Page 223 - Geochemistry of Oil Field Waters

210 ORIGIN OF OILFIELD WATERS

Petroleum and natural gas

The total amount of organic matter dispersed in the sedimentary rocks of
the earth has been estimated to be about 2,700 trillion metric tons; of this
amount 50 trillion metric tons are dispersed petroleum hydrocarbons, of
which 0.5 trillion metric tons exist in petroleum reservoirs (Hunt, 1968).
The types of hydrocarbons in a petroleum often indicate its origin; for
example, if it contains predominately odd-numbered n-alkanes in the low
molecular-weight range, it probably was formed from marine organisms.
Petroleums from the Uinta Basin contain a predominance of odd-numbered
hydrocarbons in the vax fraction, indicating a nonmarine organic source.
Waxes derived from land plants contain a predominance of hydrocarbons
with carbon numbers of C27, CZ9, and C31, while hydrocarbons derived from
marine plankton may contain more hydrocarbons with carbon numbers of
c17, and c19.
The water in the sediments containing the organic matter contains many
dissolved organic constituents such as salts of the humic, fatty, and
naphthenic acids, sugars, heterocyclics, and aromatic oxygen compounds.
Degens et al. (1964) observed that as the salt concentration in petroleum-
associated waters increases, the concentration of dissolved amino acids in-
creases.
Petroleum is generated in organic-rich shales, but the mechanisms whereby
it migrates from the shales and concentrates in porous reservoir rocks are not
understood. Petroleum precursors leave the shale with the water as the water
is expelled by compaction.
As burial proceeds, pressures and temperatures increase. With increasing
temperature, chemical changes in the solids are accelerated and the organic
matter first generates petroleum, which ultimately is converted to methane
and finally graphite. The clay minerals continue their recrystallization, and
finally metamorphism to slates, phyllites, and schists occurs: These processes
involve a continuing loss of porosity with the release of additional pore
water.
The solubility of petroleum hydrocarbons in water increases with in-
creasing temperature and pressure. However, at ambient temperature and
pressure the solubility in pure water is rather low (McAuliffe, 1969). Water-
wet shale has no permeability to immiscible fluids such as gas or oil, so the
petroleum or petroleum precursors probably do not move as droplets. Peake
and Hodgson (1966) report “accommodations” of specific hydrocarbons in
water up to about 30 ppm. Cartmill and Dickey (1970) found that a
colloidal suspension was able to pass through water-wet sands, but the tiny
droplets coalesced at points where the grain size decreased. Neruchev and
Kovacheva (1965) offered some evidence that the amount of extractable
hydrocarbon decreases in shales for the first few meters away from the
reservoir rocks, as if removed by some flushing action.
Bruderer (1956) suggested that oil deposits originated from sea water