Page 438 - Global Tectonics

420 CHAPTER 13

the Phanerozoic. In some areas, such as in parts of have the ability to absorb kerogens and remove them
North and South America, there is good evidence that from the effects of free oxygen.
the majority of kimberlites were generated during The temperature experienced by the kerogens after
times of enhanced hot spot or mantle plume activity burial is critical, and depends on the local geothermal
(Sections 5.5, 5.7), possibly associated with the break-up gradient. Temperatures of 70–85°C are required to
of the supercontinent Gondwana. The relationships develop liquids and 150–175°C for dry gas. It is also
among kimberlite magmatism, supercontinent assem- important that a critical exposure time to these tem-
bly and dispersal, the relative stability of the crust and peratures is exceeded, so the basin must be free from
mantle, and diamond productivity are discussed further tectonism and uplift during this period.
by Heaman et al. (2003). After formation, the hydrocarbons undergo primary
migration from the fine-grained source rocks and sec-

ondary migration as they concentrate and accumulate
13.2.3 Deposits of in a reservoir of high porosity. Migration occurs because
of the buoyancy of the hydrocarbons, and it follows
sedimentary basins that all hydrocarbon accumulations are allochthonous.
There are several types of oil trap, including anticline,
The majority of fossil fuels are found within sedimen- fault, stratigraphic, unconformity, and lithological traps,
tary basins whose formation can be related directly or which have the effect of providing a capping to the
indirectly to plate motions. In addition to the sedimen- reservoir with an impermeable cover which prevents
tary environment, quite stringent conditions are neces- further upward movement.
sary for the development and preservation of these Plate tectonics controls the locations of reservoirs in
resources. that it is responsible for the formation and preservation
There are four principal criteria which must be met of the sedimentary basins in which hydrocarbons are
for the development of petroleum and gas, hereinafter generated and trapped. These include:
referred to as hydrocarbons: layers rich in organic
matter within the sedimentary succession; a source of
1 intracratonic basins formed by hotspot

heat applied for a time sufficient for the maturation of
activity, Paris and Michigan basins;
organic materials into hydrocarbons; permeable path-
2 basins associated with continental rifting, e.g.
ways which allow movement of the hydrocarbons; and
the Gulf of Aden, Red Sea;
a porous reservoir whose top is sealed by impermeable
capping beds. 3 aulacogens (Section 7.1), e.g. the North Sea;
The main source of the disseminated organic 4 passive continental margin basins, e.g. the
matter, or kerogens, in sediments is plankton. The Gabon Basin;
abundance of plankton is controlled by climate, the
5 ensialic backarc basins, e.g. the Oriente Basin
quantity of nutrients available, and water body geom-
of Ecuador and Peru;

etry. The first two factors are latitude dependent, and
6 marginal seas, e.g. the Andaman Sea;
the majority of oil basins originate at low latitudes.
The latitude is obviously affected by the north–south 7 accretionary prisms, e.g. the coastal oil-fi elds
component of plate motion, while the plate confi gura- of Ecuador and Peru;
tion at any given time determines water body geom- 8 forearc basins, e.g. the Cook Inlet of southern
etry. Organic material is especially abundant along Alaska;
continental margins where there is major river runoff
9 pull-apart basins associated with strike-slip
into large deltas.
faults (Section 8.2), e.g. the Los Angeles
The preservation of kerogens requires conditions
Basin, western USA (Moody, 1973);
which are not oxidizing. These are achieved along con-
10 foreland basins (Section 10.3.2) of orogens,
tinental slopes where the production of organic matter
exceeds the availability of free oxygen to convert them e.g. the Aquitaine Basin, southwest France,
to carbon dioxide, and in closed anoxic basins. It follows 11 tensional basins associated with indentation
that the shales and mudstones produced in such envi- tectonics (Section 10.4.6), e.g. southern Asia
ronments are the most common source rocks as they and Tibet.