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 th e Muribeca Formation (Aptian) were deposited bp fan deltas, alluvial f ans, and braid deltas that prograded from low-grade metamorphic terrai ns 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 strongl y compacted, losing their primary intergranular porosity and permeabil ity, whereas massively cemented rocks show only minor compaction and f urther diagenetic modifications, Partial cementation has greatly limit ed the compaction and preserved intergranular porosity, allowing the p at-tial dissolution of carbonates and framework grains and the precipi tation of replacive ferroan dolomite/ankerite and pyrite. Offshore res ervoirs show late porosity reduction by the precipitation of quartz, k aolinite/dickite, saddle dolomite, and ferroan calcite, Experimental a nalysis of porosity and permeability reduction under pressure confirme d the importance of early cementation in the preservation of porosity in lithic rocks with ductile framework.