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PICHA, FRANK J., Chevron Overseas Petroleum, Inc., San Ramon, CA

Abstract: Exploration for Hydrocarbons Under Thrust Belts--A Challenging Frontier

New significant reserves of hydrocarbons may occur in autochthonous and parautochthonous series buried below the thin-skinned thrust belts. Typically, the thin-skinned thrust belts do not involve the crystalline basement in their frontal zones; however, the extent of depositional sequences preserved in their autochthonous position underneath these thrust belts varies greatly from very limited, as in the North American Rocky Mountain thrust belts, where the sole decollement runs at or close to the crystalline basement, to very substantial, as in the Carpathians and Apennines, where under the frontal thrust sheets there are thousands of meters of autochthonous deposits. In the Carpathians, these autochthonous deposits extend over two full plate-tectonic cycles: the Paleozoic-Hercynian cycle and the Mesozoic to Cenozoic Alpine cycle. The structural and depositional features found below the thrust belt fall into two categories, those existing prior to the orogeny, such as older structures, source rocks, and reservoirs; and those associated with thrusting, e.g., synorogenic structures or the impact of the tectonic load on the underlying autochthonous plate.

The subthrust plays have been successfully tested in several thrust belts of the world, of which the Carpathians is one of the best examples. The arcuate thin-skinned Carpathian orogenic belt, which evolved during Mesozoic and Cenozoic times from the Tethyan seaway, is thrust tens of kilometers over its Neogene foredeep and the underlying European plate. Various structural and stratigraphic settings and potential hydrocarbon plays have been recognized within the buried margins of the European plate. These include the Late Paleozoic Hercynian compressional system; the Mesozoic extensional system of rifted margins of Tethys; buried Paleogene valleys/submarine canyons; and Cenozoic synorogenic faulting of foreland. Further examples from the Apennines, Dinarides, and Urals demonstrate an existence of deeper parautochthonous, inverted, and wrench fault-related structures under the thin-skinned structures of these thrust belts. These structural and morphologic features, if combined with source rocks, reservoirs, and proper burial history, represent potential hydrocarbon plays.

The generation of hydrocarbons from sources within the subthrust plate was greatly enhanced by emplacement of the wedge-shaped thrust belt, which may also provide a regional seal. It is, therefore, the combination of the long and complex geological history of the subthrust plate, with the impact of the orogeny, that created unique conditions for generation, entrapment, and preservation of hydrocarbons in subthrust settings.

Oil and gas fields have been found in subthrust position under the thin-skinned belts, such as the Carpathians, Apennines, and Ouachitas; however, many deeper subthrust structures remain unexplored. The complexity and diversity of subthrust plays remains a challenge to exploration but also provides an opportunity for finding significant oil and gas accumulations elsewhere under the thin-skinned thrust belts. 

AAPG Search and Discovery Article #90922©1998-1999 AAPG International Distinguished Lectures