Abstract: Oil and Gas Fields Associated with Inverted Extensional Faults: A Global Review

COOPER, MARK
PanCanadian Petroleum, Calgary, Alberta, Canada

The recognition of an 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