PETROLOGY OF THE BITUMEN (MANJAK) DEPOSITS OF BARBADOS - HYDROCARBON MIGRATION IN AN ACCRETIONARY PRISM

Bitumen (manjak) deposits in Barbados are located particularly along fracture planes, including faults, and also in calcite veins, as veinlets, vug-fillings and bedding-parallel seams in sandstones, and as matrix to diagenetic carbonates. They occur particularly near the margins of the diapiric Joes River Formation and in immediately adjacent rocks, reflecting the role of diapirism in the migration of hydrocarbon-bearing fluids. The larger bitumen veins trend between E-W and NE-SW, following the strike of fault-bounded packets of the accretionary prism, the predominant fabric in the basal complex of Barbados - that is, hydrocarbon migration followed the structural grain. Petrographic relationships of bitumen with chemosynthetic 'diagenetic' carbonates suggest that hydrocarbon migration was a multi-stage process. The host sediments to the bitumens were not well consolidated and appear to have been in an overpressured state. Hydrocarbon generation may have contributed to overpressuring and buoyancy-enhanced diapirism. The association of bitumens with faults and decollement surfaces suggests that the lubricating action of the bitumen helped to accommodate deformation during diapirism and other processes related to the evolution of the accretionary prism. Widespread impregnation of sandstones in the Joes River Formation suggests that hydrocarbon migration was primarily by intergranular flow in a state of low confining pressure and did not depend on migration along the fault planes, which may be of a later date.