Geoquímica orgânica detalhada da Formação Pimenteiras (Devoniano) na área sudoeste da Bacia do Parnaíba

Abstract

Pimenteiras Formation (Devonian) is the main hydrocarbons source rock of Parnaíba Basin and is composed of intervals of radioactive shales rich in organic matter. The southwestern edge of the basin still presents few studies of organic geochemistry in the formation of interest. The region has a lack of drilled wells and previous studies used samples from outcrops, which are not highly preserved. This work differs from the others by using samples from two wells with continuous coring GASBRAS-1-RS-TO and GASBRAS-2-RS-TO drilled in the State of Tocantins. Both comprise the section of Pimenteiras Formation allowing detailed geochemical characterization of the entire formation. The new data was obtained by analysis of Total Organic Carbon (TOC), Rock-Eval Pyrolysis and Biomarkers. Radioactive shales were identified and six chemostratigraphic units were defined in the Pimenteiras Formation. Unit 3 (radioactive shales B) is around 20 m thick and is characterized by maximum values of 3.0% TOC (GASBRAS-1-RS-TO) and 3.69% TOC (GASBRAS-2-RS-TO). It has moderate to good potential for gas and condensate generation. Unit 5 (radioactive shales C) is around 40 m thick and has maximum values of 7.67% TOC (GASBRAS-1-RS-TO) and 3.95% TOC (GASBRAS-2-RS-TO). It has moderate to good potential for gas generation. Unit 5 represents the 2nd order transgressive maximum and is associated with the global anoxic event of the Frasnian (Upper Devonian). In the van Krevelen type diagram the samples from the six units behave as type II, type III and type IV, with low HI values (< 300 mg HC/g TOC). The saturated biomarkers also indicated a mixture of organic matter of marine and terrestrial origin. The samples analyzed are immature and have not reached the temperature required for the maturation of organic matter and the generation of large volumes of hydrocarbons.