ABSTRACT: The Siljan Ring Impact Structure as a Target for Hydrocarbon Exploration: Implications from the Gravberg 1 Well, Sweden

CASTANO, JOHN R., Consultant, Houston, TX, and ANTHONY GORODY, Gas Research Institute, Chicago, IL

The Siljan Ring Structure is a complex meteorite impact crater with an excavated diameter of 45 km. The Gravberg 1 well, just outside the excavated region on the northeast rim, was drilled to explore for abiogenic gas of mantle origin. The well reached a true vertical depth of 6779 m, penetrating only pre-Cambrian granites and diabase sills.

Key points for evaluation include reservoir, cap rock, and source. Porosity in the granite is related to macrofractures, microfractures, and mineral alteration. Fracture intensity is highly variable but generally decreases with depth. Below 3000 m, fracture zones are caused by regional tectonics. No good continuous reservoir was detected. Originally, calcite-cemented granite was expected to act as an impervious seal. Based on leak-off and drill-stem tests, there is no evidence for caprock at Gravberg. Hydraulic conductivity ranges from 0.07 to 7.5 x 1010 m/sec. Evidence points to deep (6 km) circulation of fresh, meteoric water, below which lies stagnant high salinity water (150,000 ppm). Contrary to expectations, no free gas was found. Water recovered from the most reliable drill-ste test is undersaturated; gas concentration was measured at 0.25 L/L. Average gas composition was 45% He, 2.5% H2, 2% Ar, 0.3% O2, 150 ppm CH4, traces of CO2, and Ne. The remainder is N2. The He, with an R/RA of 0.017, Ar, and Ne are radiogenic, originating from the breakdown of radioactive minerals in the granite. Trace CH4 in diabases is abiogenic (Carbon 13 -15 o/oo). Methane encountered in granite is of thermogenic and artificial origin (Carbon 13 -23 to -55 o/oo). There is no evidence of a dominant mantle source for hydrocarbons.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)