Trace element distributions in biodegraded crude oils and fractions from the Potiguar Basin, Brazil

Trace element distribution patterns were determined in crude oils from onshore biodegraded reservoirs along a secondary migration trend (Potiguar Basin, Brazil). Samples were fractionated into asphaltenes, paraffins, aromatics and polars and resins for the determination of 22 trace elements by ICP-MS. Most of them are strongly enriched in the resin and asphaltene fractions, but V and Ni are also associated with the aromatic and polar fractions. V, Co, Ni and Mo were influenced by biodegrading processes and accumulate in the most affected and migrated samples. Variations of the Ni/V ratio in aromatics and polars, matching those of homohopane and gammacerane indexes in paraffin, evidenced also the influence of biodegradation. Very similar elemental distribution patterns in oil and corresponding asphaltenes were observed, suggesting that the latter fraction, more refractory than the original oil, can be used as a robust inorganic geochemical fingerprint for oil characterization.