Assessment of an unusual European Shale Gas play: the Cambro-Ordovician Alum Shale, southern Sweden
- Wilfred Pool (NAM) | Mark Geluk (Shell Int. E&P) | Janneke Abels (Shell International E&P) | Graham John Tiley | Erdem Idiz (Shell International E&P) | Elise Leenaarts (Shell Global Solutions International)
- Document ID
- Society of Petroleum Engineers
- SPE/EAGE European Unconventional Resources Conference and Exhibition, 20-22 March, Vienna, Austria
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.3.4 Scale, 5.6.1 Open hole/cased hole log analysis, 5.2.1 Phase Behavior and PVT Measurements, 1.6.9 Coring, Fishing, 5.1.1 Exploration, Development, Structural Geology, 5.1.2 Faults and Fracture Characterisation, 5.1.8 Seismic Modelling, 1.6 Drilling Operations, 1.2.3 Rock properties, 5.8.2 Shale Gas, 1.11 Drilling Fluids and Materials, 4.6 Natural Gas
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In 2008 Shell obtained two licenses for unconventional gas exploration in the Skåne region of southern Sweden, with a total size of 2500 km2 (600,000 ac). The objective was the Cambro-Ordovician Alum Shale, one of the thickest and richest marine source rocks in onshore northern Europe.
The licenses covered the Höllviken Graben and the Colonus Shale Trough. In both areas the Alum Shale had been encountered in older wells, with a thickness of up to 90 m and TOC values up to 15%. Maturities of up to 2% Vre were considered encouraging for a shale gas play. Relative high quartz contents suggested good fraccability of the shales. All data was obtained through public sources. Identified risks were the uncertain timing of hydrocarbon generation and the position of the licenses adjacent to the Trans-European Suture Zone where several phases of fault movement have a risk for actually retaining the hydrocarbons.
The derisking strategy for this opportunity was based on both technical and non-technical aspects. Aim was to collect geological and geophysical data to constrain depth and thickness of the shale and to identify potential dolerite dykes. In addition, well data were needed to establish rock properties and gas content. The external environment, especially concerns from the people in Skåne regarding the visual impact of activities and potential impact of drilling activities on the aquifers and on the tourism industry have resulted in extensive engagements with stakeholders and specific requirements around seismic acquisition (low impact), site preparation and operations (e.g. small rig, different lighting).
80 km of 2D seismic was acquired in 2008 and three wells, with a final depth of around 1000 m, were drilled in 2009 to mid 2010. The Alum shale was fully cored and the well sites have been restored. Thickness, richness and maturity of the Alum were as predicted although the basin was shallower than previously anticipated. Canister desorption tests, however, indicated that the shales have only low gas saturation. This significantly increased the risk for a viable shale gas play and therefore the licenses were not renewed after the initial 3 year period.
In the last decades the oil industry has seen a global shift towards an increasing percentage of unconventional hydrocarbons (e.g. Shale Gas) in their portfolios. Building on the success of these hydrocarbons in North America these concepts are gradually spread across the globe. Unconventional hydrocarbons (UC) are currently being chased on most continents. Critical in UC exploration is a choice between an early entry of a play at low costs but considerably high risk, or a late entry which is expensive but at lower risk. The benefit of the early entry is that still a large, contiguous acreage position can be built.
In 2006/2007 a screening study for potential UC basins was undertaken in Shell. The focus was on areas with stable gas prices. An integrated team of some 30 persons, consisting of G&G but also RE, WE, PP, PT and environmental specialists screened a large number of basins outside North America. This was done largely on the basis of external databases, such as IHS Energy, proprietary well and geochemical data, reports, literature and through conferences.
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