Page 60 - Petrophysics

34 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

collision brought about the formation of the Appalachian Mountains. This
movement of North America apparently crushed a chain of ancient island
arcs and welded them onto the continent, because layers that appear
to be crushed island arcs have been located east of the Appalachian
Mountains. The junction between Pangaea and North America was
apparently weak, leading to the development of a line of rising magma
between them with the formation of spreading ocean plates on both sides
that gradually pushed the two continental masses apart and formed the
Atlantic Ocean [ 1, 2,6, 71.
The convolutions of old crustal plates and sedimentary rocks at
continental margins provide conditions for entrapment of hydrocarbons
in porous sedimentary rocks under impermeable layers that seal the oil
in place. Continental margins bordering a sea with restricted circulation
permits the collection of sediments and salt deposits, which are
associated with the genesis, migration, and trapping of oil. Margins that
are separating from one another also are zones where oil is formed and
trapped. Usually if oil is formed on one side of a continental margin, it
also will be found across the gulf, or ocean, on the margin of the other
continent. Divergent, convergent, and transform continental margins
provide the necessary conditions for sedimentation and accumulation of
hydrocarbon deposits [ 1, 8- 101.

GEOLOGIC TIME

Geologic time scales in use today were developed by numerous
geologists working independently. Different methods for subdividing
the records of flora, fauna, minerals, and radioactive decay found in
sedimentary rocks were suggested; some were repeatedly used and
have been generally accepted. Table 2.1 shows the subdivisions of
geologic time, approximate dates in millions of years, and recognized
physical events that took place during the long record of geologic
history. The Earth is estimated to be about 4.6 billion years. The
Paleozoic Era began 580 million years ago; therefore, approximately
87% of the Earth’s history occurred during the Precambrian age. The
approximate dates of most of the boundaries in the geologic time
column are established from extensive analyses of radioactive isotopes
and the flora and fauna records in sedimentary rocks. Isotopic dating
also allows estimates of the rates of mountain building and sea-level
changes [5].
Geologic age dating using radioisotopes is carried out by determining
the amount of the specific daughter isotope present with the radioactive
element and then multiplying by the rate of decay of the parent element
(Table 2.2). The rate of radioactive element decay is exponential and is