Abstract

The extensive, thick Paleogene carbonate platform (Amapá Formation) of the Foz do Amazonas Basin developed next to the ancient shelf edge through four depositional cycles. This important stratigraphic-structural unit of the basin became the natural target of the search for potential hydrocarbon reservoirs.

The Amapá carbonate platform shows six environmental belts: slope, apron, coralgal platform, large foraminifer shoal, finger coral bank and restricted lagoon. These belts consist of zones of intense bioaccumulation by red algae and large foraminifers separated by transverse channels where the products of their mechanical reworking accumulated as calcarenites. At all times, a terrigenous environment consisting of fan deltas and lagoonal sediments existed immediately behind the carbonate platform. It was connected with the open ocean by transverse canyons cutting across the carbonate platform and filled with shales containing carbonate olistoliths.

Distinct reservoir conditions were generated by underground circulation systems during episodes of subaerial exposure at the end of each depositional cycle when high-stand sea level changed to low-stand conditions. Excellent porosity exists in all microfacies except for the facies of the apron and slope belts, and consists of mainly enlarged interparticle and moldic porosity with a minor contribution of intercrystalline porosity related to dolomitization by mixing of freshwater and marine waters.

The Amapá platform is unique in the geologic record and the only modern analog is the Belize shelf. However, similar conditions of underground circulation and dolomitization by mixing waters exist today at a comparable scale in Florida and Yucatan.

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