Abstract: An Integrated Approach Using Cyclostratigraphy, Stable Isotopes and Paleoecology as a High Resolution Technique for Deep-water Reservoirs

Azambuja Filho, Nilo C. de; Azevedo, Ricardo M. L.; Rodrigues, Rene; and Koutsoukos, Eduardo A. M. - Petrobras/Cenpes

An integrated stratigraphic approach, using cyclostratigraphy, stable isotopes and paleoecological studies, has been applied to refine the temporal distribution of the Oligocene-Miocene deep-water section of the Campos basin. The succession consists of marlstones, mudstones and turbidite layers deposited in a bathyal environment.

Detailed oxygen isotope studies, using bulk samples, have allowed the recognition of significant fluctuations in d¹⁸O values for the Oligocene and early Miocene, that can be correlated to global isotopic events, such as Miller et al. (1991) and Pekar & Miller (1996) Oi₁, Oi_b, Oi₂, Oi_2a, Oi_2b and Mi₁ oxygen isotope zones (Fig. 1 and 2). The basal Oligocene succession in the Campos basin records an important event characterized by an increase in d¹⁸O values (+ 1‰). It corresponds to the Oi₁ event, dated 33.5 Ma (Bergreen et al. 1995 time scale) and is related to the formation of one of the first large continental ice sheets on Antarctica. Just before Oi2 isotope zone there is a distinctive key bed (Blue Marker) that consists of chalks, marlstones and mudstones with anomalous amount of Braarudosphaera sp. Remarkable fluctuations in the d¹⁸O and d¹³C values and in the concentrations of CaCO₃ have been recorded associated with this interval. It represents a response to oceanic upwelling of cooler waters enriched in nutrients, which has enhanced primary productivity and has an important oceanographic signature in the South Atlantic sediments.

The Oi_2a and Oi_2b events testify two important phases of d¹⁸O enrichment in oceanic carbonates, which had occurred between 30.3 Ma and about 27.1 Ma ago. The latest Oligocene was characterized by a renewed heating phase of oceanic waters, as deduced from the lowering of d¹⁸O values, both in the global curve and in the Campos basin. This tendency is modified at the Oligocene/Miocene boundary when another episode of oceanic cooling is recorded by means of a significant increase of d¹⁸O in marine carbonates. The close stratigraphic correspondence between isotopic markers of Campos basin and the main events indicated by Miller et al. (1991) for the Atlantic demonstrate the far-reaching effects of paleoceanographic changes along the Brazilian continental margin during the Tertiary.

Spectral analyses of the calcium carbonate content of marlstone and mudstone couplets and bundles indicate that the Oligocene-Miocene succession of the Campos basin has recorded in its deep-water carbonate factory the orbital beat of the precession, obliquity and eccentricity. Precession and eccentricity cycles dominated during the Oligocene, whereas for the Miocene obliquity and eccentricity played the key role in controlling carbonate cycles. Cyclostratigraphy calibrated by biostratigraphical and paleoecological studies integrated with isotopic events have led to refine the studied interval down to 100,000 years intervals allowing the prediction of the temporal distribution of deep-water reservoirs.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil