Depositional and diagenetic evolution of Cretaceous oncolitic packstone reservoirs, Macae formation, Campos basin, offshore Brazil.

Abstract

Oil accumulations in oncolitic calcarenites of the Macae formation (Lower Cretaceous) of the Campos basin were discovered by seismic methods. Reservoir trap characteristics are due to a combination of structural closure and favourable diagenetic conditions. Porosity ranges 18-30% and permeability from 5 millidarcys to 2.4 darcys. The Campos basin displays a stratigraphic evolution which expresses the tectonic stages of a rifted continental margin. The last stage of this evolution was an open-marine continental shelf, which began with the deposition of the Macae platform carbonates. These consist of juxtaposed oncolitic packstone bars and intervening mudstone bands extending parallel to depositional strike. All oncolitic bars underwent a shoaling-upward evolution. When the oncolitic bars developed into intertidal beach-rock and were exposed to dissolution by meteoric water, reservoirs of economic interest were generated and preserved after subsidence and burial beneath transgressive argillaceous mudstones. Whenever further emergence of oncolitic bars led to the development of a fresh-water lens, their beach-rock porosity was occluded upon subsidence by phreatic cementation, and the reservoirs destroyed. The shoaling-upward evolution of the Macae carbonates and the structural features of the Campos basin were strongly influenced by salt tectonics involving underlying evaporites of the Lagoa Feia formation.-J.M.H.