Page 194 - Geochemistry of Oil Field Waters

NITROGEN-FREE ORGANIC COMPOUNDS 181

ity levels, and such accommodation systems are stable for several days.
Preferential accommodation of alkanes in the C16-C20 range was found at
the expense of other 'alkanes with lower and higher carbon numbers.
A gas chromatographic method for the determination of petrol in water
was developed whereby the petrol was extracted from the water into
nitrobenzene and the extract was analyzed using a column polyethylene
glycol 1,500 on silanized Chromosorb W (Jeltes-and Veldink, 1967). The
methods were sensitive to 0.1 mg/l, and for concentrations > 0.5 mg/l the
precision was about 5% for the major components.
Low-molecular-weight hydrocarbons in the C1 -C4 range were detected in
sea water. Generally the concentration tended to decrease with depth
(Swinnerton and Linnenbom, 1967). Methane was the most abundant
hydrocarbon found, but smaller amounts of ethane, ethylene, propane,
propylene, n-butane, isobutane, and some butenes also were detected and
measured.
Hundreds of drill-stem samples of brine from water-bearing subsurface
formations in the Gulf coastal area of the United States were analyzed to
determine their amounts and kinds of hydrocarbons (Buckley et al., 1958).
The chief constituent of the dissolved gases usually was methane, with mea-
surable amounts of ethane, propane, and butane present. The concentration
of the dissolved hydrocarbons generally increased with depth in a given
formation and also increased basinward with regional and local variations. In
close proximity to some oilfields, the waters were enriched in dissolved
hydrocarbons, and up to 14 standard cubic feet of dissolved gas per barrel of
water was observed in some locations.
The ratio of toluene to benzene in 27 crude oils from various sources
ranged from 2.0 to 11.3. Toluene is less soluble than benzene in distilled
water, where the ratio is about 0.3 (McAuliffe, 1966). A method of
prospecting for petroleum, utilizing information concerning the amount of
benzene dissolved in subsurface waters, was patented (Coggeshall and
Hanson, 1956). Gas chromatographic methods proved to be good for deter-
mining the amount of benzene and other hydrocarbons in the petroleum-
associated waters (Zarrella et al., 1967). Collected information indicates that
the concentration of benzene in petroleum-associated water varies with
different types of hydrocarbon accumulations, that the benzene concen-
tration decreases with increasing distance from the hydrocarbon accumula-
tion, and that benzene is specific for detecting the occurrence of petroleum
hydrocarbon accumulation in a given geologic horizon. A brine sample taken
from a horizon separated by 27 m of shale from an oil pool contained 0.02
ppm of benzene, indicating that low-permeability shale prevents movement
of hydrocarbons.
Chromatographic techniques were developed for the determination of
sugars and phenols in sea waters and in sediments (Degens and Reuter,
1964). Biogeochemical differences were observed between the sugars in the
sea and in the sediments.