OIL AND GAS ACCUMULATIONS IN OVERTHRUST BELTS .1.

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 Earth. Their preferred habitat is the active continental margin, but intra-oceanic and intra-continental occurrences are common. Oil- and gasfields 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 or during thrusting is rare; most oil- and gasfields 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 or 'play' definition, in overthrust belts as well as in normal basins.