Page 36 - Petrophysics

1 0 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

colored volcanic rock, undergoes change principally to three types of
metamorphic rock, depending on the environmental conditions inducing
the changes: (1) gneiss, which has foliated bands of feldspars; (2) schist
or mica; and (3) slate, which is a fine-grained smooth-textured rock.
Basalt, the dark-colored volcanic rock, produces two main types of
metamorphic rock: (1) amphibolite and (2) greenschist, or green mica,
as illustrated in Figure 1.1.
On a regional scale, the distribution pattern of igneous and meta-
morphic rocks is belt-like and often parallel to the borders of the
continents. For example, the granitic rocks that form the core of the
Appalachian mountains in eastern United States are parallel to the east
coast and those in the Sierra Nevada are parallel to the west coast.
Igneous and metamorphic rocks are not involved in the origin of
petroleum as source rocks. In some cases they do serve as reservoirs,
or parts of reservoirs, where they are highly fractured or have acquired
porosity by surface weathering prior to burial and formation into a trap
for oil accumulated by tectonic events.

SEDIMENTARY ROCKS

All of the sedimentary rocks (about 66% of all rocks) are important to
the study of petrophysics and petroleum reservoir engineering. It is
possible to interpret them by considering the processes of rock degra-
dation. The principal Sedimentary rocks may be organized according
to their origin (mechanical, chemical, and biological) and their
composition, as illustrated in Table 1.3.
Mechanical weathering is responsible for breaking large preexisting
rocks into small fragments. The most important mechanism is the
expansion of water upon freezing, which results in a 9% increase of
volume. The large forces produced by freezing of water in cracks and
pores results in fragmentation of the rocks. Mechanical degradation of
rocks also occurs when a buried rock is uplifted and the surrounding
overburden is removed by erosion. The top layers of the rock expand
when the overburden pressure is relieved, forming cracks and joints that
are then further fragmented by water. Mechanical weathering produces
boulder-size rocks, gravel, sand grains, silt, and clay from igneous and
metamorphic rocks. These fragments remain in the local area, or they
may be transported by winds and water to other sites to enter into the
formation of conglomerates, sandstones, etc., as shown in Table 1.3.
Water is the principal contributor to chemical weathering, which
occurs simultaneously with mechanical weathering. Mechanical weath-
ering provides access to a large area for contact by water. Chemicals
dissolved in the water, such as carbonic acid, enter into the chemical