The Siljan Deep Well: Helium isotope results

Abstract

The Siljan Ring impact structure in Sweden is the scene of current deep-drilling exploration for methane trapped in a crustal reservoir. The initial hole, drilled to a depth of 6100 m through Precambrian granite, has been augmented by three branching “side-track” holes beginning at −4700 m that have extended the drilled depth to 6957 m. Forty-four gas samples extracted from the circulating drilling fluid have been analyzed for He, Ne, and 3He 4He ratios. Granite chips recovered from the well and eight natural gas seeps from the Siljan Ring and other localities in Sweden have also been analyzed to provide ambient helium comparisons. The 3He 4He ratios in the well gases are mixtures of three components: (1) atmospheric helium dissolved in recirculated drilling fluid, (2) “carrier” He used for well-site gas chromatography that contaminated some of the samples taken for He isotope analysis, and (3) a “crustal” radiogenic component from the granite with a 3He 4He ratio of 0.015 times the atmospheric ratio (1.40 × 10 −6). Although helium concentrations are generally low throughout the hole (up to 4800 ppm), hydrofracturing at the termination of drilling in Side-track Hole 3 yielded gas with 23,100 ppm of helium and the “crustal” isotope ratio for Siljan granite fluids. Isotope ratios in the gas seeps are in agreement with the deep “crustal” value, but the granite itself has lower 3He 4He ratios due to preferential extraction of 3He from Li-bearing micas. No evidence of a mantle helium component is observed in the isotopic data: an upper-limit calculation based on the difference between He isotope ratios in the granite and in the formation waters gives a maximum of 0.15% of MORE helium in the well gas helium.