ABSTRACT
The lithic sandstones and conglomerates of the Carmópolis Member of the Muribeca Formation (Aptian) were deposited by fan deltas, alluvial fans, and braid deltas that prograded from low-grade metamorphic terrains into the Sergipe-Alagoas rift basin during the opening of the South Atlantic. Initial carbonates in the Carmópolis reservoirs (presently at depths of 180–2200 m) were marine (high-Mg calcite/aragonite) grain rims, allochems, stromatolitic laminites, and meteoric calcite. These carbonates were subsequently replaced by dolomite/ankerite (δ18OPDB = −7.3 to −4.1‰; δ13CPDB = −15 to +16.2‰) derived from ascending thermobaric fluids prior to oil emplacement. These fluids also caused the direct precipitation of dolomite/ankerite cements and the replacement of nonferroan dolomite by ferroan dolomite/ankerite.
Rocks lacking early cements were strongly compacted, losing their primary intergranular porosity and permeability, whereas massively cemented rocks show only minor compaction and further diagenetic modifications. Partial cementation has greatly limited the compaction and preserved intergranular porosity, allowing the partial dissolution of carbonates and framework grains and the precipitation of replacive ferroan dolomite/ankerite and pyrite. Offshore reservoirs show late porosity reduction by the precipitation of quartz, kaolinite/dickite, saddle dolomite, and ferroan calcite. Experimental analyses of porosity and permeability reduction under pressure confirmed the importance of early cementation in the preservation of porosity in lithic rocks with ductile framework.
