Abstract: Oil and Gas Fields Associated with Inverted Extensional Faults: A Global Review
COOPER, MARK
PanCanadian
Petroleum, Calgary, Alberta, Canada
inversion
structure
relies on the concept of regional elevation. The regional elevation of
a marker horizon is its
structural
elevation when deformation effects are
removed. Contractional deformation will elevate beds above regional elevation
and extension will lower them below regional elevation. In an
inversion
structure, marker horizons will display different elevations above and/or
below their respective regionals in the fault hanging wall. Another key
feature is that a synrift sequence must be recognized in all
inversion
structures.
Inversion
appears to be favored by a short time interval between
the extension and compression due to a relatively weak attenuated lithosphere
that has not thermally re-equilibrated.
Inverted basins occur most commonly in intra-cratonic
areas (e.g., northwest Europe) and in back-arc areas (e.g., Sunda Arc Indonesia),
but have also been recognized within orogenic belts (e.g., the Andes of
Colombia, the Rockies, and the Appalachians) and also on passive margins
(e.g., Mid Norway Basin). In most of the areas noted above there are oil
and gas pools associated with inversion
structures.
The geometry of inversion
structures is highly
dependent on the geometry of the original fault system. In the simple case
of inverting a half graben, the
inversion
fold produced will be an asymmetric
monocline facing the footwall with the crest located above the synrift
depocenter. However, when a half graben is inverted, footwall shortcut
faults commonly result to generate a shallower dipping fault, which is
a more efficient trajectory for shortening strata.
In this review I will present a selection of oil
and gas fields associated with inversion
structures from a variety of tectonic
environments from around the world. This raises questions for an exploration
geologist of predicting when an
inversion
structure will be productive
and what the key factors for success will be. Careful analysis of the
structural
evolution and its impact on the petroleum system will often reveal the
key elements in understanding the play.
AAPG Search and Discovery Article #90927@1999-2000 AAPG Distinguished Lectures