Abiogenic origin of petroleum hydrocarbons: Need to rethink exploration strategies

retically, the second law of thermodynamics permits the genesis of natural petroleum only at depths more than 100 km at pressures of above 30 kbar well into the mantle of the earth and at higher temperatures consistent with that environment 8 . Further, the statistical mechanical model predicts that methane compressed to 30–40 kbar (pressure at depths around 150–200 km) at 1000°C (conditions in the mantle) yields hydrocarbons having properties similar to petroleum 8 . It is suggested that this petroleum of primordial material of deep origin at high pressure is transported via ‘cold’ eruptive processes into the crust of the earth until it escapes to the surface or is trapped by impermeable strata, forming petroleum reservoirs. Proponents of ‘abiogenic’ petroleum origin contend that deep microbial life is responsible for the biomarkers that are generally cited as evidence of biogenic origin. In this regard it must be noted that the microbial life has been discovered 4.2 km deep in Alaska and 5.2 km deep in Sweden 2 . The abiogenic theory on the origin of petroleum seeks to explain the origin of commercial accumulations of petrochemicals via chemical mechanisms such as serpentinite catalysis. Lipid components in hydrothermal sulphide deposits from the active Rainbow vent field (MidAtlantic Ridge at 36°N) revealed the presence of methane and possibly a minor amount of the alkanes anticipated to be of ‘abiogenic’ origin 9 . A recent study has shown the occurrence of low-molecularweight hydrocarbons in natural hydrothermal fluids, which has been attributed to abiogenic production by Fischer– Tropsch-type (FTT) reactions based on concentration, and the presence of stable radiocarbon isotopes of hydrocarbons dissolved in hydrogen-rich fluids venting at the ultramafic-hosted Lost City Hydrothermal Field 10 . These findings illustrate that the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.