Page 76 - The Petroleum System From Source to Trap

68 Klemme

and foreland basins such as the Arabian-Iranian basin oceans. Extensional or transpressional rift-sag cycles
and interior rift basins such as the West Siberian. It is that develop fault block traps whose trap integrity is
a thick sequence of source rocks most often appears to preserved if the extension fails to proceed to the drift
result in less petroleum (BOE) release. This is probably phase. In either rifted basins or Tethyan fold belt and
due to the insulation or seal effect within the central core foreland basins, fault block integrity is preserved as
of the source rock body. This lack of expulsion from the opposed to the drift phase of divergent margin basins.
central core lowers the petroleum system's recovery effi­
ciency even when the source pod is highly fractured. The
interface of source rocks with reservoir rocks or carrier CONCLUSIONS
beds thus seems an important juxtaposition. The more of
these that are interbedded ("wick" effect), the more Upper Jurassic source rocks and their petroleum
efficient the expulsion and the greater the percent of systems are present in the rift-sag stages of their basins'
potential BOE in the source that is able to migrate out structural evolution. The petroleum system recovery effi­
(Cornford, Chapter 33, this volume; Leythaeuser et al., ciency of these petroleum systems ranges from 0.1 0 to
1988a,b). These characteristic differences between thinner 0.86%. More efficient petroleum systems develop in the
( <150 m) and thicker (> 150 m) source rock sequences continental interior (rifted basins) and in those fold belt
appear to be supported by the section on present-day and foreland basins with a rift-sag cycle between the
geography, which indicates lower recovery (efficiency) platform and foredeep stages of development. Less
from coastal areas where divergent margin and rift efficient systems form in continental coastal zone
basins develop thicker source rocks pods, and by the divergent margin basins.
section on location by type of basin, which indicates that Once active source rocks are present, higher
thicker source rock sequences occur in many coastal petroleum system recovery efficiency is noted on
rifted basins and in most divergent margin basins (see average in those systems where the reservoir and cap
Figure 3.7). rocks are of high quality and cover a large area and
Upper Jurassic petroleum system recovery efficiency where the size of traps and the dynamics and timing of
is also affected by the system's petroleum realm location, these "other" plumbing ingredients are of high rank. In
by the juxtaposition of active source rock and reservoirs the limited examples in this study of petroleum systems
rock, and by the type of basin (i.e., a basin's evolution of that involve Upper Jurassic source rocks, the influence of
structural forms in Ulmishek and Klemme, 1990). In the these other plumbing ingredients are as important as, if
case of petroleum realms, a system's location within the not more important than, the source rock quality or the
low paleolatitudes of the Tethyan realm is conducive to amount of available petroleum from the system's active
deposition of evaporitic cap rocks and carbonate source rock. In several instances, both a higher recovery
reservoir rocks at moderate to deep drill depths, which is efficiency and higher recoverable petroleum (per unit
in addition to the depth-deteriorating reservoir sand­ area) are noted in systems with less available petroleum
stones found at all paleolatitudes. (per unit area) from the mature source rocks than in
Both the Tethyan and Boreal realms display tectonics petroleum systems with more available petroleum in
that incorporate the rift-sag cycle in basin development. their active source rocks. These variations in recovery
In the Tethyan realm, rift-sag cycles developed to the efficiency are also related to the geometry of the source
south or in back of the Tethyan spreading zone. For pod within the petroleum system.
example, the northern Gondwana-Neo-Tethys
Arabian-Iranian basin evolved from a Permian-Triassic
rift followed by a Jurassic sag and is preserved today,
while similar structural forms and stratigraphic
sequences are suspected to have occurred in northwest Acknowledgments The writer wishes to acknowledge and
Africa and India but appear to have been destroyed by thank I. Maycock, R. Church, and M. Nemic for aid in data
Tertiary collision. assembly; L. B. Magoon for instructive discussions and consid­
The northern Tethyan margin basins display post­ erable aid in preparation of the data; and R. Johnson for
Hercynian collision rifting (Triassic) with platform drafting.
margin sags in Jurassic time. For example, the Middle
Caspian and Amu Darya basins, while similar to basins
in China and Europe, appear to have been destroyed by References Cited
Tertiary collisions, while in the western Tethys, the Gulf
of Mexico is still in the rifted-platform to margin half-sag
stage of tectonic development awaiting the Cuban­ Bedoes, L. R., Jr., 1973, Oil and gas fields of Australia, Papua
Tethyan collision. New Guinea, and New Zealand: Sydney, Australia, Tracer
The Boreal realm, d i ffering from the southern Petroleum and Mining Publications, 382 p.
Gondwana realm, has more aborted divergent marginal Beydoun, Z. R., and H. V. Dunnington, 1975, The petroleum
geology and resources of the Middle East: Beaconsfield,
zones. For example, the North Sea grabens of the U.K., Scientific Press, 99 p.
divergent northwest European shelf and the West Carmolt, S. W., and B. St. John, 1986, Giant oil and gas fields,
Siberian-Kara Sea complex are opposed to the rift and in M. T. Halbouty, ed., Future petroleum provinces of the
drift margins of the Gondwana Atlantic and Indian World: AAPG Memoir 40, p. 11-54.