Late Paleozoic glacially related sandstone reservoirs in the Parana Basin,Brazil

摘要

Upper Paleozoic glacially related sandstones of the Itarare Group constitute thick and complex hydrocarbon reservoirs in the intra-cratonic Parana 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 glacio-marine 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 litho-facies.In subsurface,gas-bearing sandstone intervals with the same well-log response occur below unconformities,which control the overall reservoir geometry.