--> ABSTRACT: Source Rock Generative Potential of Upper Devonian Shales (Ponta Grossa Formation), Parana Basin, Brazil, by Eugenio V. Santos Neto, Jose R. Cerqueira; #91020 (1995).

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Source Rock Generative Potential of Upper Devonian Shales (Ponta Grossa Formation), Parana Basin, Brazil

Eugenio V. Santos Neto, Jose R. Cerqueira

The Ponta Grossa Formation is composed of shales and sandstones reaching a maximum thickness of about 600 m. In the northern part of the basin, such shales are of Frasnian age and have organic carbon contents of about 1.5%, hydrocarbon source potential up to 5 kg HC/t rock, and low thermal maturity. Most of these shales contain liptinitic organic matter and have hydrogen indices lower than 300 mg HC/g TOC and oxygen indices up to 100 mg CO2/TOC, suggesting deposition of organic matter under relative oxic conditions. Such interpretation is corroborated by the presence of bioturbation structures, invertebrate fossils and low hydrocarbon potential yields.

Paleogeographic reconstructions of Late Devonian (Famennian/Frasnian) suggest that most of the Parana Basin was located at latitudes of about 60-65° S. Under cold conditions circulation is likely to be invigorated and oceanic waters can become enriched in oxygen. Consequently, stratification of the water column and efficient preservation of the organic matter are not expected. These conditions would account for the low content of organic matter in the upper part of Ponta Grossa Formation.

The widespread occurrence of basaltic intrusions in the Ponta Grossa Formation in the southern part of the Parana Basin would explain the advanced stage of thermal evolution, and the generation of condensate and gas. However the generation of petroleum through the thermal influence of igneous intrusions, and the presence of oxidized organic matter decrease the significance of these Upper Devonian shales as source rocks.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995