Geochemical Investigations for Deep Gas Drilling in Siljan Impact Structure, Sweden

John R. Castano, A. W. A. Jeffrey, I. R. Kaplan, Dick Karlsson, Troels Laier, Manfred Schmitt

The geochemical analysis program for the Gravberg 1 well in the Siljan Ring (impact structure in Sweden) monitors and evaluates for scientific and commercial purposes the gas encountered in the borehole, which has penetrated Precambrian crystalline rocks to a depth of 6.6 km. At the well, three different systems measure hydrocarbons C₁-C₅. Inorganic gases, H₂, CO₂, O₂, N₂, He, and Ra, are also measured. Offsite laboratories conduct more sophisticated studies of a routine and experimental nature. These include headspace and desorbed gas analysis, and isotopic analysis of hydrocarbons, helium, and hydrogen.

Two types of hydrocarbon gases have been encountered. Hydrocarbon concentration is highest in the dolerite intrusions, about 4 to 10 times higher than in granite. In the dolerites, the hydrocarbons consist of up to 98% methane, whereas in the granite the methane content is 50-70%. The dolerite gases contain small proportions of unsaturates; in the granite, the C₂-C₄ olefins are in equal amounts with the saturates. The dolerites contain isotopically heavy methane (^dgr C¹³ generally -10 to -24 ^pmil), similar to abiogenic gases. In the granite, the methane is lighter (^dgr C¹³ generally -23 to -36 ^pmil). Experiments show that almost all the hydrocarbons in the granite are associated with the magnetic fraction; this does not hold for the dole ites. The hydrocarbons in the granites may be formed from CO and H₂ by magnetic catalysis (Fischer-Tropsch reaction) but other explanations may be possible.

Hydrogen is a prominent part of the gas mix; it exceeds methane concentration normally by an order of magnitude. The hydrogen could be indigenous. Helium is found mainly below 6 km; isotopic analyses show that the helium is of crustal origin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.