DOLOMITE DIAGENESIS AND POROSITY PRESERVATION IN LITHIC RESERVOIRS - CARMOPOLIS MEMBER, SERGIPE-ALAGOAS BASIN, NORTHEASTERN BRAZIL

The lithic sandstones and conglomerates of the Carmopolis Member of the Muribeca Formation (Aptian) were deposited bp 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 Carmopolis 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 (delta(18O)PDB = -7.3 to -4.1 parts per thousand; delta(13C)PDB = -15 to + 16.2 parts per thousand) 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 pat-tial 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 analysis of porosity and permeability reduction under pressure confirmed the importance of early cementation in the preservation of porosity in lithic rocks with ductile framework.