Abstract: Petroleum Geology of the Paraná Basin, Brazil
Milani, Edison J. and Antonio J. Catto - Petrobras/E&P
The Paraná Basin is a vast sedimentary province inside South America. It holds a stratigraphic record ranging in age from Late Ordovician to Late Cretaceous, and the hydrocarbon potential of this basin is related to two well-defined petroleum systems.
The Devonian shales of the Paraná Basin are medium to fair-quality source rocks. They have a maximum TOC content of about 3% and a mean value below 1%. Hydrogen indices are generally low, probably due to advanced levels of maturation or to weathering of its dominantly woody organic matter. The Devonian shales of the Ponta Grossa Formation, one of the preferential packages for intrusions during the Mesozoic magmatic event, are overmature in almost all domains of the basin, and sourced Upper Carboniferous-Lower Permian glacigenic sandstones with gas and condensate. Recently (1996) a wildcat tested successfully this petroleum system and opened a wide prospective acreage to be explored. In the field of Barra Bonita, the first accumulation of hydrocarbons discovered in the Paraná Basin, gas was found filling the porous space of fine grained sandstones in the lower portion of the Itararé Group. The main control of the field is structural, provided by a gentle anticline, but some stratigraphic particularities in the area were also crucial. This is certainly the case for the sandy channels in the lowermost portion of the glacigenic package, that deeply carved the Devonian section and allowed secondary migration of the gas. Another peculiar aspect of the Barra Bonita field is the sealing, in this case provided by a diabase sill.
The Upper Permian bituminous shales of the Irati Formation are well developed in the southern half of the Paraná Basin and exhibit TOC content reaching values as high as 23%, with an average of about 2% of algal, lipidic-rich, oil-prone organic matter. This source bed is immature even in its deepest area of occurrence, considering the thermal effect related to subsidence and burial alone. Oils related to the Permian source beds appear abundantly as shows in the Lower Permian coastal sandstones of the Rio Bonito Formation, and were probably generated by a mechanism of heating greatly influenced by Mesozoic intrusive bodies. In the northeastern Paraná Basin, Triassic fluvial-eolian deposits occur impregnated with heavy oil geochemically correlated with Irati's organic extracts, defining a belt of tar sandstones that guarantee the mechanisms of generation and oil expulsion from the Permian black shales.
Structural and geothermal consequences due to the presence of an intricate network of sills and dikes, as well as a lava pile up to 2 km-thick at the surface, certainly inserts some additional complexities in the petroleum geology of the Paraná Basin. Otherwise, an area in excess of 1,000,000 square kilometers and incipient exploration up to this date makes this basin one of the most promising frontiers worldwide for the next decades.
AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil