Page 214 - Geochemistry of Oil Field Waters

SEDIMENTARY ROCKS 201

erosion and redeposition of sediment deposits. The salinity of the sea may
change somewhat during transgression but probably not much.
Deltaic deposits result from a combination of environmental factors and
are related to both fluvial and regressive marine processes. During flood
times the rivers transport tremendous volumes. of material, both clastic and
organic, into delta areas. Deltaic deposition is a very important factor in the
formation of petroleum because of the tremendous amount of organic ma-
terial deposited.
Bathyal-abyssal deposits are formed in deep-water areas in the sea, and
turbidity currents are responsible for most of the clastic deposition (Emery,
1960). Lacustrine deposits are those that are formed in lakes. If the lake is a
fresh-water lake, the dissolved solids may be less than 1,000 mg/l; in a
salt-water lake, the dissolved solids may be greater than 35,000 mg/l.
Consider a simplistic sedimentation area where the borderland area is the
prime source of sediments. The coarse- to fine-grained clastics which are
weathering products of the high-mountains borderland are deposited near
their source. The clastics are detritus transported mechanically to the point
of sedimentation and are not solubilized by the water before deposition.
Primarily they are the sandstones and shales (clays). They will not be found
on the for.eland side of a depositional basin. Clay deposition can be detrital
or authigenic; illite often is detrital. There are at least two dozen clay
minerals, many of which occur in very minute grains and most of which
cannot be resolved by high-power petrographic microscopes. The electron
microscope, X-ray diffraction, and differential thermal analysis are used to
determine the type of clay.
The clays are very important in relation to petroleum and gas because
they are the major component in the shales from which petroleum and gas
are generated. The clays also possess base exchange properties which will
react with constituents in water and petroleum. The detrital clays settle in
low-energy waters and they settle more rapidly from a saline water than from
a fresh water.

Depositional environments of the carbonates

Limestones and dolomites are the dominant carbonate reservoir rocks,
while the sandstones are the dominant clastic reservoir rocks (Ham, 1962).
The carbonates were precipitated at the place where the rocks first formed,
while clastics were primarily transported grains.
Plumley et al. (1962) classified the carbonates according to an energy
index of the water from which they precipitated. They divided them into
five types. Type I is deposited in quiet water; it consists of calcite, 15-5096
clay, and < 5% detrital quartz. Type I1 is deposited in intermittently
agitated water and consists of calcite, < 25% clay, and < 50% detrital quartz.
Type I11 is deposited in slightly agitated water and it consists of calcite with
up to 50% detrital quartz. Type IV is deposited in moderately agitated water