Mineralogia da Formação Irati e suas implicações paleoambientais, paleoclimáticas e estratigráficas na Bacia do Paraná, região de Sapopema, Paraná

Abstract

This current study aims to characterise the mineralogical content of the Irati formation and interpret any observed variations in terms of the paleoenvironmental and paleoclimatic conditions, and stratigraphic context of the formation. For correlation purpose we also discuss the organic content of the Irati Formation and its economic importance. Stratigraphic information and samples of the Irati formation were obtained from two wells (SP-32-PR and SP-58-PR), within the Sapopema Municipal of north-eastern Paraná state. Despite the proximity of the two wells the geological context of the Irati formation varies greatly from one well to the other. In well no. SP-58-PR the Assistência Member of the Irati Formation is intruded by a 60 m thick diabase sill whilst this sill was not encountered in well no. SP-32-PR. Mineralogical investigations involved X-ray diffraction (XRD), Scanning Electron Microscopy (SEM)and Energy Dispersive X-ray Spectroscopy (EDS). The distribution of organic material, variations in porosity, total carbon content (TOC), sulphur content (S), insoluble residue (IR) content as well as other Rock-Eval pyrolysis data of the Irati formation is presented in this study. The analytical results for the Irati Formation sampled in well no. SP-32-PR indicate the presence of authigenic minerals particularly within the upper portion of the Assistência Member. It also contains the highest values of TOC, S content and IR content. The correlation allows us to suggest that the paleoambiental conditions of this interval was characterised by the circulation of alkaline fluids within a restricted marine environment dominated by pelagic sedimentation which allowed the accumulation of organic rich sediments. The occurrence of gypsum and the absence of kaolinite suggest possible arid condition for the studied interval within the pelitic Assistência Member. Variations in mineralogical content within the Irati Formation can be used to help identify surfaces of sequence stratigraphy such as the limit between the Taquaral and Assistência Members, or the limit with the overlying Serra Alta Formation. The organic geochemistry of the Assistência Member confirms its potential as a source rock, although this interval is considered to be thermally immature. XRD of the Assistência Member sampled within well no. SP-58-PR indicates the presence of high-temperature minerals such as talc and pyroxene related to the intrusion of the diabase sill. SEM/EDS identified enstatite and plagioclase in the samples taken from close to the intrusive contact. These minerals appear to have formed through the reaction of quartz and saponite (smectite). In contrast, hydrothermal alteration in more distant intervals led to reactions between dolomite and quartz to generate talc and calcite.