Reconstituição Paleoambiental e Potencial Petrolífero da Sucessão Siliciclástica-Carbonática Permiana da Bacia do Paraná

Abstract

The Permian age was marked by extreme paleogeographic and paleoclimatic changes with predominances of arid conditions across the globe as a consequence of the eustatic sea-level fall from the end of the Carboniferous glaciation to the beginning of the Permian. The final phases of continental agglutination of the Pangea supercontinent caused uplifts accompanied by successive regressions of the epicontinental seas, culminating with the installation of desert systems at the end of the Permian. During the marine regressive phase, Western Pangea was the depositional site of a 50m thick mixed siliciclastic-carbonatic succession under restricted and shallow paleoenvironmental conditions, widely distributed in the Paraná Basin, SE of South America. The studied succession includes the top of the Palermo, Irati, and the base of the Serra Alta Formation, comprising a total of 120 m-thick. The Irati succession is essentially composed of dolomite intercalated with organic matter gray to black shale rich, which is considered an important source rock for hydrocarbons. This unit covers the heterolithic deposits from the Palermo Formation and is overlaid by shales from the Serra Alta Formation. The high content and good quality of organic matter present in Irati shales aroused economic interest since the 19th century. Although many works have contributed to the knowledge of the depositional paleoenvironment and generating potential of this unit, mainly from organic geochemical data, gaps remain regarding the paleoenvironmental understanding, previously interpreted as restricted sea or lacustrine. This research was carried out from 125 drill cores distributed in the center-north, center-south, and extreme south of the basin, provided by the company Irati Petróleo LTDA. complemented by outcrops from the northern region. Twenty-three drill cores were selected for sedimentological and stratigraphic study from facies/microfacies analyses, aided by XRD, SEM-EDS, and cathodoluminescence images. Total organic carbon (TOC), Rock-Eval pyrolysis, and biomarker analysis were performed on 102 cores. The integration of data allowed: the lateral correlation of the Irati Formation for more than 2,000 km in the SSW-NNE direction of the basin; the paleoenvironmental reconstitution; and lateral and vertical characterization of the generating potential. Twenty-one facies/microfacies were identified and organized into about 300 siliciclastic-carbonate couplet that are grouped into 59 high-frequency cycles representative of mid-outer ramp and offshore environments dominated by distal turbidity systems. The succession consists of four third-order sequences (S1, S2, S3, and S4). Sequence boundaries are type 2, with no evidence of subaerial erosion, marked by overlapping transgressive offshore deposits. Transgressive Systems Tracts are succeeded by Highstand Systems Tracts defined by the appearance of dolomite levels with a thickening upwards tendency, which indicates high carbonate productivity under normal to hypersaline conditions, evidenced by the presence of gammaceran, halite crystals. and pseudomorphs of gypsum. The transgressive deposits of sequences S3 and S4 (Assistance Member) form the two intervals (oil-shale) with the greatest generating potential in the Paraná Basin. Oil-shale S3 has the highest values of organic carbon and generating potential. The highest TOC peaks were 19.40% for the extreme south region, 22.23% for the center north, and 27.12% for the center-south of the basin. Kerogen predominates types I and II, which also presents an increased contribution, mainly from type I converging to the center-south. Oil-shale S4 presents lower TOC values for the extreme south region (8.82%), south-central (21.7%), and north-central (14.61%). The kerogen type is similar to oil-shale S3, it predominates type II, with a high contribution of type I in the south-central region and smaller proportions of type III. The organic matter of the Irati Formation is predominantly immature, however, occurrences of maximum pyrolysis temperature (Tmax) equal to or greater than 440 ºC in samples close to diabase sills show that there was localized maturation of organic matter, which corroborates the occurrence of a non-conventional petroleum system for mixed deposits of black shales (generators) - carbonates (reservoirs) of the Irati Formation. Regarding the quantity and quality of the source rock present in the Irati Formation, the center-south portion presents the highest values of organic carbon as well as the potential for hydrocarbon generation. The analysis of the stacking patterns associated with the previous ages of SHRIMP U-Pb from volcanic ash allowed the correlation of the succession with the global sea-level curve, allowing to estimate an age of 8.0 Ma for the Irati Sea and of 2 .7 Ma for 3rd order depositional sequences. Likewise, ages of 26.6 ka were calculated for the carbonate-siliciclastic couplets, 135.5 ka and 400 ka for the high-frequency cycles, whose origin is here attributed to the climatic cyclicity induced by the terrestrial orbital oscillation, compatible with the Milankovitch cyclicity. The characterization of the cycles based on faciological and organic geochemical data also demonstrates a strong climate control in the generation of intervals rich in organic matter.