Page 83 - The Petroleum System From Source to Trap

4. Genetic Classification of Petroleum Systems 75

plays in a petroleum province. As a part of this genetic
classification, the source potential index (SPI) is intro­
duced as a useful tool for approximating regional
charging potential. Furthermore, patterns of regional
petroleum occurrence can be predicted by delineating
(}) CHARGE ® MIGRATION • @ ENTRAPMENT the pod of mature source rock and recognizing the
FACTOR DRAINAGE STYlE STYlE
1------ -- 1------....j -- 1--------i migration style and distances (Demaison, 1984). The final
- Supercharged
- Normally Charged - Vertically Drained - High Impedance result of integrating a source rock maturity map,
- Laterally Drained
- Low Impedance
- Undercharged migration style, and SPI is a more effective way to
evaluate the regional petroleum potential than by using
the tectonic analog approach alone. A prerequisite to the
A PETROLEUM SYSTEM IS CLASSIFIED
ACCORDING TO THE QUALIFIERS genetic approach, however, is that the tectonic
SELECTED FROM THESE framework, sequence stratigraphy, geologic history,
THREE FACTORS thermal history, and a reasonably adequate geochemical
database must be fully integrated using sedimentary
Figure 4.1. Flow diagram for the genetic classification of a basin analysis techniques.
petroleum system. The themes reviewed in this chapter do not address
the prediction, before drilling, of the volume and compo­
sition of petroleum trapped in individual prospects.
basin (or province) is a geologic entity containing at least
one or more petroleum systems. Advanced computer-aided methodologies for quantita­
The concept of the petroleum system is not synony­ tive prospect evaluation have already been described by
mous with play, which is defined by Bois (1975, p. 87) as Nederlof (1979, 1981), Sluijk and Nederlof (1984), and
Nijhuis and Baak (1990). Instead, the present genetic clas­
"a continuous portion of sedimentary volume which sification scheme has been devised to conduct rapid,
contains pools showing the following characteristics: (1) semiquantitative regional evaluations to outline new
reservoirs within the same productive sequence occur
throughout the zone, (2) hydrocarbons are of similar plays, clarify research objectives, or better assess
chemical composition, and (3) traps are of the same petroleum potential.
type."
A petroleum system can be described according to
three geologic factors-charge, migration, and entrap­ CHARGE FACTOR
ment (Figure 4.1). These three factors occur sequentially In every petroleum system, the principal constraint to
and are therefore conditional, that is, migration cannot the petroleum richness of a province is the adequacy of
occur without there first being a charge. The charge is the charge factor, which must be powerful enough to
dependent on the pod of mature source rock that provide sufficient petroleum charge to the
provides a certain supply of petroleum during a given migration-entrapment part of the system. The overriding
time span. The effectiveness of the charge is mainly importance of the charge factor is obvious: if there is no
controlled by chemical processes, consisting of biochem­ petroleum generation in the subsurface, then there is no
ical transformation of dead organisms into kerogen petroleum system, thus the migration-entrapment
during deposition of the source rock (Demaison et al., subsystem loses its relevance.
1984) and thermochemical kinetics, which control the Charge has been defined as the hydrocarbon volumes
transformation of kerogen into petroleum (Tissot et al., available for entrapment (Sluijk and Nederlof, 1984).
1987). The migration-entrapment part of the system Charge volume equals the volume of petroleum generated
gathers petroleum from the pod of active source rock in the drainage area of a trap minus the volume lost
and distributes it in a manner that may lead to either through migration processes. Overall migration losses
concentration of petroleum into economic accumulations result from the summation of primary migration losses
or loss of petroleum due to dispersion and destruction. (expulsion from the active source rock into the carrier
Migration-entrapment is predominantly controlled by bed) and secondary migration losses (in the carrier bed,
physical processes, including the buoyant rise of between the active source rock and the trap).
petroleum in water, fluid flow and capillary pressure in The term regional charge is used to represent the total
porous media (llling, 1939; Schowalter, 1 9 79), and amount of petroleum from a pod of mature source rock
pressure-temperature-composition relationships that is available for entrapment:
affecting phase behavior before and during petroleum
entrapment. There have been great advances made in Regional charge = Quantity of petroleum from a pod of
our understanding of petroleum migration processes mature source rock - (expulsion
since the mid-1970s (Durand, 1988). losses + migration losses)
The purpose of the proposed genetic classification of
petroleum systems is to describe and predict (1) the The regional charge is dependent on the initial source
relative charging potential of a petroleum system and rock richness and the volume of the mature source rock
thus segments of the petroleum province, and (2) the pod. Source rock richness is most accurately expressed in
geographic location of zones of petroleum occurrence or terms of the hydrocarbon genetic potential (51 + S2