Page 278 - Geochemistry of Oil Field Waters

26 2 CLASSIFICATION OF OILFIELD WATERS

iodide and bromide concenbrations may point to a bitumen accumulation);
(3) ratio Cl/Br < 350; and (4) SO4 x lOO/Cl< 1.
In addition to indicating the degree of alteration, bromide and iodide as
biophile constituents play a decisive role in the classification Bojarski
adopted. This followed because of the increased concentration of biophile
elements in the waters accompanying a petroleum deposit. The concen-
tration of iodide in the ground waters depends mainly on the organic sub-
stances, whereas the’concentration of bromide up to a certain limit takes
place in an inorganic medium, but an increase in bromide must be evaluated
as a positive indication. In many waters accompanying petroleum deposits,
large amounts of bromide and smaller amounts of iodide were detected, or
vice versa. This probably is related to the type of bituminous substances
which absorb the individual biophile elements in different amounts.
Chebotarev’s classification

Chebotarev (1955), an Australian geochemist, classifies waters on the basis
of dissolved bicarbonate, sulfate, and chloride, and he does not consider the
acid waters or those that contain free sulfuric or hydrochloric acid. His
fundamental assumption is that the anions are independent variables while
the cations are dependent.
The geochemical types of waters are related to the products of weath-
ering. Table 8.111 illustrates the cycles and products that are produced by
weathering. During the first cycle the igneous rocks are weathered allowing
chloride, sulfate, calcium, sodium, silica, and magnesium to go into solution.
The second cycle is the weathering of sedimentary rocks with the solution of
more of the same products. The third cycle is the weathering of recent drift
and yields of the above constituents plus aluminum and iron.
Table 8.IV illustrates Chebotarev’s (1955) geochemical classification of
subsurface waters. The phase of weathering corresponds to four phases of
the solution and redistribution of the chemical constituents in the earth’s
crust and correlates with their relative mobilities. He plotted the relative
mobilities of nine chemical constituents using the mobility of chloride as
100%. From this four phases were obtained, namely: (1) chloride and sulfate
100% to about 58% mobility; (2) calcium, sodium, magnesium and potas-
sium 3% to about 1.2% mobility; (3) silica about 0.20% mobility; and (4)
iron oxide and aluminum oxide, less than 0.05% mobility. The four phases
of weathering correspond to the products of weathering shown in Table 8.IV
and also to the cycles and products of weathering in Table 8.111. For
example the fourth phase in Table 8.IV corresponds with the first cycle in
Table 8.111.
The genetic types of water shown in the upper portion (A) of Table 8.IV
do not correspond directly with the weathering phases since the genetic
types overlap the phases. These genetic types are related to the accumulation
products shown in Table 8.111. In the lower portion (B) of Table 8.IV are the