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Centre for Earth and Environmental Science Research

Hydrocarbons in alkaline and carbonatite intrusions: geochemistry and distribution, origin and evolution, environmental, geological and metallogenic implications

Executive Summary
ResearchersDr Peter J. Treloar
Professor Andy Rankin
Ms Bettina Beeskow (PhD student, now at GFZ, Potsdam, Germany)
Dr Jo Potter (University of Western Ontario)
Dr Valentin Nivin, Kola Science centre, Apatity, Russia
Dr Torsten Vennermann, University of Lausanne
Dr Mark Lespinasse, Nancy
Funding Body/SourceINTAS, Brussels
Duration2002 - 2005
Project SummaryTo identify the distribution of gaseous, liquid and solid hydrocarbons in igneous peralkaline, alkaline and carbonatite intrusions on regional, local and micro-structural scales; to constrain the sources of the hydrocarbons and the conditions and mechanisms of their origin and evolution; and to evaluate the extent to which such hydrocarbons may form a viable economic future energy resource.

The alkaline rocks of the Kola Peninsula in NW Russia have long been known to host large volumes of methane and other hydrocarbon gases. It is possible to distinguish between occluded gas (OG), which is contained in sealed fluid inclusions and micro-cracks, and free gas (FG) which occurs within interconnected fractures. OG and FG compositions are similar in that both contain the same gas species. However, ratios between the gas species are different. This may be the result of the way that the FG have evolved. They likely represent a mixed gas that includes aliquots of gas formed elsewhere, including subsequently released OG, which have migrated and mixed during migration. These gases will have been oxidised by reaction with the wall rocks to the fractures through which they have migrated.

The main hypothesis to test at the start of the project was that the hydrocarbons were abiogenic in origin and had formed through a post magmatic Fischer-Tropsch like reaction in which H2 released by hydration reactions combined with residual post-magmatic CO2 to form CH4 and higher hydrocarbons. Our chemical and isotopic data confirm the abiogenic nature of the main hydrocarbons present in the Kola rocks. However, textural and fluid inclusion data suggest that Fischer-Tropsch like reactions were not the main driver of hydrocarbon gas generation (Beeskow et al, 2006). In particular, as we find no evidence of primary CO2-rich fluid inclusions in syn-magmatic minerals, it is unlikely that CO2 was a primary gas phase within the magma system. By contrast, all primary inclusions are CH4-rich and, in cases, are attached to discrete crystals. These data suggest that CH4 was not produced by post-magmatic reactions. They also suggest that the methane was not produced by late-magmatic respeciation. The only remaining option is that the methane exsolved from methane rich alkaline magmas as they crystallised.

Our data suggest a heterogeneous distribution of HCG within the complexes meaning that HCG distribution is not constant across the massifs. In the Khibiny massif, for instance, HCG concentrations are highest in the apatite-rich rocks of the "Central Arch". The Central Arch is a horseshoe-shaped structure which separates the inner and outer parts of the complex and is characterised by high REE concentrations. This structure is marked both by high HCG concentrations and by high gas fluxes in both underground workings and sub-soil atmospheres. To form a viable economic resource there needs to be both a sufficient quantity of hydrocarbon gases (HCG) contained within the igneous complexes and a high degree of fracture connectivity, assuming that HCG-rich free gases are contained within the fracture system.

Gas mobility data collected to date document variable release rates of free gas from fracture systems opened during mining. Release rates of up to 15 to 20 ml min-1 have been measured. Gas overpressure in pre-stressed rocks is up to 1000hPa. These data are consistent with consistent leakage of methane into workings within alkaline complexes. HCG concentration appears to be independent of rock-type but is instead linked to fracture and fault systems within which we identify an array of inter connected fractures of various ages. We are using a novel approach to model palaeo- and present porosities and permeabilities to constrain the degree of interconnectivity in the alkaline rock system. Preliminary results suggest a high degree of interconnectivity suggesting that the fracture system can be opened that methane can be extracted from the system.

Publications

Beeskow B., Rankin, A,H., Murphy, P.J. & Treloar, P.,J. 2005. Mixed CH4-CO2 fluid inclusions in quartz from the South Wales Coalfield as suitable natural calibration standards for microthermometry and Raman spectroscopy. Chem, Geol., 223, 3-15.

Beeskow, B., Treloar, P.J., Rankin, A.H., Vennemann, T.W., Spangenberg, J. 2007. A reassessment of models for hydrocarbon generation in the Khibiny nepheline syenite complex, Kola Peninsula, Russia. Lithos ***,***

Nivin V. A. and Treloar P J. (2005) Free-phase hydrogen-hydrocarbon gases in the Lovozero peralkaline intrusion: origin, occurrence and environmental implications of their continued seepage. In: Peralkaline Rocks: Sources, Economic Potential and Evolution from Alkaline Melts. Abstract Volume. Compiled by Michael Marks. P. 79-81.

Nivin V. A., Treloar P. J, Konopleva N & Ikorsky S (2004). Occurrence and genesis of carbonaceous species in the Khibina alkaline igneous complex, Kola Peninsula, NW Russia. 32nd IGC, Italy, Florence, August 20-28, 2004. Abstracts, part 1, pp. 518-519.

Nivin, V.A., Treloar, P.J., Korlopleva, N.G. & Ikorsky, S.V. 2005. A review of the occurrence, form and origin of C-bearing species in the Khibiny alkaline igneous complex, Kola Peninsula, NW Russia. Lithos, 85, 93-112.

Nivin V.A, Treloar P. J., Konopleva, N, Ikorsky S. Occurrence and genesis of carbonaceous species in the Khibina alkaline igneous complex, Kola Peninsula, NW Russia. 32nd IGC, Italy, Florence, August 20-28, 2004.

Nivin V.A Treloar P.J. and Beeskow, B. Free-phase hydrogen-hydrocarbon gases in the Lovozero peralkaline intrusion: origin, occurrence and environmental implications of their continued seepage. Peralk2005: PERALK 2005: sources, economic potential and evolution from alkaline melts. Tubingen, Germany, March 4-6, 2005.

Nivin, V., Belov, N.I., Treloar, P.J. & Timofeyev, V.V. 2001. Relationships between gas geochemistry and release rates and the geomechanical state of igneous rock massifs. Tectonophysics. 336. 233-244.

Potter, J., Rankin, A. H., Treloar, P. J., 2004. Abiogenic Fischer-Tropsch synthesis of hydrocarbons in alkaline igneous rocks; fluid inclusion, textural and isotopic evidence from the Lovozero complex, N.W. Russia . .Lithos. 75, 311-330.

Associated Research Groups

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