Estratigrafia e ambiente deposicional do Grupo Itararé (Permocarbonífero), Bacia do Paraná (parte.2)

Abstract

The present paper offers a petrographic and reservoir analysis of the Itararé Group, a Permo-Carboniferous glacigenic deposit in southern Brazil's Paraná Basin. Part 1, dealing with stratigraphy-and the depositional environment, appeared in Volume 2, n. 2-4 of this bulletin. The Itararé Group contains basically two types of sandstones: one clay-rich, displaying no porosity and gamma ray values of over 50 API units; the second contains little or no clay, displays less than 50 API units, and porosity ranging from five to 30%. Porosity is mostly secondary intergranular, due to the dissolution of early carbonate cement, with minor contribution of the dissolution of feldspars and lithics. The development of secondarv porosity in the Itararé Group was probably contemporaneous with, or later than, the Gondwana break-up, when the Paraná Basin was affected by a regional thermal anomaly caused by the stretching anil thinning of the lithosphere during the continental rifting and opening of the Atlantic Ocean. The thermomaturation of organic matter present in the Devonian black shales released carboxylic acids, promoting the dissolution of carbonate and feldspar and producing secondary porosity. Extensive dissolution observed in sandstones from shallow wes (e. g., 700 m deep) near the Paraná Basin's eastern outcrop be/t is probably associated with the percolation of meteoric water through reservoirs in outcrops near the recharge zone. The most common cements found in Itararé Group reservoirs are anhydrite and Fe-dolomite, both probably precipitated following the appearance of secondary porosity. Minor cements include dolomite, Fe-calcite, chlorite, and quartz overgrowths. Chlorite occurs preferentially below 3,000 m deep.