DEPOSITIONAL-ENVIRONMENTS OF THE ORGANIC- MATTER OF APTIAN-ALBIAN SEDIMENTS FROM THE ARARIPE BASIN (NE BRAZIL)

The geochemical study of Aptian and Albian sediments from the Araripe Basin allows the recognition of three organic-rich intervals. The olde r corresponds to black argilites from the Fundao Unit (Late Aptian), t he second occurs in the argilites and laminated limestones oi the Crat e Member (Latest Aptian-Early Albian) and the latest is recorded in ar gilites close to the gypsum mass from the lpubi Member (Middle to Late Albian). Pyrolysis data show that the organic content is highly varia ble according to the facies, but some levels yield 45.9% TOC. High hyd rogen indices and elemental analysis of kerogen concentrates indicate mainly type I or II organic matter. The petroleum potential reaches up to 350 kg hydrocarbons per ton of rock, but Tmax and the abundance of biomarker, suggest that the organic matter is too immature for hydroc arbon generation. The sedimentological and palaeoenvironmental study p ermits the definition of the conditions suitable for the preservation of organic carbon-rich strata. The uniform lithology oi some black sha le level suggests a low bottom relief morphology. Depth is unknown but the water column must have been thin to very thin, as indicated by th e recurrence of desiccation cracks. During low runoff phases, the surf ace waters were highly productive, whereas a stratification of bottom- waters aids the preservation of organic matter. During high runoff pha ses, these conditions disappeared and a siliciclastic sedimentation to ok place.