Prospects for commercial abiogenic gas production: Implications from the Siljan Ring Area, Sweden

Journal Article · Fri Jan 01 00:00:00 EST 1993 · United States Geological Survey, Professional Paper; (United States)

OSTI ID:6814469

Castano, J R ^[1]

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1. DGSI, The Woodlands, TX (United States)

The Siljan Ring structure in central Sweden is a 360-Ma 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 as a commercial venture to explore for abiogenic gas of mantle origin. The well reached a true vertical depth of 6,779 m, penetrating only Precambrian granite porphyry, granite, quartz monzonite, and diabase (dolerite) sills. The sills correspond to prominent, low-dipping, high-amplitude seismic reflectors that afford the only good correlative horizons. Key points for evaluation include reservoir, cap rock, and source. Porosity in the granite is related to macrofractures, microfractures, and mineral alteration. On the basis of well data, the influence of the impact drops off dramatically below 2,000 m and is absent below 4,000 m. Fracture intensity is highly variable, but generally decreases with depth. Below 3,000 m, fracture zones are caused by regional tectonics. No continuous reservoir units were detected. Originally, calcite-cemented granite was expected to act as an impervious seal. Stable isotope studies of calcite show that mantle methane is not involved in any of the fractionation equilibria for these cements; instead there is equilibration with modern meteoric water. At depth, magmatic CO[sub 2] is the likely carbon source for the heaviest carbon isotopes measured. On the basis of leak-off and drill-stem tests, no caprocks were penetrated by the Gravberg 1 well. The pore-pressure gradient is essentially hydrostatic; the expectation model had predicted pressures approaching lithostatic. Hydraulic conductivity ranges from 0.07 to 7.5x10[sup [minus]10] m/s. Evidence points to deep (6 km) circulation of fresh, meteoric water below which lies stagnant, high-salinity water (150,000 ppm total dissolved solids). Contrary to expectations, no free gas was found. 71 refs., 11 figs., 1 tab.

• OSTI ID: 6814469
• Journal Information: United States Geological Survey, Professional Paper; (United States), Vol. 1570
• Country of Publication: United States
• Language: English

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