Page 121 - Introduction to Petroleum Engineering

ROCKS AND FORMATIONS 107
6.2 ROCKS AND FORMATIONS

The three primary rock types are igneous, sedimentary, or metamorphic. Rocks can
transition from one rock type to another when the physical conditions change.
The transitions are part of the rock cycle shown in Figure 6.5. The rock cycle begins
with the formation of magma, or molten rock, in the mantle. Convection cells in the
mantle carry the magma toward the surface of the Earth where it encounters lower
temperatures and pressures. At sufficiently low temperature and pressure, the magma
cools and solidifies into igneous rock.
Igneous rock on the surface of the Earth is subjected to atmospheric conditions.
Erosion can break the igneous rock into smaller particles which can be transported
by wind, water, and ice. The particles can become finer as they collide with other
objects during the transport process. Eventually, the particles will be deposited when
the energy of the transporting agent dissipates.
The deposition of particles can lead to sizable accumulations of sediment. The loose
sediment is subjected to increasing temperature and pressure as it is buried beneath
additional layers of sediment. The combination of increased temperature, pressure, and
chemical processes can transform loose sediment into sedimentary rock. If the
sedimentary rocks are buried deep enough, they can encounter pressures and tempera-
tures that can change the character of the rock in a process called metamorphism. Given
enough time, pressure, and heat, rocks will melt and start the rock cycle again.
Sedimentary rocks are important because they are often the porous medium associ-
ated with commercially important reservoirs. The key attributes used to classify
sedimentary rock are mineral composition, grain size and shape, color, and structure.
Rocks are composed of minerals. Each mineral is a naturally occurring, inorganic
solid with a specific chemical and crystalline structure. Mineralogy is the study of
the set of minerals within the rock. The source of the minerals, the rate of mineral
breakdown, and the environment of deposition are important factors to consider in
characterizing the geologic environment.
Grains which form sedimentary rocks are created by weathering processes at the
surface of the Earth. Weathering creates particles that can be practically any size,

Sedimentary rock

Erode, transport, Bury, apply
deposit, cement heat, pressure

Igneous rock Metamorphic rock

Cool and solidify Heat and melt

Magma
(molten rock)

FIGURE 6.5 The rock cycle.