Aliphatic and aromatic biomarkers of the Devonian source rocks from the Western Parnaiba Basin Brazil: Pimenteiras Formation

摘要

Previous geochemical studies carried out in the Parnaiba Basin (Brazil) show that natural gas deposits, whose source rock is formed by the shales of the Pimenteiras Formation (PF; Devonian), stimulated new research to understand the petroleum system. Despite previous studies of PF, ifs characterization with emphasis on the biomarker content is still incipient. In this work, the organic matter (OM) of outcrop samples of the Pimenteiras Formation outcrop samples was investigated through diagnostic biomarkers, to evaluate paleoenvironmental conditions of deposition, the thermal evolution, and the organic matter type. The general distribution of hydrocarbons suggested that the samples present low thermal evolution, marine depositional paleoenvironment, and dysoxic conditions. The molecular parameters based on steranes, hopanes, Pr/C-17 and Ph/C-18, presence of nC(25), n-C-27 and n-C-29 n-alkanes showed strong evidence of marine origin OM with higher plant input. The data also suggested that OM was not affected by igneous intrusions, being below the oil window. Also, the isomers 4 alpha(methyl)- and 4 beta(methyl)-24-ethylcholestane and 24-n-propylcholestane have been detected, which are indicative of a marine depositional system. Aromatic carotenoids and derivatives suggest that euxinic conditions have reached the photic zone, an environment conducive to the preservation of organic matter.