SWEDEN'S SILJAN RING WELL EVALUATED

Thomas Gold
Ithaca, N.Y.

The final results are in from the first major drilling operation undertaken to explore the deeper levels of the Siljan ring impact crater in Central Sweden.

The results demonstrate that hydrocarbon gases from methane to pentane-as well as a light, largely saturated oil-are present deep in the granitic rock.

The impact crater, generated 360 million years ago by a major meteorite, is now a circular area about 44 km across.

EARLY INVESTIGATIONS

Investigation of this area as a possible oil and gas prospect began in 1982 on the basis that the fracturing caused by the impact may have facilitated the upward movement of hydrocarbon fluids from much deeper levels.

It may also have generated sufficient fracture porosity in the otherwise very dense granitic rock to allow formation of reservoirs at levels that could be reached by the drill.

That first round of investigations demonstrated that indeed just this area showed very high levels of methane in the soil, in lakes, in water wells (to the extent that some wells drilled for water produced enough gas to be used for house heating), and also that there were a number of oil seeps visible in several stone quarries.

The ring shaped depression that surrounds the crater area contains some sediments of Ordovician age, but these are nowhere deeper than 300 m.

It seemed most improbable that oil seeps in the stone quarries could be attributed to sources from such small amounts of sediments, especially since the oil seeps are very active and one had to assume they were not limited just to the areas in quarries where they could readily be identified.

GEOCHEMICAL ANALYSES

U.S. prospecting companies skilled in surface chemical prospecting were brought in and investigated the area for all the chemical signs of hydrocarbon seepage including:

Direct observation of soil gas;
Formation of certain carbonates in the soil known to result from the oxidation of hydrocarbons; and
Presence of trace elements associated with hydrocarbon seepage.

Five small test holes were drilled in the area. All produced methane as well as other hydrocarbon gases, and hydrogen sometimes at combustible levels. The U.S. prospecting companies declared the area to be clearly a major hydrocarbon prospect.

1 GRAVBERG WELL

It was on the basis of this that a large drilling operation was commenced in June 1986.

it was funded initially by a combination of the Swedish State Power Board (Vattenfall) and private investors.

The U.S. Gas Research Institute also made a contribution in return for which it was to receive detailed scientific and technical information.

Drilling was straightforward and fast down to approximately 5,000 M. The target depth chosen by the indication of increased porosity obtained by seismic measurements was 7.5 km.

At the deeper levels, the drilling became very difficult with breakouts, hole deviation from vertical, and pinching of the drill pipe and bit.

Altogether, four branches of the hole were drilled below 5 km, the deepest reaching a true vertical depth of approximately 6.7 km.

GASES ENCOUNTERED

During the drilling with a water based drilling fluid, good measurements were obtained of hydrogen, helium, methane, and the other hydrocarbon gases up to pentane.

In general, the volumes brought up in the returning drilling fluid increased as the depth increased.

There was a clear correlation of all the gases with each other, which excluded the possibility that they were in any way the result of drilling additives.

The gases were also seen to increase at times of lowered pressure in the wellbore, such as occurs when the drill pipe is being withdrawn, and the gas quantities also increased at levels in the rock at which independent measurements indicated a greater porosity. Carbon isotope values of the methane were analyzed and showed unusually high 13C values in coincidence with the more porous regions (813C of - 15% in the usual notation).

So far as the gas measurements were concerned, they seemed to confirm that hydrocarbon gases were indeed streaming through the pore spaces of this granitic rock.

The only rock other than granite that was encountered during the entire operation was thin layers of volcanic intrusive rock around which the granite was more heavily fractured.

Gas readings obtained during drilling from the returning drilling fluid were confirmed by gas readings in the drill cuttings.

HEAVY OIL PASTE

In June 1987, when drilling was at a depth of approximately 6 km, a new phenomenon was observed.

The drill string stuck for approximately 10 days, and when it was finally freed 10 m of drill pipe were solidly filled with a thick paste.

On analysis, the paste turned out to be very fine grained magnetite, mostly submicroscopic, bound into the paste consistency by a light oil.

The drilling fluid at the time was water based, and lubricants that had been used during drilling were oils of totally different chemical compositions.

A small amount of diesel oil had been used 4 months earlier, but after 4 months of water circulation one could not understand that small remains of this could have become concentrated underneath the water at the bottom of the hole, and to the complete exclusion of water.

one also had no explanation of the origin of the magnetite grains which, on examination, turned out to be quite different in chemical detail from the ordinary magnetite crystals that were in small concentration in the granitic rock.

This magnetite-oil sludge seemed to exist in large amounts at deep levels in the granite. Similar magnetite-oil sludges had been known in other petroleum areas and, in several cases, had been identified as a product of bacteria that use more highly oxidized iron in the rock as the oxygen donor together with hydrocarbons as their metabolic intake.

The recrystallized magnetite so produced is thereby shaped into these very small particles.

As drilling proceeded, several more occasions of the entry of fine grained magnetite were observed, and when later an oil based drilling fluid was used it acquired many tons of this magnetite into the circulating system.

It became clear that the magnetite oil sludge was a major component of the pore spaces at the deeper levels.

The last stages of drilling and an attempt at hydraulic fracturing led to the deeper levels below 5.8 km becoming inaccessible due to broken pipe and possibly the presence of heavy sludge.

OIL IN MUD RETURNS

During the period when an oil based drilling fluid was used, a further interesting observation was made.

Not only did the drilling fluid acquire fine grained magnetite, but it also appeared to acquire substantial quantities of oil.

This was noted in the following manner: the drilling fluid was an emulsion consisting of 20% water, 80% diesel oil. Whenever losses had to be made up, it was noted that more than 20% of water had to be added to keep the ratio at 20-80.

A selective loss of water into the granite from this stable emulsion seemed very improbable and, therefore, the addition of oil seemed a better explanation.

This clearly would go together with the entry of the magnetite grains, which presumably entered again as a magnetite-oil sludge of the same kind that had been seen in the event of June 1987.

When the drilling of this hole had to be stopped in September 1989 because of the technical drilling difficulties, a test procedure was done in which the fluid pressure on segments of the wellbore was substantially reduced and the fluids so extracted were analyzed.

This analysis indicated the presence of another oil than the diesel that had been in use. It was, however, an oil that showed some similarity to diesel, although it had a significantly different distribution of the different molecular species.

PUMPING OPERATION

It was then decided to introduce a pump to examine better what fluids would come into the open well bore when the pressure was lowered.

The pumping operation was started at the beginning of May 1990 and finished at the beginning of July 1990.

During that period, approximately 15 cu m (85 bbl) of oil were pumped up with more than 10 tons of solids in granular form, again mostly fine-grained magnetite. In addition, about 150 tons of very saline water (15% sodium and calcium salts) were pumped up.

It was the detailed analysis of this oil that now proved conclusively that it was oil from the formation and that it could not have been derived from the drilling fluids.

FLUID ANALYSIS

The pumped up fluid contained the same component of heavier saturated hydrocarbons that had already been identified in the smaller amounts of oil brought up during the testing period.

In addition, a major component of asphaltenes was then present that could not be accounted for by any of the drilling additives.

Further, the pumped oil contained a volatile component that made it much more flammable than any of the drilling fluids; it also had a strong smell, quite different from that of the diesel oil or any of the additives, and thought to be characteristic of natural crude oil by the experts.

Finally, a detailed study of the content of metals in organo-metallic form showed this oil to have remarkably high values of nickel, many times higher than could be accounted by any fluids that had been used in the well.

Nickel is commonly present in crude oil. The values found here were not outside the range that is seen elsewhere, but definitely among the higher values.

The other metal commonly detected in crude oils, vanadium, was significantly present in the oil but was below detectable levels in the drilling fluids.

SURFACE SOURCE DISCOUNTED

These observations seem to have shown conclusively that hydrocarbons including liquids are in the pores of granitic rock and at such depths that any supply from the surface can be ruled out.

A supply from below is indicated also by a number of other observations: the very saline water, certainly a formation product, cannot have reached this salinity just by the solution of salts from the local granite.

Very saline water is known to be frequently associated with hydrocarbons. The noble gas, helium, was observed prominently during drilling and seen at record concentration at times during the pumping tests.

Fully 50% of the gas at times was helium, a figure that probably represents the highest natural concentration that has even been reported.

POROSITY CONDITIONS

Can these results be used to make any prognosis about the possibilities of a large commercial production of gas and oil from the Siljan Ring area?

Is there likely to be sufficient permeability at any level for adequate flow rates into the well bore, and is there likely to be an adequate seal or caprock in the formation at a level that the drill can reach and penetrate, so that good reservoir conditions can be met?

There are two effects that would help to generate these favorable circumstances.

Firstly, at a sufficient depth, the overburden weight of the rock will tend to crush the pore spaces if the fluid pressure in the pores corresponds only to an overburden of water (the hydrostatic pressure).

Beneath this level of crushed pores ("the critical layer") fluids coming from below may then be at a pressure approximating that of the rock overburden and thus support increased porosity.

This effect has been seen in numerous deep wells. It is clearly a widespread phenomenon, and theoretical considerations make clear that it must occur.

RESERVOIR SEAL

The second phenomenon that assists in producing a seal above an oil or gas reservoir is the deposition of pore filling substances produced or transported by the existing hydrocarbons.

Among those are the calcite cements identified as a product of reactions of methane with rock minerals (by the characteristic wide scatter in carbon isotope values pointing to a process of isotopic fractionation, which only the lightest carbon bearing molecule can suffer).

Other pore filling minerals are also known to lead to an improved containment.

In the present case, another pore filling material served the same function: it is the magnetite-oil paste.

This material cannot have been transported through the rock in the concentrated form in which it was found. It must have been transported as a dilute admixture of fine grained magnetite in oil.

Upon being forced through small pore spaces, the solid component tends to become more and more concentrated while some of the liquid component escapes. The resulting thick paste will then tend to be pressurized with the pressure of the deeper liquids, and that will be a close approach to lithostatic pressure.

Again, this is not new in the oil and gas drilling industry where salt, also effectively a highly viscous fluid, is often seen at near lithostatic pressure.

It may be expected, therefore, that magnetite paste would contribute substantially towards the generation of a seal above a hydrocarbon reservoir.

RESERVOIR IMPLICATIONS

The size of the Siljan structure is very large by comparison with major oil and gas fields in the world.

If the first hole drilled, necessarily in an almost arbitrary location within this formation, showed clear evidence of oil and gas, then the probability is that a major fraction of this large formation contains these hydrocarbons also.

Since the seismic and gravity information indicate the existence of enhanced porosity at deeper levels, one may expect to be dealing with an oil and gas field of world class dimensions. There is no question that similar information in a similarly large structure, but one composed of sediments, would be generally regarded as a very major prospect worthy of a large effort for its exploitation.

Having now seen so clearly that sedimentary material is not a prerequisite for the existence of oil and gas, there is no reason to regard the Siljan structure differently.