “maturation,” from organic material in the shale is
ongoing to some extent. The rate of maturation increases with formation temperature,
which increases with depth at about 0.01–0.02°F per ft. Hence, a formation’s tempera-
ture could exceed 200°F even at 8000 ft below the surface. As a result of increasing
maturation rate with depth, the size of the hydrocarbon molecules in oil decreases with
increasing depth. Thus, deep formations are more likely to contain gas and not oil.
The compositions of hydrocarbon resources are quite variable from location to
location around the world. Here are some interesting and unusual examples:

1. In the Athabasca oil sands of northern Alberta, the oil is usually called “tar.”
The density oil in the oil sands is greater than water density and its viscosity is
very high. Much of the oil produced from these sands is obtained by mining
and then transporting the oily sand to a central location for separation.
2. Gilsonite, which is mined from formations in northeastern Utah, is a solid with
a composition similar to the very largest molecules found in oils.
3. Natural gas hydrates are solids formed when water and gas mix at low temper-
atures and high pressures, such as in the oceans off the continental shelves
where natural gas that has seeped from subsurface formations is trapped on the
ocean floor.
4. A smelly wax mined from fractured formations, ozokerite is thought to be
produced by natural processes that have stripped away all but the waxy portion
of oils.
5. Gas associated with most coal formations was generated by thermal matura-
tion of the coal, which was formed from compressed plant matter. As a result,
the composition of “coalbed” gas differs from gases that originated from the
accumulation of algae and plankton in sediment.
6. Natural gases from formations in western Kansas and Oklahoma and northern
Texas contain up to 2% helium, which is extracted from the natural gas. The helium
is produced by alpha decay of radioactive minerals in the formations of that area.