Abstract

Litharenites composed of sedimentary and low-grade metamorphic rock fragments are rarely good hydrocarbon reservoirs. Most of their primary porosity is lost by ductile grain deformation during mechanical compaction. However, the lithic Carmópolis Member of the Muribeca Formation from the Sergipe–Alagoas Basin, NE Brazil, is one of the most important oil reservoirs of the country. The high reservoir quality of the Carmópolis Member, despite its unusual composition rich in low-grade metamorphic fragments, indicates the occurrence of effective porosity preservation processes. The understanding of the occurrence and distribution of these porosity preservation processes are of paramount importance to optimize the recovery from producing oilfields, for the further development of these reservoirs, and for the exploration for similar reservoirs. The Carmópolis Member consists of alluvial conglomerates and sandstones intercalated with fine-grained lacustrine and alluvial-plain sediments deposited during the Aptian, in an arid environment. Detailed petrographic analyses were performed on 135 thin sections taken from cores of six wells, with support from SEM and EDS analyses. The reservoirs correspond mainly to conglomerates and to medium- to coarse-grained conglomeratic sandstones, classified as litharenite phyllarenites, subordinately feldspathic litharenites, and lithic arkoses. Dolomite is the most abundant cement. Other volumetrically significant processes are K-feldspar and calcite precipitation, feldspar dissolution, and deformation of ductile grains due to compaction. Twenty reservoir petrofacies were defined, based on the main aspects of primary composition and texture and diagenesis that control the porosity. Although partial dolomite cementation was previously considered as the main process of porosity preservation in the Carmópolis reservoirs, the present study suggests that other mechanisms such as early oil saturation may have been important for preserving their quality. Variations in depositional texture and composition, and in the distribution of these processes, resulted in the various defined reservoir petrofacies. Oil and the fluids that precipitated dolomite migrated up along major faults from deeply buried fine sediments to the shallow reservoirs.