Page 92 - The Petroleum System From Source to Trap

4. Genetic Classification of Petroleum Systems 83

2000 mL

Shale seal
Shale s ource rock)
(
20 Km

Figure 4.8. Example of a supercharged, vertically drained, high-impedance petroleum system, patterned after the Central
graben of the North Sea. The basin fill of a rift basin tends to be vertically drained due to petroleum transfer along faults and
fractures. (Adapted from Thomas et al., 1985.)

of tectonic activity (e.g., faulting) after the petroleum has absolutely essential to laterally drained systems, seal
been generated and entrapped is critical to the preserva­ continuity is also important in vertically drained
tion of seal integrity. systems. Deeper seals cause petroleum to converge on
Basin fill that contains laterally drained petroleum the faults, fractures, or other "windows" or "chimneys"
systems share many common features: through these seals, where focused vertical drainage
transports the petroleum upward until effective top seals
1. Oil accumulations tend to occur in immature sedi­ are reached, thus charging associated traps.
mentary strata located far away from the pod of Extensional, wrench, and thrust tectonics produce
mature source rock. Accumulations containing faults and fractures that function as avenues for focused
long-distance migrated oil commonly account for vertical migration, particularly if tectonic activity keeps
more than 50% of the entrapped oil volume within them open for much of their geologic history. Petroleum­
the petroleum system. Lateral migration distances rich basin fill of a rift basin owes much of the petroleum
in excess of 160 km have been observed. transfer to vertical movement along faults, which are also
2. A single reservoir rock of the same age, present responsible for much of the entrapment. Examples
under the most effective regional seal, usually hosts include the North Sea (Figure 4 . 8), Gulf of Suez,
most of the entrapped oil and gas within the Gippsland, Reconcavo, and Sirte. Passive Atlantic-type
petroleum system. margins affected by salt tectonics and extensional
3. Faulting of the effective regional seal rock is minor faulting, such as Campeche-Reforma (Figure 4.9), Lower
or insignificant. Congo, and Campos, also tend to be vertically drained.
4. In supercharged, laterally drained petroleum The highly productive Tertiary deltas of Nigeria and
systems, large accumulations of heavy oil are often the U.S. Gulf Coast are typical vertically drained systems
found near the margins of the system in shallow due to the formation of listric faults in an extensional
regime (Figure 4.10). The structure may also be compli­
immature sedimentary strata. Examples include cated by piercement salt domes, such as along the Gulf
the heavy oil belts of eastern Venezuela (Figure 4.7) Coast, that facilitate vertical drainage. Multiple
and western Canada (Demaison, 1977; Creany and sandstone reservoirs containing identical oil types
Allan, 1990; Krause and James, 1990). migrated from deep-seated, overpressured source rocks
are commonplace in these deltaic settings. Similarly,
Vertically Drained Petroleum Systems
hydrocarbons entrapped in more ancient deltaic
Focused vertical drainage of petroleum is associated sequences (e.g., the Lower Cretaceous reservoirs of the
with a moderate to high degree of structural deformation Jeanne d'Arc basin in Canada and the Barrow-Dampier
capable of creating selective breaching and local basin in Australia) have migrated vertically through fault
petroleum leakage through stratigraphically widespread zones from an underlying source rock (Upper Jurassic
seal rocks (Price, 1980). Although continuous seals are source rock in both examples).