Paleoenvironmental reconstruction of the Permian coal beds in the Parana & PRIME; Basin (Brazil): evidence from organic geochemical and sedimentological analyses

The Rio Bonito Formation (Paran & PRIME;a Basin) contains the thickest coal beds in Brazil, that are related to peat-swamp developed in coastal depositional systems during the early Permian. This study aims to understand the source of the coal organic matter and to distinguish depositional sub-environments along the coal-bearing interval from the southern border of the basin. The analysis was performed on sedimentological and stratigraphic data from 10 wells combined with organic petrology and geochemistry of the three coal beds. Samples were collected in different key beds from the SG-14 well, in different and strategic depths at the base (218.0 m), at the intermediate (209.0 m), and at the top (181.0 m) of the coal-bearing interval. The vitrinite reflectance measurements of the selected samples, Tmax values, C29 & alpha;& alpha;& alpha; 20 S/(20 S + 20 R) ratio, and the absence of peak Ts (and presence of peak Tm) revealed the organic matter in the coal-bearing interval is thermally immature, suggesting low burial depth for the Rio Bonito Formation in the study region. Maceral composition showed predominance of vitrinite and inertinite in the three coal samples. Rock-Eval pyrolysis data indicate that the coal beds are composed of type III kerogen in the three samples, while the biomarkers indicate type II-III kerogen mixtures. The n-alkane distribution profiles suggest dominance of terrestrial organic matter in the samples, with a possible marine contribution at the top of the coal-bearing interval. The pristane/phytane (Pr/Ph) and C27/C29 & alpha;& alpha;& alpha;20 R ratios (regular steranes) revealed sub-anoxic condition and land plants as major organic matter source for basal and intermediate parts of the coal-bearing interval. They also indicate an increased anoxic conditions and possible marine algal contribution toward the uppermost coal level. The presence of cone-in-cone-shaped structures (SG07 well, depth 157.5 m) and Orbiculoidea fossils (SG-14 well, depth 192.5 m) are indicative of brackish water and support river-marine source mixtures at the top of the coal-bearing interval. The evolutive sedimentation model comprises four paleoenvironments for the Rio Bonito Formation along geological time, in which the basal part of the coal interval is interpreted as overbank deposits in a fluvial depositional system. On the other hand, the intermediate part of the coal-bearing interval was formed in bay-head delta, and the upper part was deposited in the central estuary, both in a tide- and wave-influenced estuarine context.

Automated summary

The Rio Bonito Formation in Brazil contains the thickest coal beds in the country, which were formed in coastal depositional systems during the early Permian. This study analyzed sedimentological, stratigraphic, organic petrological, and geochemical data to understand the source of the coal organic matter and distinguish different depositional sub-environments. The results suggest that the organic matter in the coal-bearing interval is thermally immature and derived from land plants, with possible marine contributions at the top.