Page 95 - The Petroleum System From Source to Trap

86 Demaison and Huizinga

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- .. 11-'1 ..
.--------- High Impence - - - - - - 1-- - - Low Impednce _ _ ___,�
(no seals)

Shale seal
f� -s-5� Shale (source rock)

Figure 4.13. Example of a nonnally charged, vertically drained, high-impedance petroleum system, patterned after the
Reconcavo basin, Brazil. The petroliferous portion of the basin fill (left side) contains the petroleum system, which is best
characterized by high impedance. However, the basin fill also contains a low-Impedance sector (right side) that lacks
effective seals, resulting in vertical dispersion and loss of petroleum, regardless of structure. The boundary betwen the
high and low impedance defines the geographic extent of this petroleum system.

In contrast, purely stratigraphic traps that lack any vertical migration focusing and entrapment. This is a
partial structural control tend to be imperfectly sealed commonly encountered situation in Tertiary deltas, rift
and, consequently, are generally small in size. systems, and other extensional regimes.
In a natural system, any change or transfer of energy Low-impedance entrapment style is characterized by
brings about a statistically increasing degree of random­ either a high degree of regional seal continuity and a low
ness in that system. Therefore, it is compatible with ther­ degree of structural deformation, or a low degree of
modynamic principles that, given geologic time, the regional seal effectiveness coupled with a high or low
ultimate fate of petroleum in sedimentary rocks is disper­ degree of structural deformation. In the latter case, the
sion and destruction by thermal or bacterial processes. degree of structural deformation is irrelevant to defining
Each petroleum accumulation, regardless of size, corre­ the impedance because a deficiency in seals results in
sponds to a local, temporary entropy reversal requiring vertical dispersion and loss of petroleum, regardless of
an improbable set of geologic circumstances. The larger structure (Figure 4.13).
the accumulation, the exponentially higher the degree of
geologic improbability, resulting in the well-known log
normal distribution of field sizes observed in most CONCLUSIONS AND IMPLICATIONS
petroleum basins (Klemme, 1983; North, 1985). A critical FOR EXPLORATION
geologic factor responsible for lowering entropy is the
degree of physical resistance working against wholesale The genetic classification of petroleum systems
dispersion of petroleum as it tends to migrate toward the consists of applying the following three geologic factors:
surface. This physical factor is called impedance. (1) charge factor (supercharged, normally charged, or
The degree of structural deformation and the seal undercharged), (2) migration drainage style (vertically
integrity are the two key factors used to qualify entrap­ drained or laterally drained), and (3) entrapment style
ment styles. Together, these factors control the degree of (high impedance or low impedance). A petroleum
impedance working against the natural tendency for system can be classified by using a combination of quali­
petroleum to become randomly dispersed and lost in fiers, selected from each of these three categories (Figure
sedimentary rocks. Entrapment style can be classified as 4.1). Provinces containing known petroleum systems
high impedance or low impedance (Figure 4.1). have been classified to illustrate the use of this scheme
High-impedance entrapment style is characterized by (Figure 4.14).
laterally continuous seals coupled with a moderate to This genetic classification scheme describes petroleum
high degree of structural deformation. Regional seal systems in terms of their process-driven attributes.
continuity is essential to the integrity and retention char­ Discrimtion between petroleum systems is based on
acteristics of traps and to the collecting efficiency of differences in regional charging capacity and in the
drainage areas surrounding the traps. However, local geologic styles controlling hydrocarbon concentration
breaching of seals in faulted regimes results in combined and retention. This classification provides explorationists