Porosity Preservation in the Lithic Carmópolis Reservoirs, Sergipe-Alagoas Basin, Brazil

Abstract

Aptian lithic sandstones and conglomerates of the Carmópolis Member, Muribeca Formation, constitute the reservoirs of the giant Carmópolis and adjacent oil fields of Sergipe-Alagoas Basin, NE Brazil. The reservoirs were deposited by alluvial-deltaic systems in the margin of a “pre-salt” hypersaline lake, and are highly porous, despite being rich in ductile, low-grade metamorphic rock fragments. A systematic petrographic study of 135 thin sections prepared from core samples from 6 wells revealed that early partial cementation by Fe-dolomite, previously regarded as the main porosity preservation process through compaction inhibition, is clearly not the only factor responsible for the quality of these reservoirs. Several samples with little or no dolomite cementation still present unusually high porosity values. Therefore, other mechanisms of framework sustaining, including early oil saturation and overpressure, are being evaluated. The systematic study of the variation of depositional texture and composition, and of the distribution of dolomite cementation and other diagenetic processes in relation to facies and stratigraphic sequences and boundaries is revealing the controls on the quality of Carmópolis reservoirs. Understanding the distribution of these controls will contribute for improving production from these unconventional and complex reservoirs, as well as for identifying new potential prospects with similar characteristics.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016