Page 66 - The Petroleum System From Source to Trap

3. Petroleum Systems of the World Involving U p per Jurassic Source Rocks 59

The same source rocks were deposited on the by a postrift-predrift sag, while with drift, a prograding
northwest Europe shelf as well as in the Jeane d'Arc half-sag structural form is developed. Some rifts in
subprovince (Kimmeridgian "hot shales") and both divergent margin shelf areas remain as rifts and sags, as
include sectors with structural forms resembling both little drift has yet to occur in the vicinity of the rift (e.g.,
rifted basins and divergent margin basins. These similar Jeane d'Arc subprovince and parts of the Norwegian
source rocks sequences were probably part of a much shelf). Generally, the source rocks are deposited in the
larger contiguous source rock sequence that extended early stage of basin evolution, while the reservoir and
from the Arctic to the temperate Newfoundland-Grand seal rocks, and especially the overburden rocks, are
Banks to the Iberia sector of Laurasia, which was later deposited in the later stages of basin evolution.
separated by the late Mesozoic drift opening of the
Atlantic Ocean. In the Jeane d'Arc subprovince, the Location by Structural Form
Kimmeridgian hot shale was deposited, as was the Verril
Canyon facies, in an anoxic environment as type I and II Structural form reflects the evolutionary tectonic stage
kerogens in the half-sag phase of the Scotia shelf. The of various basin types (Kingston et al., 1983; Klemme,
Verril Canyon source rocks were deposited in response 1983; Ulmishek and Klemme, 1990). Nearly all Upper
to the opening (drift phase) of the Atlantic south of the Jurassic source rocks were deposited in the structural
Newfoundland fracture zone, which eventually form of the rift-sag cycle of a basin's tectonic evolution,
separated central North America from North Africa. At with 1 0 % of the BOE coming from the source rocks
the same time, the area north of the fracture zone deposited in the rift stage and nearly 90% from the sag
remained in the rift (predrift) phase of tectonic develop­ stage. A minor amount of the BOE (Scotia shelf) came
ment (von der Dick, 1989; Foster and Rofenson, in press). from source rocks deposited in the prograding sequence
The Vienna basin contains an anomalous petroleum into a half-sag.
system in that it, like the Tuxedni-Hernlock(!) petroleum Other major world source rocks (Klemme and
system of the Cook Inlet area (Magoon, Chapter 22, this Ulmishek, 1991) include Silurian source rocks deposited
volume), has its source rocks in a terrane that is tectoni­ only on platform structural forms, Upper Devonian
cally exotic to the reservoir rocks and traps. In the Vienna source rocks with only 12% deposited on the rift-sag of
basin, the Upper Jurassic source rocks were deposited in Laurasia, and Pennsylvanian-Permian source rocks with
a rifted divergent margin that was later overthrust by the 47% deposited on the rift-sag structural forms of
Alpine collisional terrane in Late Cretaceous-early Laurasia. In contrast, 93% of middle Cretaceous source
Tertiary time. On this convergent margin, the collisional rocks were deposited on rift-sag structural forms located
terrane was then superposed by the transform rifted late in Laurasia and northern Gondwana. It is likely that the
Tertiary movement to form the Vienna basin, which absence of the rift-sag as a depositional structural form
contains the system's Miocene traps and reservoir rocks in the Silurian source rocks and its limited presence in
(Ladwein et al., 1991). In the case of the Cook Inlet, the Devonian source rocks are due to both Caledonian and
Middle Jurassic source rocks are believed to have been Hercynian destruction of early and middle Phanerozoic
deposited in a forearc structural form which was then source rocks deposited in the structural forms of the
incorporated into an accreted zone (Ott, 1992). In both of rift-sag cycle and to the progressive biologic evolution
these basins, the source rocks appear to be in what, for from shallower to deeper water ecological niches as the
many years, was considered basement rock. habitat of bottom scavengers evolved (Ulmishek and
All of the basin types in which Upper Jurassic source Klemme, 1990).
rocks were deposited include in their structural Most of the Upper Jurassic source rocks occur strati­
evolution a time when a rift or rifts were followed by a graphically close to the interface of the rift where it
sag, either linear or circular (Ulmishek and Klemme, changes to a sag. The Bazhenov and Khodzipaik source
1990). The fold belt and foreland basins (of the Tethyan rocks of West Siberian and Amu Darya and the Upper
realm) have a rift--sag followed by a foredeep structural Dingo of the northwest shelf of Australia are generally at
form. Those basins on the craton of northern Gondwana the base of the sag. The Kimmeridgian hot shale source
also have a prerift platform structural form, which is rocks of the northwest European shelt the North Sea,
followed by a rift-sag-foredeep sequence of structural and the Jeane d'Arc are in the rift immediately below
forms. The Neuquen basin has a tectonic evolution in and in transition with the overlying sag. The
which the preforedeep sequence has been labeled as Hanifa-Sargelu source rocks in the Arabian-Iranian
either a backarc or a passive margin. In its present fold basin are located at some stratigraphic distance above the
belt and foreland form, the foreland resembles a passive Permian-Triassic saglike rift or riftlike sag (Koop and
margin, but the backarc nature of a rift-sag tectonic Stonely, 1982).
sequence seems the most likely first stage for this basin's The linear nature of rifts and their overlying sags often
tectonic evolution. In the Papua and possibly the have structural cross warps that provide barriers or sills.
Neuquen basins, the rift sequence may have been These barriers can cause euxinic basins where sapropelic
partially or totally destroyed by cratonward thrusting marine and lacustrine continental deposits can accumu­
and either orogenic uplift or subduction of much of the late in anoxic environments at all paleolatitudes. In low
rift-sag sequence (a condition that might apply to the or equatorial paleolatitudes, type I and II kerogens,
Indian continent as opposed to the Arabian subconti­ salt-evaporite seal rocks, and carbonate reservoirs are
nent). Divergent margins develop a rift or rifts followed favored. High paleolatitudes tend to have more humic