Geochemical characterization and origin of kerogens from source-rock of Devonian in the Amazonas Basin, Brazil

Abstract

Thirty-seven devonian shale samples from the Barreirinha Formation were collected from the northern and southern borders of the Amazonas Basin and were geochemically analyzed to evaluate their hydrocarbon source potentials and depositional paleoenvironments and to estimate the thermal effects of igneous intrusions on kerogens. The results showed that samples from the northern border exhibited total organic carbon (TOC) and Rock Eval pyrolysis parameters that indicated excellent hydrocarbon source potential and type III kerogen. These TOC values were residual due to the intense hydrocarbon generation from heat from intrusive rocks present in that area. Samples from the southern border had poor to excellent hydrocarbon source potential and type II and III kerogens. Chromatograms of organic extracts and saturated biomarkers suggested marine anoxic to oxic and suboxic to oxic depositional paleoenvironments in the northern and southern borders, respectively. The saturated biomarkers and stable carbon isotope ratios (δ¹³C) suggested that the original organic matter was marine with some continental contributions. The maximum pyrolysis temperature (Tmax), Spore Color Index (SCI), kerogen color, Pr/n-C17 and Ph/n-C18 ratios, and biomarker data indicated that the northern border shales are at the end of the oil generation window, while those from the southern border are at the beginning of the oil generation window.