Page 122 - Introduction to Petroleum Engineering

108 PETROLEUM GEOLOGY

Very angular Sub-angular Sub-rounded Well rounded

FIGURE 6.6 Grain shapes.

shape, or composition. A glacier may create and transport a boulder the size of a
house, while a desert wind might create a uniform bed of very fine sand. The parti-
cles, also known as sediments, are transported to the site of deposition. Particles
transported by wind or water roll and bump along the transport pathway. The edges
of a grain give information about the depositional environment.
Figure 6.6 illustrates different grain shapes. Grains with sharp edges probably did not
get transported very far, which suggests the grain was transported in a low‐energy environ-
ment such as a slow‐moving stream. Rounded grains suggest transport in a high‐energy
environment such as a fast‐moving stream. Rocks made up of rounded grains tend to have
larger permeability than rocks composed of grains that are flat or have sharp edges.
Sorting refers to the uniformity of grain size. The size of grains in well‐sorted rock
is relatively uniform, while there is considerable variation in grain size in poorly sorted
rock. Fluids will typically flow better through well‐sorted rock than poorly sorted rock.
The ability to flow is characterized by a rock property called permeability.

Example 6.3 Canyon Formation

It has taken the Colorado river about 8 million years to carve a mile deep canyon as
the surface of the Earth uplifted. Use this observation to estimate the rate of uplift of
the Earth’s surface at the Grand Canyon. Express your answer in inches per year.

Answer
Note: 1 mi = 5280 ft and 1 foot = 12 in.
6
The rate of uplift = (1 mi)/(8 × 10 yr) = 1.25 × 10 mi/yr.
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Convert to in./yr: (1.25 × 10 mi/yr) (5280 ft/mi) (12 in./ft) = 7.92 × 10 in./yr.
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This is less than 1 in. per century.
Convert to mm/yr: (7.92 × 10 in./yr) (2.54 cm/in.) (10 mm/cm) = 0.2 mm/yr.
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6.2.1 Formations
The environment where a rock is deposited is called the environment of deposition.
As the environment, such as a shoreline, moves from one location to another, it leaves
a laterally continuous progression of rock which is distinctive in character. If the
progression is large enough to be mapped, it can be called a formation.