Page 215 - Geochemistry of Oil Field Waters

202 ORIGIN OF OILFIELD WATERS

and consists of calcite with up to 50% detrital quartz. Types I11 and IV are
similar in percentage of minerals; however, they are further differentiated
according to grain size, sorting, roundness, and fossils, as are the other types.
Type V is deposited in strongly agitated (high-energy) water and it consists of
calcite, < 5% clay, and < 25% detrital quartz.
The calcium in the carbonates is released during rock weathering and goes
into solution as bicarbonate. The solubility of calcium carbonate in water is
also dependent upon the amount of carbon dioxide in solution. If the
amount of dissolve carbon dioxide decreases, calcium carbonate is precipi-
tated; therefore, the amount of calcium carbonate precipitation increases in
warm water because the amount of carbon dioxide in solution is less than in
cold water. Considerable amounts of carbonate precipitation occur in warm
environments (Illing, 1954). Aquatic plants also absorb carbon dioxide and
cause carbonate precipitation. The deposited carbonates can be pure or
mixed with sand, clay, iron, manganese, and organic matter.
Modern carbonate deposition occurs as deep-water oozes, and reefs, and
on shallow shelves. The deepwater oozes form along the flanks of ocean
basins at depths of less than 6,000 m. They do not form at depths greater
than 6,000 m because the calcium carbonate solubility increases with the
increased pressure. On the flanks of the basins, terrigenous material mixes
with the calcium carbonate. Often the mixture is 65--89% percent calcium
carbonate with silt making up the remainder (Gevirtz and Friedman, 1966).
Reef carbonates develop in open oceans on shallow platforms forming
atolls, as isolated patches on the ocean shelves, or along the margins of shelf
areas as fringing reefs. Fringing reefs generally occur in tropical regions on
the western side of the ocean basin. Reefs form as a result of living organisms
which form the framework of the sediment (Ginsburg and Lowenstam,
1958).
Present-day shelf carbonates are developing in Florida Bay, on the Bahama
Banks, on the Australian shelf, and on shelves off British Honduras, Yucatan,
and India. The precipitation often occurs as shallow carbonate mud banks.
The sediment in the shallow shelf areas often consists of about 10% skeletal
material mixed with oolites, mud aggregates, grapestone, aragonite needles,
calcareous algae, etc. The average rate of carbonate accumulation on the
Bahama Banks is 50 mg cm-2 yr-I (Broecker and Takahashi, 1966). The
salinity of the water ranges from 36,000 mg/l of dissolved solids to 46,500
mg/l. The more saline waters occur in lagoonal areas. The pH ranges from
about 8.0 to 8.2 and is lowest at the end of the day because of C02
extraction from the water by marine plants.
In the simplistic depositional basin shown in Fig. 7.3 (Moore, 1969),
limestones and reef limestones will be deposited on the foreland side, formed
from water soluble constituents that precipitated from the saline solutions.