Page 210 - Geochemistry of Oil Field Waters

SEDIMENTARY ROCKS 197

The volume of the earth is about 1,100 billion km3 and the volume of the
oceans is about 1.3 .billion km3; however, the oceans with an area of 360
million km2 cover 70% of the surface of the earth. The average composition
of some of the igneous and sedimentary rocks of the earth’s crust is shown in
Table 7.11, which was taken from Clarke and Washington (1924) and
Rankama and Sahama (1950). The resistate rocks referred to in Table 7.11
are composed of residues not chemically decomposed in the weathering of
the parent rocks. Hydrolyzate rocks are the insoluble products formed by
chemical reactions during weathering of parent rocks. Precipitate rocks are
those formed by chemical precipitation of minerals from aqueous solution.
Evaporite rocks are marine evaporites which were produced when the water
in which they were dissolved was evaporated.
Sedimentary rocks comprise about 5% of the lithosphere, while the
igneous rocks form 95%. The three main types of sedimentary rocks are
shale, sandstone, and iimestone, and their relative abundance determined
from geochemical data ranges from 70 to 83% shale, from 8 to 16% sand-
stone; and from 5 to 14% limestone (Pettijohn, 1957). Levorsen (1966)
noted that oil and gas are found in reservoir rocks consisting primarily of
sandstones, limestones, and dolomites.

Weat hering

Weathering is a most important factor in producing the source material for
the creation of sedimentary rocks. Processes that cause weathering are
chemical, physical, and biological (Ross, 1943).
The weathering of rock by physical methods includes temperature changes
brought about by climate changes. Examples are the breaking of rock by
thermal expansion (heat), the breaking of rock by the expansion of freezing
water in the pores or cracks, or the mechanical breaking of rock as a glacier
moves over it. Breaking the rock causes the surface area to become larger
without significantly changing the chemical composition.
Biological weathering includes the cracking of rock as a result of plant
roots and the action of acids derived from plants, animals, and bacteria. The
biotic factor includes bacteria, algae, protista, protozoa, higher animals and
plants, during both life and subsequent necrotic decomposition which
furnish Ht ions, colloids, complexing agents, and dispersants.
Chemical weathering involves the action of water upon the parent rock
and upon the products of physical and biological weathering. In chemical
weathering the composition of the source rock is changed by solution,
hydrolysis, oxidation, and reduction reactions. The Ht ion when concen-
trated in aqueous solution is a very important energy factor because it will
cause rapid chemical reactions with parent rocks. The redox potential in-
fluences the rate of removal of elements, such as iron and manganese from
the parent rock, and if it is a reducing potential, these elements are more
likely to remain in solution after solubilization.