Page 213 - Geochemistry of Oil Field Waters

200 ORIGIN OF OILFIELD WATERS

Sorting of weathered products

Products of weathering are transported by water by rolling along the
streambed, by suspension of the smaller particles, and by solution of soluble
components. As the larger rocks and pebbles roll along the streambed, they
are abraded by bumping against each other and by abrasive action with the
rocks in the streambed. The size of the clastics, which are detritus trans-
ported mechanically to the point of sedimentation and portions solubilized
by water before sedimentation, decreases in the downcurrent direction
(Pettijohn, 1957), and this change in grain size is primarily a sorting effect.
The effect is noted in both fluvial and marine deposits.

Mobile belt
f
Geosynclinal trough Borderland
r
For eland
(stable) Shelf area

Fig. 7.3. Idealized depositional basin. (After Moore, 1969.)

Knowledge of the sorting of the clastics is used in reconstructing the
ancient environment (Visher, 1965).This knowledge can be applied to ex-
ploration for petroleum and other valuable minerals. A simplistic deposi-
tional basin is shown in Fig. 7.3; the deposited clastics will be found on the
borderland side of the basin and not on the foreland side.

Depositional environments of clastics

Depositional environments of the clastics include eolian, fluvial, regressive
marine, transgressive marine, deltaic, bathyal-abyssal, and lacustrine. Eolian
deposits are sands that are drifted and arranged by currents of air or wind.
Fluvial deposits are those related to streams, rivers, and ponds. Water in
these environments usually contains less than 10,000 mg/l of dissolved
solids.
Regressive marine deposits are land-derived sediments that are transported
seaward and settle in the ocean. The salinity of the water transporting these
sediments will vary, it is fresher at its source and becomes more brackish as it
nears the sea. The dissolved solids in contemporary sea water are about
35,000 mg/l, while some estuary waters contain about 20,000 mg/l of dis-
solved solids.
Transgressive marine deposits usually are small in volume compared to
fluvial and regressive-marine deposits. Such deposits are formed mainly by