Characterization of organic matter in the Oligocene (Chattian) turbiditic fine grained deposits, offshore Angola

Abstract : Variations in the abundance and gross composition of the organic matter (OM) content of fine grained turbiditic Chattian sediments off Angola were explored with Rock-Eval pyrolysis of 216 core samples from four wells. The study revealed that the OM in the sediments is immature but is present in sufficient abundance [>1% total organic carbon (TOC)] and with a good enough quality [hydrogen index (HI) around 185 mg hydrocarbon/g TOC on average] to constitute a potential source rock. A deeper insight into the composition of the OM was provided by the extraction and analysis of the kerogen from 15 samples. These samples were submitted to visual inspection and the counting of organic particles under the microscope (palynofacies analysis), elemental analysis and pyrolysis-gas chromatography-mass spectrometry (GC-MS). Although individually the various observations did not provide evidence clearly diagnostic of the origin of the OM, they confirm that it has a mixed marine-terrestrial origin, a notable part of the terrestrial contribution very probably originating from leaves. The good buoyancy of the OM and its richness in lipid components might explain the spreading of terrestrial OM over a large area and its relatively good oil potential.