Page 191 - Geochemistry of Oil Field Waters

178 ORGANIC CONSTITUENTS IN SALINE WATERS

tained minute quantities of hydrocarbon, to be squeezed out of the con-
solidating sediments, and subsequently the oil was accumulated in sands and
left behind in structural traps (Kidwell and Hunt, 1958). Obviously, the
waters associated with petroleum play a very important part in the origin,
migration, accumulation, and subsequent production of petroleum - the
accumulation and production of petroleum being totally dependent upon
hydraulic flow in response to geostatic and hydrostatic pressures.
Consider briefly that 99% of the oil found and produced typically occurs
within the pore spaces of sedimentary rocks (Hedberg, 1964). About 59% of
the production comes from sandstone reservoirs, 40% from carbonates, and
1% from other types of rock. Petroleum in igneous and metamorphic rocks
occurs primarily in fracture pore spaces and probably has migrated to these
rocks from its place of origin.
The solubilities of petroleum hydrocarbons in water increase with
temperature and decrease as the salinity of the water increases. A tempera-
ture drop from 150” to 25°C reduces the solubility of petroleum in water by
a factor of 4.5-20.5. Such a mechanism can account for the accumulation
of petroleum because as upward moving subsurface waters containing
dissolved hydrocarbons decrease in temperature and pressure and meet more
saline waters, they will release dissolved hydrocarbons (Price, 1973).
Information concerning dissolved organic matter in sea water was
published as early as 1892 (Duursma, 1965). Palmitic acid, stearic acid,
acrolein, and organic nitrogen were tentatively identified. The dissolved
organic matter was found to be about 2 mg/l in the open sea, increasing to
about 15 mg/l in water taken near the coast of Greece, all of which was
attributed to saponification of the fats of dead organic organisms. Phyto-
plankton organisms comprise most living marine organic matter, 10% of
which eventually becomes animal matter. The bulk of the organic particulate
matter in the sea results from dead animal matter, but the dissolved organic
constituents appear to be derived from dead phytoplankton and detritus
rather than excretions from living cells.
Decomposition of organic matter results primarily from the activities of
heterotrophic bacteria. Organic matter decomposes more rapidly in a near-
shore environment, where there is an abundance of such matter and bacteria,
than in a deep-sea environment, where both the matter and bacteria are
diluted. The dissolved organic matter can be classified into groups as follows:
(1) nitrogen-free (for example, carbohydrates); (2) nitrogen-containing (for
example, proteins); (3) lipids (for example, esters of fatty acids); and (4)
complexes comprised of mixtures of the preceding three groups (for
example, humic acids).

Nitrogen-free organic compounds

Many petroleum-associated waters contain methane; however, in Japan,
there is a type of natural-gas deposit called “suiy6sei-tenynengasuy’, a dry gas,