NATURAL POROSITY, DIAGENETIC EVOLUTION AND EXPERIMENTAL POROSITY DEVELOPMENT IN MACAE CARBONATES (ALBIAN-CENOMANIAN), CAMPOS BASIN, OFFSHORE BRAZIL

Albian-Cenomanian carbonates represent one of the major stratigraphic units of the Brazilian continental margin and, in the Campos Basin, constitute an important target for petroleum exploration. Oil has been discovered in the Macae Formation but, due to adverse properties of the reservoir rocks, only some of the discoveries show commercial oil production.; This investigation pertains to reservoir rocks in the upper 200 meters of oncolitic calcarenites of the Macae Formation, in the Garoupa Field and in non-producing wells. One hundred and fifty samples from cored intervals were used, representing all types of reservoir rocks.; Two approaches were followed in this study. In the first, the pore structure and diagenetic evolution of the reservoirs was studied using petrographic and cathode luminescence microscopy of thin sections, scanning electron microscopy of pore casts and natural rock surfaces, stable isotope compositions of oncolites and cements, sample porosity and permeability, and geophysical well log interpretation. The second approach was experimental, using a circulating triaxial apparatus where samples were submitted to porosity and permeability enhancement, under conditions simulating deep burial.; The pore structure of the producing rocks in the Garoupa Field shows three types of pores: a continuous phase of mesopores and macropores that provides transmissibility; a spongy network of micropores that provides most of the storage capacity; and lamellar micropores that connect the spongy micropores to the mesopores. Mesopores and macropores vary from interparticle to intraparticle, they are rarely vuggy or moldic, or associated to fractures and stylolites. The spongy network of micropores occurs between micrite-size calcite grains in oncolites, matrix and ooids. The lamellar micropores occur between calcite cement crystals that rim almost all available mesopores and macropores. Oil saturates both mesopores and micropores and oil saturations are normally in excess of 80%. No direct correlation exists between porosity and permeability in producing rocks.; Non-producing reservoir rocks are exclusively microporous. Porosity can reach 25% when permeability is of the order of one millidarcy. Oil saturation is around 60%. A direct correlation of porosity and permeability exists and permeability can be predicted to be in excess of 10 md for porosities greater than 30%, estimated for depths less than 2,000 meters. Under such conditions, microporous calcarenites can represent exploration targets.; The diagenetic evolution of the reservoir rocks includes a sequence of phases in a shallowing trend of early diagenesis, followed by the gradual burial and compaction of the section. Major dissolution occurred under freshwater vadose conditions while main cementation occurred in a phreatic freshwater environment.; During the porosity enhancement experiments, from 60 to 500 cc of acid solution were circulated for each test. An average increase of 4.4% in rock porosity and 4.8 times in rock permeability was produced. The vertical distribution of experimental porosity reveals a trend of decreasing porosity from top to bottom of the samples, and solution voids developed equally inside oncolites and micrite matrix but rarely inside sparite cement. Porosity features produced were the selective solution of micrite mud with respect to dolomite rhombs and the selective dissolution of oncolites with respect to sparite cement.