Major Paleoceanographic Events Of The Brazilian Continental Margin: Relationships With The Giant Oil Fields Of The Oiigo-Miocene Of Campos Basin, Brazil

Abstract

ABSTRACT Major sequence boundaries that occur in Campos basin during the Oligocene and early Miocene are related with cooling of bottom waters and basinward shift of coastal onlap, as indicated by the isotopes values. These boundaries occur at 35.8 (Albacora, Bicudo, Bonito, Carapeba, Enchova, Garoupinha, Vermeiho, and Vioia fieids), 30. 1/29.8 (Albacora fieid), 28.6 (Albacora and Enchova fields), 27.4, 25.2/24.7 (Mariim, Albacora, Moréia, Vermelho, and West Enchova), 22.3, 21.3, 17.4 (Albacora fieid), and 16.6 Ma (Albacora field). They show a close relationship with the sequence boundaries marked as global events. INTRODUCTION The Campos and Santos basins are located in the Southeast portion of the Brazilian continental shelf. The studied sections comprise the lower Oligocene to lower Miocene in Campos basin, and the lower Miocene in Santos basin. The main objective of this paper is to correlate the Campos basin Tertiary deep-water giant oil reservoirs with the major paleoceanographic events that affected the Brazilian continental margin during Oligocene and early Miocene times. This research was developed through the determination and time calibration of the Campos and Santos basins main isotopic events.1,2 The isotopic record was compared with the sequence stratigraphy of Campos basin, the Cenozoic eustatic and onlap curves,3 the Neogene erosive events of Santos basin,4 and the isotope stratigraphy for the Atlantic Ocean.5 The ?13C positive events correspond to periods in which the water temperature was milder, reflecting spans of time of major productivity and preservation of organic matter. These positive events can be correlated to phases of coastal onlap landward shift.6 The negative events of ?13C represent epochs of minor organic productivity and higher organic matter oxidation, due to the presence of cold and oxygenated bottom currents. These negative events of the carbon isotopic values can be correlated to phases of coastal onlap basinward shift. The Cenozoic paleoclimatologic and paleoceanographic evolution was basically controlled by the progressive coding and glaciation on the pales and perhaps by some warming in the tropics.7,8 Thus the accumulated ice volume record, potentially evaluated through the ?18O record, is of special interest, being related to global events such as sea-level variations. It is possible through seismostratigraphic analysis to establish chronohorizons to these glacio-eustatic events, since the isotopic variations are abrupt and occur in a short period of time. The importance of the detailed study of these glacio-eustatic variations, obtained from the integrated analysis of isotope stratigraphy, seismostratigraphy, and biostratigraphy, lies on the possibility of getting a more accurate timing determination of erosive events and associated turbidite deposits of potential economic interest. METHODOLOGY The analytical procedures and the utilized equipment for the geochemical analyses are better described elsewhere.1,2 The sample preparation technique used in this work was defined by McCrea.9 The sample analyses were made at the Geochemistry Section of the PETRO-BRÁS Research Center. The isotopic values expressed are related to the PDB pattern.