Page 81 - The Petroleum System From Source to Trap

Magoon, L. B, and W. G. Dnw, eds., 1 '1'1-!, The petroleum
system-from source to trap: AAPG :'l.lemoir 60.

Chapter 4

Genetic Classification of Petroleum

Systems Using Three Factors:

Charge, Migration, and Entrapment

Gerard Demaison Bradley J. Huizinga
Capitola, Cahfomia, U.S.A. ARCO Intcmatimwl Oil and Gas Co.
Plano, Texas, U.S.A.

Abstract
Our genetic classification of petroleum systems is founded on a simple working nomenclature
that consists of combining qualiers from each of the following three categories: (1) charge factor
(supercharged, normally charged, or undercharged), (2) migration drainage style (vertically
drained or laterally drained), and (3) entrapment style (high impedance or low impedance).
The charge factor is estimated using the initial richness of the source rock and the volume of
mature source rock. The source potential index (SPI), which combines source rock richness and net
source rock thickness into a single parameter, is a convenient shortcut for comparing the petroleum
potential of diverse source rocks containg dissimilar kerogen types and for rapidly estimating
regional charging capacity. In extensively explored petroleum provinces that contain a single
petroleum system, a positive correlation exists between the magnitude of the SPI and province­
wide petroleum reseres.
Migration drainage style is detennined from the structural and stratigraphic framework of the
basin fill. Vertical migration drainage, which occurs mainly through faults and fractures that breach
a seal, is characteristic of petroleum systems in rift basins, deltaic sequences, salt dome provinces,
wrench basins, and fold and thrust belts. In contrast, lateral migration drainage is dominant
wherever stratigraphically continuous seal-reservoir "doublets" or carrier beds extend over a large
area in a tectonically stable province (e.g., foreland or intracratonic platform basins). Recogtion of
the dominant migration style helps to predict the location of zones of petroleum occurrence in
relation to the pOd of mature source rock.
Entrapment style, which is also dependent on the structural framework and the presence and
effectiveness of seals, describes the degree of resistance (impedance) working against dispersion of
the petroleum charge. Application of these working concepts should help to reduce geologic risk
significantly, particularly in new ventures exploration.

INTRODUCTION (Klemme, 1975, 1980, 1983; Bois et al., 1982; Carrnalt and
St. John, 1 9 86). Also, field size distributions in petroleum
During the past 40 years, several tectonic classifica­ basins generally show some relationship to basin classifi­
tions of sedimentary basins have been developed by cation and basin size (Klemme, 1 9 83). Furthermore,
petroleum geologists to provide a common framework recognition of tectonic style helps to predict regional
of reference for the earth's sedimentary basins (e.g., retention efficiency (e.g., in the evaluation of thrust belts;
Weeks, 1952; Perrodon, 1972; Klemme, 1975, 1980, 1983; Vann, 1 9 89) or abnormally high heat flow regimes
Bally and Snelson, 1980; Kingston et al., 1983; Helwig, affecting source rock maturation and oil preservation
1985). These classifications have been developed with the (e.g., in proto-oceanic rifts such as the Red Sea or Afar
purpose of finding relationships between tectonic style triangle).
and petroleum occurrence. The most useful outcome of Nevertheless, any degree of petroleum richness,
these studies has been to demonstrate the statistical ranging from near barren to highly prolific, can be
predominance of giant fields and major petroleum observed among specific examples of each basin type.
reserves in cratonic sags and foredeep basins, in contrast Bally and Snelson (1980, p. 71) recognized this point by
to lesser reserves in pull-apart and thrust-belt settings concluding that "the classification of basins does little to

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