Abstract:
The most abundant trace metals in petroleum are vanadium and nickel, which chelate porphyrins derived from chlorophylls as demonstrated first by Treibs (1934). We analyzed trace metals in 78 oil samples distributed throughout Brazil's sedimentary basins along the Atlantic margin and in the Upper Amazon basin, using inductively coupled plasma-mass spectrography. We found that for the Fe-Cr-Mn triad and separately for the Co-Ni-Ga-Cu-La-Ce-Nd group, median trace element ratios in the oils are equal or very close to primitive mantle values, but far removed from those of the continental crust. This relationship holds true both for the entire dataset and for individual basins. Thus, for the entire dataset, median trace element ratios of the oils are: Co/Ni 0.042 (mantle 0.054, crust 0.49); Ga/Ni 0.002 (mantle 0.002, crust 0.31), and Fe/Cr 24.3 (mantle 23.8, crust 434.0). We suggest that, in the basins studied, metals with mantle-like ratios in petroleum are leached from peridotites as they become serpentinized by synrift hydrothermal activity beneath a tectonically thinned continental crust. The presence of serpentinites is indicated by geophysical data under these basins and observed directly by Leg 173 of the Ocean Drilling Program along the Iberian continental margin. Hydrogen formed during serpentinization in the absence of free oxygen may increase hydrocarbon yields in maturing source rocks of the overlying sedimentary sequence, possibly by catalytic hydrogenation of unsaturated organic compounds. Trace metal ratios reflect also later evolution of the oils. Thus in the Upper Amazon (Solimoes) basin, intruded by Mesozoic diabase sills, relative Ni and Co abundances in the oils are low because of the thermal breakdown of heat-sensitive porphyrin rings. V/Ni ratios are mantle-like in oils derived from lacustrine source rocks of the Reconcavo rift basin, but rise to crustal values in basins along the continental margin, reaching highest levels in the most biodegraded oils.