Page 276 - Geochemistry of Oil Field Waters

26 0 CLASSIFICATION OF OILFIELD WATERS

TABLE 8.11
Coefficients characterizing the genetic types of waters
~~
Type of water Na+/Cl- (Na’ - Cl-)/S04-2 (Cl-- Na+)/Mg+’
Chloride-calcium <1 <O >1
Chloride-magnesium < 1 <O <1
Bicarbonate-sodium > 1 >1 <O
Sulfate-sodium >1 <1 <O

related to water types in this order: chloride-calcium > bicarbonate-sodium
> chloride-magnesium > sulfate-sodium. Most oilfield waters of the
chloride-calcium type belong to the S1 S2 A2 class with a few in the S2 S,
A2 class, while most oilfield bicarbonate-sodium waters belong to the S, A,
A2 and A, S1 A2 classes.
Other significant indicators were grouped by Sulin; however, none of
them can assure the existence of a hydrocarbon deposit, and certainly they
cannot provide definite evidence of the size of the accumulation. The groups
are as follows:
Group I: direct hydrocarbon indicators; for example, naphthenic acid salts
and iodide. The naphthenic acids are more soluble in bicarbonate-sodium
type waters and are related to the composition of the hydrocarbon accumu-
lation. Iodide is related to oil because it must have an organic origin. Sulin
also noted the dissolved gases in the waters and considered the heavier
hydrocarbons such as ethane and butane and the absence of oxygen as direct
indicators.
Group 11: highly mineralized chloride-calcium or bicarbonate-sodium
types of water containing reduced forms of sulfur are important indirect
indicators of oil. The sulfate content should be low to indicate interaction
with bituminous constituents and/or sulfate-reducing bacteria.
Group 111: in this group are constituents which have no genetic relation-
ship to hydrocarbons but appear characteristic of waters that are related to
hydrocarbon accumulations. The constituents are bromide, boron, barium,
strontium, radium, and possibly fluoride.

Modification of Sulin’s system by Bojarski

Bojarski (1970) studied 400 water analyses and differentiated hydro-
chemical zones within basins in Poland that appear suitable for preservation
of hydrocarbon deposits. He distinguished the waters as follows:
(1) Waters of the bicarbonate-sodium type. Such waters occur in the
upper zone of a sedimentation basin, with “intense water exchange” (that is,
a hydrodynamic situation where the waters are moving at a relatively fast
geological rate), which promotes unfavorable conditions for the preservation
of petroleum and natural gas deposits. The waters are defined by the ratio