The country is host to several plays that could yield trillions of cubic feet of gas.

The organic-rich mudrocks that we tend to think of as the source rocks for conventional hydrocarbon accumulations can also play host to a variety of less conventional deposits. At shallow depths when still immature, organic shales may contain biogenic gas accumulations or their organic matter (kerogen) can be converted to synthetic shale oil by thermal methods. At higher temperatures within the oil window, both liquids and gas are generated, the relative proportions depending on the type of organic matter preserved in the shale. And depending on the expulsion efficiency of the generated hydrocarbons, substantial volumes of hydrocarbon liquids can remain trapped in the shale source rocks. In fractured shale oil plays, this trapped oil is recovered through natural or induced fractures.

At still higher temperatures outside the oil window, thermogenic gas is generated from organic-rich shale through the breakdown of organic material or the thermal cracking of pre-existing hydrocarbon liquids, creating potential shale gas plays.

“Unconventional” gas plays are no longer unconventional. The exploitation of gas resource plays in North America has now developed to such an extent that 50% of US gas production comes from coalbed gas, tight sand gas, and shale gas. Of these, shale gas is the fastest-growing. In the five-year period between the beginning of 2004 and the end of 2008, US shale gas production almost trebled, from 1.88 Bcf/d to 5.52 Bcf/d. In 2008, US shale gas production exceeded coalbed gas production for the first time.

In that same five-year period, fractured shale oil production from the Bakken Formation in Montana, North Dakota, and Saskatchewan rose from 23,600 b/d in 2004 to 153,000 b/d in 2008.

What lies behind this dramatic growth in gas and oil production from shale resource plays? In part it reflects declining production from traditional reservoirs and a desire to increase energy security by producing from indigenous sources. The principal drivers, however, have been technological advances in drilling and completion. With little exploration risk, the development of resource plays has become very much like a manufacturing process.

Shale gas in Europe

Europe is particularly well-suited to gas resource play exploitation because of its large market, established pipeline infrastructure, increasing demand, and current dependence on gas imports. Relatively high natural gas prices add to the attraction.

Shale gas exploration in Europe is in its infancy. The first exploratory well was spudded in Germany in 2008, and drilling commenced in Sweden in November 2009. As a consequence, little is known about Europe’s ultimate potential. In 1997, H-H Rogner estimated the in-place resource of Europe (including Turkey) at 550 Tcf by extrapolating from estimated US resources of gas per ton of shale and applying this to the worldwide distribution of oil-shale. He admitted that this approach was “quite speculative.” His remarks appeared in “Annual Review of Energy and the Environment.”

More recent studies indicate significantly larger in-place resources. OMV has suggested a potential recoverable shale gas resource of 15 Tcf in the Vienna Basin, Austria, from an in-place resource of 200 Tcf to 300 Tcf. TNO’s “best estimate” for “producible gas in place” in “high potential” areas of the Netherlands is 198 Tcf from an estimated in-place resource of 3,950 Tcf.

Given the potential size of the in-place resource, it is not surprising that investigations are currently under way in at least 10 countries. Company interest extends from super-majors such as ExxonMobil and Shell through major independents to small niche players and coalbed gas explorers who may have some shale gas potential on their acreage.

Major shale gas plays

There are three potentially major regional shale gas plays in Europe plus a number of others with local potential (Table 1).

The oldest is a Lower Paleozoic play that occurs in northwest Europe running from eastern Denmark through southern Sweden to north and east Poland. The organic-rich shales with shale gas potential lie on the southwestern margin of the Baltica palaeo-continent and tend to thicken in toward the bounding Trans-European Suture Zone.

In Denmark and Sweden, the principal target is the kerogenous Alum Shale of Middle Cambrian to Early Ordovician (Tremadoc) age. No drilling has taken place in the Fennoscandian Border Zone and Norwegian-Danish Basin in Denmark, but on Nov. 28, 2009, Shell spudded the Lövestad A3-1 well, the first in a three-well test program, in the Colonussänkan permit (Fennoscandian Border Zone). The well has a planned total depth of about 3,300 ft (1,007 m).

Further to the southeast, in Poland, the main Lower Paleozoic target is Silurian-age graptolitic shale, with the Upper Ordovician a secondary target. The Silurian in particular thickens towards the southwest in the area of the Gdansk Depression (Baltic Depression) and the Danish-Polish Marginal Trough, which defines the southwest margin of the Baltic Depression. In parts of the trough such as the Warsaw Trough and Lublin Trough, more than 10,000 ft (3,050 m) of Silurian section may be present.

To date, this play has been the most sought-after in Europe (Table 2). Some 15 concessions have been awarded in the Gdansk Depression, five in the Danish-Polish Marginal Trough, and six on the East European Platform Margin northeast of the Marginal Trough.

A number of small niche players are active in the Gdansk Depression, but of the 11 concessions on the Platform Margin and Marginal Trough, 10 are held by ExxonMobil, Chevron, or Marathon.

The first test of the Polish Lower Paleozoic is scheduled for April 2010, when Lane Energy (a subsidiary of 3Legs Resources) will spud a well, probably in the L?bork concession (Gdansk Depression). Lane’s initial seismic and drilling program on its six Gdansk Depression concessions is being funded by ConocoPhillips, giving the latter the option to earn up to 70% interest in the concessions.

The second major play is a Carboniferous basinal marine shale play that extends from the Cheshire Basin in northwest England across the Anglo-Dutch Basin and Northwest German Basin to the Fore-Sudetic Monocline (Northeast German-Polish Basin) in southwest Poland. The age of the most prospective shales appears to young westwards from the Visean (Middle Mississippian) Kulm facies of southwest Poland and northeast Germany to the Namurian (Upper Mississippian to Lower Pennsylvanian) of northwest Germany, the Epen Formation of the Netherlands, and the Bowland Shale in northwest England.

Lane Energy, the 3Legs Resources subsidiary, has interests in southwest Poland, but unlike the Gdansk area, this activity is not funded by ConocoPhillips. The nature of German E&P reporting is such that it can be difficult to establish the activity taking place on long-held licenses. It is assumed that ExxonMobil, both directly and indirectly through the BEB ExxonMobil/Shell joint venture, will be examining the potential of Visean shale in eastern Germany and Namurian shale in the west.

In the Netherlands, Cuadrilla Resources was recently awarded a license on the margin of the London-Brabant High and West Netherlands Sub-basin of the Anglo-Dutch Basin. The Epen Formation shale (Namurian) is likely the primary target in this location. Cuadrilla Resources, through its Bowland Resources subsidiary, also has interests in the Cheshire Basin in northwest England. Spudding in March 2010, the company’s Preese Hall 1 well will test the Namurian-age Bowland Shale. The depth to top shale is estimated at 4,300 ft (1,312 m) with a gross prognosed shale thickness of some 4,000 ft (1,220 m). This well is the first known test of the Carboniferous shale gas play in Europe.

The third major regional play comprises Liassic (Lower Jurassic) bituminous shales that are being targeted in the Weald Basin (southern England), Paris Basin, the Netherlands, northern Germany, and Switzerland’s Molasse Basin. In continental Europe, the principal target is the Lower Toarcian Posidonia Shale. In eastern Germany and Poland, the Lower Toarcian grades into a terrestrial facies and loses its source potential. In southern England, the principal bituminous shales are older and occur in the Lower Lias.

These bituminous shales are clearly oil-prone. Therefore, the principal limitation regarding their shale gas potential lies in finding locations in which they have been sufficiently deeply buried to have entered the gas window. Locations where this may have occurred include the flexural foreland basin of the Swiss Molasse, the Mesozoic depocentres of the Lower Saxony Sub-basin (Northwest German Basin), and the offshore Broad Fourteens Basin and Central Graben of the Netherlands. Whether the Liassic shales will be within the gas window in the Weald and Paris basins remains to be seen.

A number of companies are thought to be investigating Lower Jurassic shale gas potential. These include EurEnergy Resources in the Weald and Paris basins, Cuadrilla Resources/AJ Lucas in the Weald Basin, and Schuepbach Energy in Switzerland’s Molasse Basin. In Germany, ExxonMobil commenced shale gas exploratory drilling in 2008 in the Lower Saxony Basin, drilling Damme 2/2A and 3 in the Munsterland concession and Oppenwehe 1 in Minden. Niedernwöhren 1 was spudded in the Schaumburg permit on Oct 13, 2010. The Posidonia Shale is presumed to have been at least one of the targets for these wells.

Other plays with shale gas potential

Austria. OMV is investigating the potential of the Upper Jurassic Mikulov Formation in the Deep Vienna Basin.

France. Permo-Carboniferous basins in the Languedoc such as the Stephanian-Autunian Lodève Basin may have some potential in bituminous Autunian (Lower Permian) shale. Schuepbach Energy has applied for a permit in the Landguedoc-Provence Basin that also incorporates part of the Lodève Basin. A number of other companies have also applied for permits in Languedoc-Provence, many of them overlapping. The results of some of these applications should be known by April 2010.

Germany. The Upper Devonian Kellwasser shale has been touted as having potential in northern Germany, as have Wealden paper shales of Berriasian age in the Lower Saxony Sub-basin. In the Bodensee Trough north of the Swiss-German border, Parkyn Energy, another 3Legs Resources subsidiary, has taken out two licenses in which the principal prospect appears to be Lacustrine shale of Permian age.

Netherlands. The Upper Jurassic Kimmeridge Clay is sufficiently deeply buried in the Central Graben in the northern Dutch offshore to have reached the gas window.

Spain. Applications that are presumed to be for shale gas exploration have been submitted in the Pyrenean Foothills (Cuadrilla Resources) and the Campo de Gibraltar (Schuepbach Resources/Vancast).

Switzerland. In addition to the Lower Jurassic Posidonia Shale, Schuepbach is also targeting the Aalenian (Middle Jurassic) Opalinuston in the Molasse Basin.

United Kingdom. The Upper Jurassic Kimmeridge Clay has been proposed as a possible target in the Weald Basin, but there is considerable doubt that it will be mature for significant gas generation in this basin. If there is to be shale gas potential it seems more likely that it will come from older shales (Rhaetic or older).

And finally a “play” that probably isn’t: Secondary sources and press reports have suggested that the unconventional gas prospects being investigated in Hungary’s Makó Trough by Falcon Oil & Gas/ExxonMobil/MOL are a shale gas play. The companies themselves, however, have not mentioned a shale gas play, referring only to tight gas sand or basin-centered gas prospects. Furthermore, it seems unlikely that the young Neogene shales of the trough will have achieved the requisite degree of brittleness to support a fractured shale gas play.

Since exploration for shale gas in Europe is in its infancy, little is known about the characteristics of potential gas shales. Some of the parameters that have been reported are shown in Table 3.

Fractured shale oil play

In France’s Paris Basin, Toreador Resources is investigating the fractured shale oil potential of a Liassic (Lower Jurassic) analog to the Bakken Formation of the Williston Basin. The Liassic section is similar to the Bakken Formation in that the bituminous shales also contain a middle calcareous member (Banc de Roc). Shows have previously been detected in 11 conventional exploration wells drilled from the 1950s onwards, and six wells have produced oil on test. The company hopes to commence a three-well pilot exploration program in Q3 2010.

Click here to view tables mentioned in this article.