Almério Barros França

Stratigraphy and reservoir potential of glacial deposits of the Itarare Group (Carboniferous-Permian), Parana basin, Brazil

Drilling in the Parana basin of Brazil in the mid-1980s discovered gas and condensate in the Itarare Group, and showed that glacial deposits in Brazil can contain hydrocarbons. The reservoir potential of the Carboniferous-Permian Itarare Group of the basin is analyzed using new subsurface data from 20 deep wells drilled in the early to middle 1980s. Central to the analysis was the construction of over 3000 km of cross sections based on more than 100 wells, the description of more than 400 m of core, and study of 95 thin sections. Subsurface exploration and mapping of the Itarare are greatly aided by the recognition of three recently defined and described formations and four members, which are traceable for hundreds of kilometers. These units belong to three major glacial cycles in which the pebbly mudstones and shales are seals and glacially related sandstones are reservoirs. The best sandstone reservoirs in the deep subsurface belong to the Rio Segredo Member, the uppermost sandy unit of the Itarare. The Rio Segredo Member is the best petroleum target because it is overlain by thick seals and massive pebbly mudstones and shales, and because it is shallower and less compacted than underlying, more deeply buried sandstones. This member has little detrital matrix and much of its porosity is secondary, developed by carboxylic acid and CO[2] generated when Jurassic-Cretaceous basalts, sills, and dikes were intruded into the Parana basin as Gondwana broke up.

Ocorrência de dolomita em sela de rochas carbonáticas na mina Rio Bonito, Grupo Açungui (Paraná)

The dolomitization process which affected the carbonate rocks in the Acungui Group, exposed in the Rio Bonito quarry, occurred in two distinct episodes, namely of replacement and void-filling types. In the first case, the ancient limestone had its matrix largely substituted by dolomite with finely to averagely-grained crystals, primarily unimodal and secondarily polimodal distribution, in individuals of cloudy aspect. The void-filling type of dolomitization, responsible for the precipitation of micritic material (Dp1) of dolomitic and Fe-rich composition, originated from fluid in chemical disequilibrium with the fluid responsible for the previous dolomitization. The dolomitic cement Dp2 is constituted of saddle dolomite with non-planar individuals, coarsely-grained and with strong undulate extinction. Moreover, this type of dolomite occurs in the process of fracture-filling, in the breccia as well as in the dolomite marble, with generation posterior to that of the previous type. The dedolomitization process occurred in the dolomitic breccia, under superficial conditions, with the precipitation of late calcite as a void-filleing in association with the dolomite from Dp2 generation. Quartz completes the final phase of the void-filling process

NOTA BREVE: Ocorrência de dolomita em sela de rochas carbonáticas na mina Rio Bonito, Grupo Açungui (Paraná)

The dolomitization process which affected the carbonate rocks in the Acungui Group, exposed in the Rio Bonito quarry, occurred in two distinct episodes, namely of replacement and void-filling types. In the first case, the ancient limestone had its matrix largely substituted by dolomite with finely to averagely-grained crystals, primarily unimodal and secondarily polimodal distribution, in individuals of cloudy aspect. The void-filling type of dolomitization, responsible for the precipitation of micritic material (Dp1) of dolomitic and Fe-rich composition, originated from fluid in chemical disequilibrium with the fluid responsible for the previous dolomitization. The dolomitic cement Dp2 is constituted of saddle dolomite with non-planar individuals, coarsely-grained and with strong undulate extinction. Moreover, this type of dolomite occurs in the process of fracture-filling, in the breccia as well as in the dolomite marble, with generation posterior to that of the previous type. The dedolomitization process occurred in the dolomitic breccia, under superficial conditions, with the precipitation of late calcite as a void-filleing in association with the dolomite from Dp2 generation. Quartz completes the final phase of the void-filling process.