Paleoenvironmental characterization of the Ponta Grossa formation (Devonian) at the northwest border of the Paraná Basin, Brazil

Abstract

Previously uncharacterized Devonian shale samples from the Ponta Grossa Formation (Paraná Basin) were collected from three outcrops in Mato Grosso (DA01 and DA02) and Mato Grosso do Sul (PG02) along the northwest of the basin. The depositional paleoenvironment and hydrocarbon source potential were assessed using Rock-Eval pyrolysis, saturate and aromatic biomarkers, and carbon isotope data. For the DA01 shales, poor total organic carbon (TOC) contents and low hydrogen index (HI) indicate a high predominance of type III kerogen. The biomarker analysis results suggest a transitional depositional paleoenvironment with a greater contribution of continental than marine organic matter, pointing to a regressive marine event. In DA02, the TOC contents ranged from 0.24 to 4.46%, with kerogen types II and III, S2 ranging from poor to good, HI ranging from 78 to 410 mg HC/g TOC, and biomarkers indicating a marine transitional environment and transgressive marine event. Shales from outcrop PG02 had the highest TOC contents, particularly at the top, with predominantly kerogen types II and III, S2 varying from low to excellent, and HI ranging from 100 to 350 mg HC/g TOC. Two types of paleoenvironments were identified based on the biomarkers of PG02: regressive at the base and transgressive in the middle and top of the outcrop. The DA01 outcrop was located at a more proximal position to the coast, the DA02 were at more distal position, while PG02 was located in a transitional position between DA01 and DA02. Ratios of C29αββ/(αββ+ααα) and 20S/(20S + 20R) steranes, the presence of unresolved complex mixture, and a higher proportion of polar compounds to the detriment of saturated and aromatic compounds indicated that the shales were immature for the generation of hydrocarbons. This study represents the first broad organic geochemical evaluation of the northwest Ponta Grossa Formation.