Indicadores de Zona Fótica Anóxica em Petróleos da Bacia de Sergipe-Alagoas: Distribuição de Aril Isoprenoides e Aril Carotenoides
biomarkers, aromatic carotenoids, Sergipe-Alagoas Basin.
Four crude oil samples from Sergipe-Alagoas Basin, Northeastern Brazil, were analyzed using Gas Chromatography coupled to Mass Spectrometry (GC-MS, full Scan), in order to detect and characterize the oils based on classical biomarkers, and correlate with the aromatic carotenoids and use them as indicator tools of the settings of depositional paleoenvironment. The chromatograms and/or geochemistry parameters of the oils’ saturated fraction analyzed from Sergipe-Alagoas Basin indicated that the oils showed low levels of biodegradation, similar maturation (on terpanes Ts/(Ts + Tm)), C29 αββ/(αββ + ααα), C27, and C29 20S/(20S + 20R) steranes, deposition environment reducer (Pr/(Pr + Ph) < 1), and saline (presence of gammacerane and β-carotane). Moreover we identified a large range of diagenetic and catagenetic products of the aromatic carotenoid isorenieratane with a variety of structures including C40, C33, and C32 diaryl isoprenoides and aryl isoprenoides derivatives of short chain and/or additional rings. These compounds presumably are degradation products of isorenieratene, a pigment of the photosynthetic green sulphur bacteria Chlorobiaceae. This organisms live at/or below the oxic/anoxic boundary layer and require both light and H2S. The presence of diagenetic and catagenetic products of the isorenieratene in crude oils is therefore an excellent indication for photic zone anoxia in the depositional environment.