A review of the occurrence and origin of abiogenic hydrocarbons in igneous rocks
Published:January 01, 2003
J. Potter, J. Konnerup-Madsen, 2003. "A review of the occurrence and origin of abiogenic hydrocarbons in igneous rocks", Hydrocarbons in Crystalline Rocks, N. Petford, K. J. W. McCaffrey
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Reports on the presence of hydrocarbons in igneous rocks have been on the increase and generating greater interest in the scientific community over the last 20 years. Most of the occurrences are due to the incorporation of organic material into the magmatic systems. However, reports on the presence of hydrocarbons formed by abiogenic processes have also increased in recent years, suggesting that these hydrocarbons may not be as rare as previously thought and may have implications for natural gas resources in the future. This paper reviews these occurrences and the models proposed for the generation of these hydrocarbons, in particular the nature of the hydrocarbon-bearing fluids in the alkaline complexes Khibina, Lovozero and Ilimaussaq. The origin of these hydrocarbons remains controversial, whether they are (1) derived directly from the mantle, (2) formed during late crystallization stages by respeciation of a C-O-H fluid below 500°C, or (3) formed during postmagmatic alteration processes involving Fisher-Tropsch type reactions catalysed in the presence of Fe-oxides and silicates. The reports suggest that a direct mantle origin for the hydrocarbon fluid is unlikely. A model involving near-solidus reequilibration of a C-O-H fluid to a CH4-rich composition is possible, although only for extreme melt compositions that have large crystallization temperature ranges (i.e. hyperagpaitic melts). The Fischer-Tropsch synthesis of hydrocarbons in igneous rocks seems to be a more applicable model for a wide variety of igneous rocks.
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Hydrocarbons in Crystalline Rocks
‘Commercial oil deposits in basement rocks are not geological “accidents” but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement rocks should be explored with the same professional skill and zeal as ccumulations in the overlying sediments’, Landes et al. (1960), AAPG Bulletin.
Given that most OPEC countries are currently at or within 5% Production capacity, there is a growing need to look for ‘new oil’ and other hydrocarbons in non-traditional sources.
While oil and gas fields in crystalline basement are still discovered mostly by accident, as shown in this book, such reservoirs can be very prolific, especially if the basement rock is highly faulted or fractured. The chapters in this volume cover a diverse range of topics related broadly to the theme of hydrocarbons in crystalline rocks, and challenge explorationists’ definition of basement rock, which needs to be less narrow and more responsive to new geological ideas.