--> Abstract: Turbidite Reservoir Distribution in Roncador Field, Campos Basin, Brazil, by P. R. S. Santos, H. D. Rangel, C. M. S. P. Quintaes, and J. M. Caixeta; #90933 (1998).

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Abstract: Turbidite Reservoir Distribution in Roncador Field, Campos Basin, Brazil

Santos, P. R. da S.; H. D. Rangel; C. M. S. P. Quintaes; J. M. Caixeta - Petrobras/E&P

Roncador oil field is located in the northern area of Campos Basin, 130 km from the coast in a bathymetric contour from 1,500 to 1,900m, The discovery well, 1-RJS-436A, drilled 153m of Maastrichtian turbidite reservoirs divided Into five main zones, Interbedded with shales related to the nannofossil biozones N-290 and N-280. Only the uppermost zone can be associated with seismic amplitude anomaly and can be traced laterally on seismic profiles. The other four zones do not present significant acoustic Impedance contrast with encasing shales. A second well, drilled in the southwestern part of the field reached Maastrichtian, turbidite reservoir presenting a different sandstone distribution. This well is located near to the pinch out of the Maastrichtian turbidite sequence that onlaps toward western against an Albian structural high. The sands and shales don?t present any significant seismic amplitude differences among them. In the southwestern area of the field most of the N-290 turbidite sandstone reservoir are amalgamated, so the depositional cycles are incomplete, without interbedding shale among the cycles. A third well also drilled in the southwestern area of the field presented similar reservoir distribution than the area drilled by the second well, except that it presented preserved depositional cycles in the upper part of the sequence, with three additional reservoir levels, interbedded with encasing shales, and it also sampled N-280 Maastrichtian turbidite sands, similarly to the eastern area of the field (1-RJS-436 A). Two other wells drilled in the northern area of the field presented a different reservoir distribution than the southwestern and the eastern areas. In the northern area N-290 turbidite reservoir continuity can be traced on seismic profiles for over 10km laterally for the uppermost and the second reservoir levels.

Two main factors control reservoir distribution differences in the three areas of Roncador field: rate of subsidence related with halokinesis and preservation of sediments from erosion. In areas with more intense subsidence associated with halokinesis it was possible to deposit and preserve more complete depositional cycles of turbidite sandstones and capping shales, such as in the eastern area of the field. In areas where subsidence was not so intense, there was turbidite sandstone deposition but each cycle was partially eroded and the sands were amalgamated with the following cycle, not preserving the finer sands and shales. This type of reservoir distribution occurs in the southwestern area of the field. In areas where the same amount of subsidence was more regionally distributed it was possible to spread turbidite reservoir for greater areas, depositing similar thickness of sandstones and capping shales. Although the upper part of the sandstones are locally eroded the amount of subsidence was enough to preserve both of them and to characterize them on seismic profiles and well logs, such as occurs In the northern area of the field. The structural and seismic attribute maps Illustrate the depositional pattern of the sands and the effects of the halokinesis in the distribution of the turbidites. Several seismic facies identified in the turbidite sequence are related to Maastrichtian depositional and erosional aspects.

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