Abstract: High-Resolution Stratigraphy of a Miocene/Oligocene Turbidite Reservoir from Deep-Water South-Central Campos Basin, Brazil

Becker, M.R; O.C. Assis; R.R.P. Alves; R.A. Santos; A.P. Barros; M.R. Rodrigues; O. G. Souza, Jr.; C.R. O. Rodrigues - Petrobras/Cenpes/E&P

An Upper Oligocene/Lower Miocene basin-floor fan formed the sandstone reservoir of a giant oil field in deep water (600 to 1200 m) south-central Campos basin area. This dip-elongated (NW/SE oriented), sand-rich fan originally covered an area of 130 km² and reached thickness up to 25 m. However, the submarine fan was heavily dissected by moderate to low sinuosity, 200 to 900 m-wide, deep-sea channels (Fig. 1). The original fan was compartmentalized in several NW oriented, 250-2000 m-wide, 4-10 km-long sandbodies.

The fan was built by a retrogradational stacking of 3 to 5 m-thick turbidite units. In mud-dominated areas of the field, the turbidite units show a crescent-shaped gamma-ray log-character. Radioactive, thin shale layers form the base and top of the units. In sand-dominated areas of the field, turbidite units are sharp-based, with frequent amalgamation of sandstone layers and a general fining-upward trend.

The reservoir is formed by friable, well-sorted, fine- to very fine-grained sandstones. Structureless or normally-graded, sharp-based sandstone are the most frequent reservoir facies. They form the lower portion of the sandstone beds and can reach up to 3 m-thick. Gradually, the upper portion (5 to 10 cm-thick) of these sandstone beds passes to laminated sandstone or displays low-angle cross-stratification. Silt and mud content is lower than 10% in unstratified sandstones and can reach more than 10% in laminated sandstones. The reservoir sandstones show homogeneous average porosity (around 30%) and high permeability (generally above 1000 mD). Non-reservoir facies intercalated with reservoir beds are up to 2 m-thick intervals of bioturbated marls, shales and siltstones.

Non-confined, high-density turbidite currents deposited the sandstone beds as amalgamated or laterally-retrogradationally compensated lobes to build the submarine fan. Deep basin currents may have partially reworked the turbidite sand deposits and probably generated part of the laminated and cross-stratified sandstones. Associated dilute turbidite flows and hemipelagic deposition produced the shale-marls beds intercalated with the sandstones.

Mud-filled, late channels and vertical intercalations of reservoir and non-reservoir facies observed among the turbidite units produced the complex stratigraphic framework and the most impacting compartmentalization of the reservoir.

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