Microfacies, depositional environments, and diagenesis of the Amapá carbonates (Paleocene - Middle Miocene), foz do Amazonas Basin, offshore NE Brazil / Breno Wolff.

Tese (Ph.D.) - University of Illinois, 1983

A total of 2,795 thin sections were studied, representing the carbonate section of 18 wells drilled in the Foz do Amazonas Basin, offshore northeast Brazil. The Amapá Formation (Paleocene-Middle Miocene) consists of a carbonate platform adjacent to a landward fluvio-deltaic system which periodically prograded over the platform. The latter became discontinuous in the central portion of the basin after the Oligocene, when transverse canyons were developed connecting the deltaic complex to the open sea. The Amapá carbonates have been divided into 22 distinct microfacies combined into two depositional models separated by a major unconformity. The paleoenvironmental reconstruction of model 1 (Paleocene to Early Eocene) consists of five juxtaposed microfacies classified into three environmental subdivisions. Model 2 (Middle Eocene to Middle Miocene) comprises 17 microfacies classified into seven subenvironments. A sequence of twelve diagenetic stages was established for model 1 based on textural relationships. The most important reservoir rocks in this model are due to primary porosity locally reduced by later cementation. These potential oil reservoirs, which locally reach 20% porosity consist of lagoonal deposits of dasycladacean calcarenites with common intraparticle and interparticle porosity. The diagenetic sequence determined for model 2 comprises 13 distinct events. The best reservoirs in this model are due to secondary porosity by both selective and nonselective dissolution, the underground freshwater circulation being the main control factor of porosity generation. In the basinwide diagenetic interpretation it is suggested that four episodes of sea level lowering disrupted the dynamic equilibrium existing between rivers and ocean, and triggered distinct reactivations of the groundwater system. These disruptions would have activated extensive underground circulations of freshwater through the carbonates, causing both porosity generation in the undersaturated zone and dolomitization in the mixed water zone.