A petrological and organic geochemical study of Permian coal seams east of Maracajá, South Santa Catarina, Paraná Basin, Brazil

Abstract

Coal samples from the Permian Rio Bonito Formation, collected from borehole MML-09 east of Maracajá, South Santa Catarina, Paraná Basin, Brazil, were characterized by petrological (vitrinite reflectance measurements, maceral analyses) and geochemical (Rock-Eval, Total Organic Carbon (TOC) and GC–MS) methods. The mean vitrinite reflectance values range from 0.73 to 0.87% Rrandom, indicating a rank level of high volatile B/A bituminous. The maceral composition showed predominance of vitrinite (49.4 to 61.2vol.%) and inertinite (9 to 24.5vol.%). The Rock-Eval analysis presented the following values: S1 and S2 ranging from 0.74 to 2.51mg HC/g coal and 34.1 to 108.87mg HC/g coal, respectively; Hydrogen Indices between 158 and 240mg HC/g TOC and Tmax (°C) between 440 and 448°C. The TOC contents of the coal samples present values ranging from 18.85 to 45.31wt.%. Vitrinite reflectance and Rock-Eval Tmax values indicate that the coals are in the beginning of the oil generation window. The potential for the formation of liquid hydrocarbons is rather low, due to the nature of the humic coal, whereas the rank level indicates that generation of gas from the coal has occurred. The biomarker analysis, using GC–MS and GC–MS–MS, allowed the interpretation of the depositional environment and provided another way of determining the degree of maturity of the coals, besides vitrinite reflectance and Tmax. The pristane/phytane (Pr/Ph) ratio is >1, mainly controlled by organic matter type, implied suboxic conditions during sedimentation. The vitrinite reflectance measurements, Tmax values and 20S/(20S+20R) and αββ/(αββ+ααα) ratios for C29 regular steranes revealed the marginally mature stage of the organic matter, although the distribution of hopanes indicated a mature stage of the organic matter, due to the absence of ββ hopanes and predominance of the 22S epimer over the 22R epimer for the hopanes from C-31 to C-35.