Page 63 - Petrophysics

SEDIMENTARY GEOLOGY 37

SOLUTION

=
(8.3
t = -In (g) lO5)(l.O5l)
1
x
C
Age = 8,723 years
(refer to Equation 2.3 and U-238 in Table 2.2).
Several important events in the geologic history of the Earth already
have been mentioned, and others are shown in the geologic column
of Table 2.1. The Appalachian Mountains were formed by collision
of North America with Pangaea about 500 million years ago, and the
climax of their growth coincides with the birth of the Atlantic Ocean
at the beginning of the Mesozoic Era about 255 million years ago. The
Mississippi River and the Rocky Mountains originated at about the same
time (63-65 million years ago), and Yellowstone Park volcanism is
estimated to have begun about 40 million years ago. Several ice ages
occurred in the Recent or Holocene Epoch that began about 2 million
years ago [3, 51.

SEDIMENTARY GEOLOGY

Sedimentary geology is fundamental to the exploration and develop-
ment of petroleum reservoirs. It establishes the criteria for petroleum
exploration by providing the geologic evidence for prediction of the
location of new petroleum provinces. Petroleum is found in many areas
in a variety of sedimentary basins. Hydrocarbons may occur at shallow
depths along the edges of the basin, the deep central areas, and in the
far edges where tectonic motion may have provided sealed traps for oil
and gas [l-101.

BASINS

Sedimentary basins differ in origin and lithology. Each is individually
unique, but all share several common characteristics. Basins represent
accumulations of clastic and evaporite materials in a geologically depres-
sed area (an area that has undergone subsidence with respect to the
surrounding land mass) or an off-shore slope. They have thick sedimen-
tary layers in the center that thin toward the edges. The layers represent
successive sedimentary episodes.