Page 94 - The Petroleum System From Source to Trap

4. Genetic Classification of Petroleum Systems 85

T o p Oil Window

Shale seal
s
� -s -53 Shale (source rock)

Figure 4.11 . Example of a supercharged, vertically drained, high-impedance petroleum system, patterned after the Los
Angeles basin, U.S.A. Basin fill in wrench-fault basins typically shows vertical migration of petroleum through faults .

....c:--- Low Impedance �---High Impedance ---:
(no seals)

....__ _ _ _ _ Overmature Sequence --

---- -
T o p Oil
T o p Gas Window
Window
Shale seal
� 5_ - s_-�:1 Shale (source rock)

Figure 4. 2 . Example of a vertically drained fold and thrust belt. The presnce of an effective top seal causes part of the
1
petroleum system to show high impedance (right side� whereas the lack of seal rock results in a low-impedance sector of
the fold and thrust beH (left side).

2. Multiple, vertically stacked reseroir rock, Entrapment Style
sometimes of grossly different ages, often contain
the same genetic type of oil, such as the Lower Traps are subsurface loci where petroleum can no
Cretaceous-Tertiary reservoirs of the Campos longer continue its migration toward the surface because
basin (Mohriak et al., 1990). The oils may show the buoyant movement of oil and gas has been arrested.
varying degrees of postentrapment alteration (e.g., The foremost scientific basis for finding petroleum is the
biodegradation and water washing). Biological identification of traps by geophysical methods. Convex­
markers can distinguish between compositional upward structural traps are physically the most efficient
variations caused by secondary alteration processes in retaining petroleum because any layer serving as a top
and those resulting from source rock and thermal seal is also a lateral seal (Downey, 1984). That is why the
maturity differences. bulk of the world's oil reserves has been found in four­
3. Faulting remains active until the last effective way closures. Although structural traps are the most
regional seal has been laid down. efficient on the whole, a structural trap reliant on a fault
4. In supercharged, vertically drained petroleum for closure carries substantial seal risks (Downey, 1984).
systems, surface seepages are abundant wherever Structural-stratigraphic traps, featuring three-way struc­
tectonic activity, persisting to the present day, has tural closures in combination with reservoir wedge-outs,
breached the regional top seal in places (e.g., San account for a few of the world's supergiant oil accumula­
Joaquin basin of California, salt domes in the U.S. tions, including Prudhoe Bay (Figure 4.5), Venezuela's
Gulf Coast, Zagros fold belt, and Magdalena Valley &livar Coastal fields, and the East Texas field, as well as
of Colombia). many other accumulations of substantial economic merit.