Abstract: Oil and Gas Accumulations in Overthrust Belts

By

Johann-Christian Pratsch

Overthrust belts are zones of high crustal mobility leading to major vertical and lateral movements of rock units. They developed throughout the geologic history of the globe. Their preferred habitat is the active continental margin but intra-oceanic and intra-continental occurrences are common too, indicating that interactions of crustal blocks are a more general setting. Oil and gas fields exist only in small portions of the known overthrust belts. Their distribution is closely related to the present existence of generative depocenters. Preservation of hydrocarbon fields formed prior to or during thrusting is rare at best. Most oil and gas fields in overthrust belts have been formed after the major thrust deformation.

Definition of present effective depocenters and prediction of preferred lateral and vertical hydrocarbon migration are the main parameters in any basin evaluation of play definition, in overthrust belts as well as in normal basins. A comparison of successful and unsuccessful exploration campaigns in different overthrust belts shows that such present effective depocenters exist in portions of thick overthrust sections as well as in overthrusted foreland arms. Mainly because of destruction of older accumulations, Neogene times of hydrocarbon generation, migration and entrapments are most important. Examples from the American Cordilleras, the Alpine-Carpathian belt, the Pindus trend, North- and West-Africa, Papua New Guinea all follow comparable geologic principles.

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