Page 240 - Geochemistry of Oil Field Waters

224 ORIGIN OF OILFIELD WATERS

mation water exist in the Western Canada Sedimentary Basin, that the flow
systems affected accumulations of hydrocarbons in the basin, and further
that pressure and salinity variations might be explained by membrane
properties of the shales. He saw a need for studies of the hydrodynamics of
the basin.
Hydrodynamics and geochemistry of the Paradox Basin were studied by
Hanshaw and Hill (1969). The ground-water movement in the basin is
generally southwestward from the high outcrop areas in western Colorado,
flowing toward the Colorado River discharge areas. Hydrodynamic con-
ditions exist in lower Paleozoic strata which are favorable to accumulations
of petroleum in stratigraphic traps. Paleozoic aquifers in northwestern New
Mexico have very high potentiometric surfaces and these aquifers may be the
outflow receptors of an osmotic membrane system operating within the San
Juan Basin. This regional study was excellent and of value in exploration for
petroleum and gas.
Parker (1969) studied brines and waters in five aquifers of Cretaceous age
in the East Texas Basin. He found that the composition of the waters in the
older, more deeply buried aquifers were modified more than waters in
younger, less deeply buried aquifers. Most of the modifications were made
by exchange reactions, dilution by meteoric waters, and loss of sulfate be-
cause of bacterial reduction. Hydrodynamic movement of the waters in the
Woodbine formation contributed to the giant oil accumulation in the East
Texas Basin. Much of the stratigraphic trapped oil probably was trapped in
part because of this type of flow.

Sabkha sediments and transport of valuable ores

Bush (1970) discussed the origin of chloride-rich brines from Sabkha sedi-
ments and how they are related to inclusion brines and lead-zinc deposits of
the type found in the Mississippi Valley. He noted that Helgeson (1964) and
Barnes and Czamanske (1967) have shown that chloride-rich scrlutions can
transport lead and zinc as chloride complexes. According to Bush (1970),
Sabkha brines free of sulfur are expelled by sediment compaction, migrate,
and become enriched in base metals until they contact a zone of higher
temperature and pressure. In this zone, sulfides are present as a result either
of inorganic reduction of sulfate, anaerobic reduction, or hydrocarbon
reduction of anhydrite. The sulfides cause the base metals to precipitate.
Formation brines are the medium in which several metals, in addition to
hydrocarbons, migrate prior to deposition in ore deposits. A current theory
is that the metals travel primarily as chloride complexes in solutions that are
depleted in reduced sulfur species (Dunham, 1970). The metals subsequently
are precipitated when a source of reduced sulfur is met. An example of a
source of reduced sulfur is an area where anaerobic bacteria are reducing
sulfate. This occurs in waters near petroleum-bearing formations, and such
waters in carbonate reservoirs often contain considerable amounts of sulfide.