Page 219 - Geochemistry of Oil Field Waters

206 ORIGIN OF OILFIELD WATERS

The organic matter produced by photosynthesis in the oceans is estimated
to be sufficient to produce 11 million metric tons of hydrocarbon precursors
annually (Riley, 1944). A very small amount of this organic material pre-
served in sedimentary rocks each year through geologic time would supply
all of the known oil and gas fields plus many undiscovered giant fields.
Shales consisting of organic material, siltstones, claystones, and limy mud
mixtures are found in the trough of a basin (Fig.7.3.). A simplistic idealized
view of the trough area is that organic matter deposited in the trough is
preserved in the stagnant low-Eh environment. If rapid subsidence occurs,
the organic material later is transformed into hydrocarbons which move up
and out of the trough into stratigraphic traps on the foreland side or struc-
tural traps on the borderland side of the basin.
The time interval between deposition of the organic material and con-
version to petroleum is millions of years, during which time the trough or
ocean basin is filled with sediment and buried, and the sediment is com-
pacted to rock. Some of the water in which the sediments deposited will
remain in the rocks as interstitial water.

Deposition of silica

Most of the silica in sediments is of the detrital variety but some is
authigenic. Silica is dissolved by waters with high pH potentials and precipi-
tated from water with a low pH. The precipitated silica often acts as a
cement.

Sediment compact ion

Sediments compact or consolidate in response to an imposed load, and in
the natural environment, the load is the weight of overlying sediments
(Weller, 1959). Compaction of a sediment results in a reduction of the
interstitial volume concurrent with expulsion of interstitial water and defor-
mation of the sediment skeleton (the solid granular framework exclusive of
bound interstitial water). The grains and the interstitial water are almost
incompressible, and the rate of expulsion of interstitial water is about
identical with the rate of compaction (Taylor, 1956).
Terzaghi and Peck (1968) studied the expulsion of pore water from un-
consolidated clays, and determined that the rate at which clay compaction
occurs is dependent upon the clay permeability, its volume compressibility,
and the square of the thickness of the bed which is compacted. Shales
decrease in porosity when compacted; for example, their porosity can
decrease about 80% as they compact during burial.
White (1957) noted that very large quantities of water are removed from
sediments during their compaction. For example, water-saturated shale
decreasing in porosity from 20 to 10% loses 10" liters of water per km3 of
sediment. Such sediment could yield per km3 a water supply of 20 liters per