Deep drilling project Siljan (lake)

the gravberg-1 location, in north-northeast section of ring, chosen because gravity anomaly indicates less dense rock below, , 4 seismic reflectors may represent cap rock above natural gas reservoir. electrical resistance lower in ring outside it, suggesting rocks contain fluid. soil in area showed clear traces of methane , heavy hydrocarbons, halo around ring coincident vanadium , nickel trace elements; sediment shown not source of hydrocarbons due location, ice movement pattern, , low level of maturity. although greatest gas leakage in north-northwestern corner, lack of leakage in central area suggested underlying cap.

drilling began on 1 july 1986. september 1989 reached 6,957 m (22,825 ft) in depth.

four seismic reflectors detected @ 1,440 m (4,720 ft), 2,800 m (9,200 ft), 4,740 m (15,550 ft) , 7,400 m (24,300 ft). first 3 penetrated , found dolerite sills, fourth had been identified before drilling having best potential gas production. dolerite believed have existed as 1/2 billion years before meteorite impact. granite extensively fractured, calcite principal cement down 3,200 m (10,500 ft) depth, lesser amounts 5,000 m (16,000 ft).

traces of methane found @ depths, higher levels in dolerite. although appear of abiogenic origin, near dolerite sills points probable mantle origin. hydrogen found in sections of hole, correlated helium. helium isotopic pattern consistent radiogenic, crustal origin.

a black sticky substance found blocking drill pipe when circulation stopped several days @ depth of 5,520 m (18,110 ft). substance black, sticky when wet, powdery when dry, showed no fluorescence, , magnetic. none of drilling materials showed gas chromatographic patterns corresponded black substance. contained no ordinary rock minerals, none of granite fines expected in drilling fluid, made of microscopic magnetite held oils.

the did not penetrate fourth seismic reflector, did not produce large quantities of fuel, , never entered commercial production. 84 barrels (13.4 m) of oil produced; later shown originate organic additives, lubricants , mud used in drilling process.

a second hole, stenberg-1, drilled in center of ring, 12 km (7.5 mi) distant ring sediments , gravberg-1. went depth of 6,500 m (21,300 ft). hole produced gases , oily magnetite sludge similar found in gravberg-1.