The occurrence, distribution and orgin of hydrocarbons in the Khibiny Nepheline Syenite Complex, Kola Peninsula, Russia

The occurrence of hydrocarbon-bearing fluids in alkaline igneous rocks has been known for many years, but their origin is still controversial. The fluids may be of biogenic origin, derived from the host rocks, or the result of abiogenic processes. Three mechanisms for abiogenic production have been proposed: mantle derivation, late-magmatic C-O-H respeciation and post-magmatic Fischer- Tropsch generation. Here, new petrographic, micro thermometric and laser Raman data, combined with data of carbon and bulk gas content and isotope signatures as well as permeability and porosity data of material from throughout the Khibiny intrusion are presented. These are discussed in the context of previously published work in order to characterise the occurrence and distribution of hydrocarbon-bearing fluids and to reassess the hypotheses for hydrocarbon generation for the Khibiny pluton. Bulk gas data show a dominance of methane and only minor concentrations of higher hydrocarbons in whole-rock samples. On average, 12.8 cm" of hydrocarbons are released from one kg rock of the Khibiny pluton. Solid carbon is present at levels between 0.21 and 0.06 wt%. Both, methane and carbon contents are highest in the marginal areas suggesting a possible contribution from the host rocks. The fluid inclusion study shows that about 90 % of the inclusion population is methane-dominant and only 10 % water-dominant. The majority of the fluid inclusions are secondary and were trapped over a wide range of P-T conditions within subsequently healed microfractures. P-T entrapment conditions for primary fluid inclusions are around 600 MPa and 600°C. Melt inclusions often contain a methane-rich fluid phase. Isotope data for methane indicate a largely abiogenic origin, but the decrease of o13C towards the margin of the complex from - 5.4 %0 to - 22.4 %0 suggests addition of biogenic material derived from the host rocks. This is in agreement with the results of the fluid inclusion plane and mieroerack study that indicates a high degree of fluid movement throughout the history of the pluton, up to the present day. Average palaeo and modem porosity estimates are 5.56 % and 4.80 %, respectively. Average palaeo and modem permeability values are 2.38 mD and 1.95 mD, respectively. Basedon these porosity values the methane content in the Khibiny pluton is estimated at between 416 km' and 7649 km" In summary, isotope data, the presence of primary Ctla-dominated fluid inclusions and melt inclusions, which also contain a methane-rich gas phase, indicate an abiogenic mantle origin of the hydrocarbons. There is little evidence for operation of a Fischer-Tropsoh-type reaction in methane generation. An increase in carbon and methane content, together with decreasing O13C isotope values towards the pluton margin suggests that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks.EThOS - Electronic Theses Online ServiceGBUnited Kingdo