Unconventional Liquid Rich Shale (LRS) and shale gas (SG) plays are self-charged continuous systems relying on their indigenous organic matter as the source of hydrocarbons. The residual pore space of fine-grained organic rich sediments themselves or similarly tight deposits in their close vicinity act as the reservoirs. Pressure (P) and temperature (T) can significantly increase during burial of the sediments and can likewise undergo a substantial instant P but time-deferred T decrease during erosion and uplift. These changes in P and T strongly influence the phase behaviour of the generated and trapped hydrocarbons (HC), thereby affecting the retention capacity of the source rock, the HC flow properties, potential migration routes, and the predicted HC volumes initially in place (GIIP/OIIP). Shell's integrated Cauldron Shale Gas Simulator calculates resource density on a regional to basin scale, typically during the opportunity identification and screening phase of a project. Informed decision-making in exploring for unconventional opportunities relies on the integration of all available (i.e., sometimes scarce) data and their associated uncertainties for basin scale physical elements and processes into a working geologic model.
Pool, Wilfred (NAM) | Geluk, Mark (Shell Int. E&P) | Abels, Janneke (Shell International E&P) | Tiley, Graham John (Shell International E&P) | Idiz, Erdem (Shell Global Solutions International) | Leenaarts, Elise
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.