Shale Gas Reservoirs in the Western Canadian Sedimentary Basin
ultimate recoveries expected to be 5BCF estimated ultimate recovery/well and initial production of 4.5mmcf/day.
The lower Cretaceous Buckinghorse Formation and lateral equivalents (Moosebar) in northeastern BC were evaluated in 2008 by Chalmers and Bustin, who estimated more than 200BCF/section GIP potential. The shales have TOC contents ranging from 1 to over 16wt%, thermal maturities between immature and overmature and maximum thicknesses of 1,600m, representing huge resource potential. Due to the high detrital quartz content in many of these shales and silts, porosities of up to 16% have been reported.5
In east-central Alberta and western Saskatchewan, the Cretaceous Colorado group shales provide major unconventional gas fields from the Milk River, Medicine Hat and Second White Specks formations. The lower Colorado group shales consist of non-calcareous shales separated by a wedge of coarser, siliciclastic material that thins northeastwards. The upper Colorado sequence is composed of fine- to coarse-grained clastics, highlighted by the wide-spread marker of the white speckled shales, which have been identified as two major gas sources. TOC contents range from 2 to 12wt% and it is believed that in some areas the gas can be sourced biogenically (by bacterial decay).
Technical Challenges
To quantify shale gas resources and develop an efficient exploration programme, the effects of the physical properties of shale on gas content and transport need to be understood. However, factors that influence GIP and reservoir processes remain unclear, and favourable
Figure 3: Impact of Temperature on Sorption Capacity of Devonian– Mississippian Shales in the Western Canadian Sedimentary Basin
0.8 1.0 1.2 1.4
0.6 0.4 0.2
0.0 040 20
3.45 wt % TOC 3.83 wt % TOC 2.05 wt % TOC 4.01 wt % TOC 80
60 Temperature (ºC) TOC = total organic carbon. Source: Ross, unpublished.
shale gas reservoir properties are often poorly defined. Significant gas accumulations undoubtedly exist in WCSB shales, but their exploitation has been inhibited by four factors:
•
Insufficient/incomplete data sets on organic-rich facies that are deemed excellent source rocks but uneconomic reservoirs. Shales have not been the primary target for sub-surface exploration, therefore shale samples are scarce. Most published reservoir exploration data relate to source-rock analysis, hydrocarbon seal efficiency or specific problems such as shale sensitivity to drilling fluids.
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Gas Capacity
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