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VEAR, ALWYN, BP Exploration, Caracas, Venezuela

ABSTRACT: Controls on the Distribution of Cretaceous Source Rocks in South America

More than thirty South American basins, exhibiting a variety of structural styles, contain petroleum source rocks of Cretaceous age. However, the presence of truly "world-class" source rocks, capable of supplying multi-billion barrel oil provinces, is restricted to relatively few basins and appears to be primarily a function of large scale Cretaceous tectonic setting. In Early Cretaceous times the best source rocks were preserved in both a southern ocean and in the rift between South America and Africa. By the Late Cretaceous, these southern and eastern continental limits had become narrow passive margins. In contrast, on the northern continental margin a wide shelf to a restricted tropical sea was developing at this time. Periodic upwelling enhanced surface productivity on this shelf which led to development of some of the world's richest source rocks. On the tectonically active western margin moderate quality source rocks were forming in a series of back-arc basins, whilst further west, in the Pacific fore-arc, organic-rich intervals were rarely deposited. This article documents what is known about each of the explored basins (including the volume and character of discovered petroleums), it investigates the geologic factors which governed the richness and quality of petroleum source rocks and it assesses how continued tectonic activity has modified or even destroyed primary source quality. Finally it predicts which of the as yet underexplored basins should contain good quality source rocks and could become prolific petroleum provinces of the future.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.