Petrographic characterization of organic matter and solid bitumen from the Pimenteiras Formation (Devonian), Parnaíba Basin (Brazil), potential for hydrocarbon generation

Abstract

The Paleozoic-Mesozoic Parnaiba Basin is an intraplate volcano-sedimentary basin which the Pimenteiras Formation (Devonian) has the higher potential for hydrocarbon generation, mainly gas and condensate. The present study aims to make a petrographic characterization of organic matter and solid bitumen from the Pimenteiras Formation (Devonian), Parnaíba Basin (Brazil), potential for hydrocarbon generation. The study observed the presence of solid bitumen, formed as a result of the cracking of terrestrial organic matter (gas-prone) and marine organic matter (oil-prone) during diagenesis and catagenesis. These solid bitumens were classified according to the reflectance values such as glance pitch, grahamite and epi-impsonite. The solid bitumens were observed in all analyzed wells with a significance variation in terms of thermal maturity from top to bottom of the Pimenteiras sequence. High reflectance values (epi-impsonite) were observed in samples located at the bottom of the sequence in relation to the top samples, mainly in the BP-77 well. On the other hand, significance variations in terms of vitrinite reflectance values were not observed in samples close to igneous intrusions. Greater presence of macerals such as vitrinite and inertinite was identified, and rarely were observed liptinites. Large particles of solid bitumens were observed, being mostly biostructured and characterized by a homogeneous gray surface, mostly well preserved and isotropic. Similar biostructuring was also observed in palynofacies data, through the phytocasts. The solid bitumen reflectance ranges from 0.46 to 1.16 %R_o in the BP-77 well samples, with vitrinite reflectance equivalent (VRE_o) ranging from 0.68 to 1.12 %R_o, from 0.33 to 0.50 %R_o (with VRE_o varying from 0.60 to 0.71 %R_o) in the BP-59 well samples, and between 0.36 and 0.7 %R_o with VRE_o varying between 0.62 and 0.84 %R_o in the BP-22 well samples, respectively. These data suggest that the organic matter ranges from early to late mature in terms of thermal maturity. On the other hand, the solid bitumen versus vitrinite reflectance, and solid bitumen versus T_max suggest that the OM (organic matter) varies from immature to early mature. The presence of solid bitumens suggests a thermal maturity of the organic matter and potential for hydrocarbon generation.