Evaluation of the hydrocarbon generation potential of the Pimenteiras Formation, Parnaiba Basin (Brazil) based on total organic carbon content and Rock-Eval pyrolysis data

Abstract

The Parnaiba Basin is a Paleozoic to Mesozoic intraplate volcano-sedimentary basin where the Pimenteiras Formation (Devonian) is the main sequence with potential of hydrocarbon generation, mostly natural gas. The present paper evaluates the potential of hydrocarbon generation of Pimenteiras Formation based on Total Organic Carbon (TOC) and Rock-Eval pyrolysis parameters. In this work, 1077 shale samples of the Pimenteiras Formation distributed in 32 wells were evaluated. The TOC content varies between 0.1 to 4.7 wt.%, partially reflecting the accumulation and preservation rates of the organic matter in marine and coastal depositional environments controlled by regressive-transgressive cycles. The oxic and anoxic conditions vary significantly with deposition in this situation, which were evidenced by HI and OI variations through sample profiles. In the north and center of the basin, the Pimenteiras Formation has a higher potential for hydrocarbon generation relative to the south, probably due to higher anoxic conditions during deposition. The Hydrogen Index indicates the predominance of kerogen types II and III with minor occurrences of types I and IV. The Tmax values indicate general immature conditions and locally postmature, where the lowest temperatures represent the basin´s burial history, whereas the higher ones were influenced by igneous intrusions and thermogenic anomalies related to the Transbrasiliano Lineament. In addition, the excessive heat around the intrusions altered the Rock-Eval pyrolysis parameters as well as the type of organic matter, resulting in a relative increase of the kerogen types III and IV, which explains the great potential for gas generation in this basin.

New U–Pb geochronology for the Central Atlantic Magmatic Province, critical reevaluation of high-precision ages and their impact on the end-Triassic extinction event

“…The Parnaíba Basin presents a similar feature to the other two Brazilian basins, where mafic sills can extend continually for 200 km, have an estimated maximum thickness of ca 500 m 56 , 57 , and intrude the Canindé (fine sandstone, siltites and black shales), Serra Grande (sandstone and shales), and Balsas (clastic-evaporitic complex) groups 57 . The Pimenteiras Formation (Canindé Group) has more than 500 m in thickness and is composed of dark shales rich in organic matter 58 , 59 , where total organic carbon reaches up to 16.60 wt.%, while rock–eval pyrolysis was measured to a high of 51.43 mm HC/g 59 . Additionally, vitrinite reflectance values show an increase in T max (451 °C) and the presence of talc, chlorite and illite can be associated with the thermal influence of igneous intrusions 58 , 59 .…”

“…The Pimenteiras Formation (Canindé Group) has more than 500 m in thickness and is composed of dark shales rich in organic matter 58 , 59 , where total organic carbon reaches up to 16.60 wt.%, while rock–eval pyrolysis was measured to a high of 51.43 mm HC/g 59 . Additionally, vitrinite reflectance values show an increase in T max (451 °C) and the presence of talc, chlorite and illite can be associated with the thermal influence of igneous intrusions 58 , 59 . Overall, there is a direct correlation between hydrocarbon generation and magmatic events in the Parnaíba Basin 56 , 58 – 60 .…”

“…6 ). Therefore, degassing of sediments in Brazilian intracratonic basins, such as the Parnaíba, Amazon and Solimões 2 , 3 , 8 , 9 , 56 , 59 , could have all joined forces to play a role in the End-Triassic crisis, but it remains necessary to establish the widespread occurrence of the first CAMP magmatic pulse that seems to account for most of the greenhouse gases released around the ETE and TJB.…”

“…Additionally, vitrinite reflectance values show an increase in T max (451 °C) and the presence of talc, chlorite and illite can be associated with the thermal influence of igneous intrusions 58,59 . Overall, there is a direct correlation between hydrocarbon generation and magmatic events in the Parnaíba Basin 56,[58][59][60] .…”