Page 216 - Geochemistry of Oil Field Waters

SEDIMENTARY ROCKS 203

Depositional environments of evaporites

Calcium carbonate precipitates from sea water after it begins to evaporate
(Usiglio, 1849). Removal of calcium ions from solution increases the Mg/Ca
ratio in the residual brine, and the precipitated calcium carbonate reacts with
the magnesium enriched brine to form dolomite (Deffeyes et al., 1964). The
common order of evaporite deposition in a basin cut off from the open sea is
limestone > dolomite > gypsum > halite > potash. Evaporite deposition can
be stopped in a basin by a change in climatic regimen or tectonism. If it were
changed and new water were allowed to enter, the already deposited salts
probably would be effectively protected by the superadjacent high-density
brine, and the lighter waters either meteoric or sea water would float on top,
developing euxinic conditions similar to a situation found in the Black Sea.
Toxic conditions in euxinic areas tend to preserve deposited organic
matter. The preserved organic matter later can be transformed to hydrocar-
bons and petroleum. Hypersaline lagoons often are very prolific in the
production of organic matter (Phleger and Ewing, 1962). Algal pads and an
abundance of organic organisms are characteristic of a high-pH, high-salinity
environment (Carpelan, 1957) such as exists before euxinic conditions
develop.
The deposition of evaporites occurs when water evaporates under re-
stricted environmental conditions. The restrictions usually are caused by
tectonism such as an uplift, regression of the strand line leaving a relict sea,
or biological building of a reef.
Sloss (1953) outlined five environments from which evaporites deposit;
they are normal marine, euxinic, penesaline, saline, and supersaline. Primary
carbonates and dolomitized carbonates precipitate from the normal marine
environment where the water contains about 35,000 mg/l of dissolved solids.
Limestones rich in organic matter and black shales often are related to
euxinic environments.
In the penesaline environment carbonates and primary dolomite form.
The salinity of the water is toxic to normal marine life but not sufficiently
saline so that halite precipitates. The dissolved solids in the water range from
about 250,000 mg/l to 350,QOO mg/l.
From a saline environment carbonates, sulfates, and halites will precipi-
tate. From a supersaline environment potassium compounds will precipitate,
and the amount of dissolved solids in the solution will be about 500,000
mg/l.
A simplistic model of an evaporite basin is a closed basin initially filled
with sea water and evaporated to dryness. In the isochemical system a layer
of calcium carbonate is deposited over the entire basin floor as evaporation
proceeds. Next, gypsum is deposited and when the water is reduced to about
10% of the original volume, halite precipitates. Halite will deposit only in the
deeper parts of the basin and if the solution goes to dryness the more soluble
salts will deposit in any remaining depressions and on top of the halite.