Organic geochemistry of margin sediments and its significance for hydrocarbon generation

Abstract

Organic-rich shales and mudstones have been drilled during the D.S.D.P. legs 47a off northwestern Africa and legs 47b and 48 off western Europe. Early Miocene black shales of the Tarfaya Basin contain an organic matter of marine origin and deposited in a reducing environment. Cretaceous dark shales and mudstones are widespread in the north Atlantic Ocean, but they contain mainly detrital organic matter of terrestrial origin. Thus, their potential for petroleum generation is rather low. An immature stage of thermal evolution can be assigned to all Miocene and Cretaceous cores. In other parts of the Atlantic Ocean black shales containing abundant organic matter of marine origin have been found in the same series of Lower Cretaceous age. The widespread occurrence of Lower Cretaceous organic rich sediments from very different sources suggests that the conditions of preservation may be the controlling factor for black shale sediments.