Geoquímica orgânica e palinofácies da formação barreirinha, Bacia do Amazonas, Brasil: paleoambiente e maturação térmica

Abstract

The Amazon Basin is an intracratonic basin of Paleozoic age located in northern Brazil. It is known for being a pioneer area in oil exploration, being considered an area of new exploratory frontier. The shales of Fm. Barreirinha (Devonian) are hydrocarbon source rocks, with Mb. Abacaxis (Frasnian) being the most important generators. At the end of the deposition of the Paleozoic sequence, and with the beginning of the opening of the North Atlantic Ocean, the Penetecaua magmatism allowed extensive sills and diabase dykes to intrude into the sedimentary rocks of the Basin, mainly in the shales. The objective of this research was to evaluate the depositional paleoenvironment and the influence of heat from igneous activities on the maturation of organic matter, and on the potential for generating hydrocarbons in samples from two distinct outcrops of the Fm. Barreirinha: I) near the Monte Alegre Dome on the Northern border of the Basin, under the influence of igneous rocks; II) on the Southern border, with less effect of igneous rocks. Thirty-five samples were collected systematically, 12 located on the Northern border and 23 on the Southern border, of the Amazon Basin. The samples were submitted to determination of total organic carbon (TOC), Rock-Eval pyrolysis and analysis by gas chromatography with ionization detector (GC-FID), gas chromatography-mass spectrometry (GC-MS) (biomarkers), petrography and determination of stable isotopes of carbon (δ 13C). The results of the analyzes showed that the samples collected at the Northern border have excellent values of TOC, S2 ranging from moderate to excellent and are at the end of the oil generation window and beginning of gas generation. The kerogen was classified as type III, however, as the area generated light hydrocarbons, the TOC, S2 and IH values are residual. The results of saturated biomarker determinations indicate the marine origin of the organic matter, which was deposited in an anoxic to suboxic marine depositional paleoenvironment. The samples collected in the Southern border, where the influence of the heat of the igneous rocks is smaller, the TOC and the S2 varied from poor to excellent, with kerogen types II/III and III, being with low maturation for the generation of hydrocarbons. The results of the determination of saturated biomarkers indicated the marine origin of the organic matter, with some contribution of organic matter of continental origin. These shales were deposited in a suboxic to oxic environment. Still on the southern border, it was possible to individualize two distinct palynofacies (I and II). The results of the determinations of palynofacies and palynological associations indicated that the sedimentation of the Barreirinha Formation occurred, initially, in this section, in a shallow deltaic marine environment, grading to a distal marine depositional environment. The ICE (Spore Color Índex) values ranged from 3.5 to 4.5, characterizing the immature stage for hydrocarbon generation. The values of the stable carbon isotope ratio (δ 13C), for all samples, despite being very negative (less than -28‰), together with the results for the biomarkers, indicated a marine depositional paleoenvironment.