Abstract

Upper Paleozoic glacially related sandstones of the Itararé Group constitute thick and complex hydrocarbon reservoirs in the intracratonic Paraná Basin of southern Brazil. Because these sandstones were deposited in both glacial and nonglacial environments, they show a high degree of variability in facies. In this article, we analyze outcrop and shallow well data to describe facies heterogeneities and their reservoir-quality implications. A regional subsurface sequence-stratigraphic framework was built, based on both strike- and dip-oriented well-to-well correlations. A well-exposed sandstone succession correlative with the main reservoir interval in the subsurface was selected in the eastern part of the basin. The Vila Velha sandstone comprises an up to 100-m (328-ft)-thick, unconformity-bounded, sand-rich succession composed of four stacked facies assemblages. Vertical facies succession shows a transition from glacial and glaciomarine to periglacial shallow-marine and nearshore settings, which define a retrogradational-progradational succession. Channel-mouth bars and wave-modified shoreline sandstones in the progradational section contain stratigraphically predictable reservoir-quality lithofacies. In subsurface, gas-bearing sandstone intervals with the same well-log response occur below unconformities, which control the overall reservoir geometry.

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