SAGD Screening Study

Regional Screening Study Of McMurray Formation (Lower Cretaceous) SAGD Reservoirs, Northeastern Alberta, Canada

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Summary:
South Half: T69-84, R4W4-12W4
North Half: T84-97, R4W4-12W4
Strata: McMurray
Year of Study: 2006

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Introduction

This SAGD screening study will be of interest to firms either considering an investment in SAGD or who already have assets undergoing development. Those looking for an appropriate lead will find this evaluation useful in screening out sites of optimal reservoirs. Those with assets undergoing development will see how their properties compare to those being pursued by others. Lastly, those with an interest in a variety of related subjects, from future oil supply calculations through to infrastructure planners, will derive value from knowing where the most likely SAGD development sites will be.

Background to the Area

Reservoir quality is a fundamental aspect controlling the technical and economic viability of thermal recovery projects. Thus, there is a need to screen available geological data in order to both evaluate prospective trends and pinpoint the sites of initial development. Steam-assisted gravity drainage (SAGD) techniques are currently being applied to several Lower Cretaceous tar sand reservoirs in northeastern Alberta. If successful, to the degree expounded by the proponents, the resulting production of crude bitumen will significantly boost Canada’s position in the global energy business. The greatest focus of SAGD applications is currently the McMurray Formation where several projects are currently active, under application, or in various stages of delineation. In spite of the potentially productive capability of currently identified projects, there remain vast areas of the greater Athabasca region that are poorly evaluated and, as a consequence, remain unleased. Although there is a widespread perception that nearly all technical risk associated with tar sand development is related to the production process and economics, it will be shown subsequently that there remains a significant role for the exploration process in finding new SAGD opportunities as well as high-grading potentially productive assets.

This report provides a multi-scale evaluation of McMurray SAGD reservoirs in northeastern Alberta between Townships 69-97 and Ranges 4-12 W4M. At the local scale, studies of several specific SAGD projects, actual and proposed, have been made so that comparisons can be made with respect to the attributes of different projects. At the regional scale, new exploration leads and fairways have been determined for the region assessed so that new, potentially economic, SAGD reservoirs can be pursued either by new players or by established operators looking to high-grade SAGD assets. In addition, the details of the more localized, currently active projects can be seen within a larger regional context.

Geologic Setting

Four main structural elements are present in northeastern Alberta that influence the disposition of the McMurray Formation and its retained fluids: a regional dip to the southwest; a regional plunge oriented west-northwest by south-southeast (up-plunge is to the north); an eastern zone of linear to circular sags, oriented north-northwest by south-southeast, that originated from the dissolution of evaporites within Devonian strata; and localized structural highs and lows that are related to compaction features above the thickest sandstones and mudstones.

The regional southwesterly dip originated in response to Laramide (Late Cretaceous-Early Tertiary) tectonism and subsidence. In contrast, the role of salt dissolution appears to have had a much longer history. In the easternmost portions of the study area, and beyond, the McMurray succession thickens dramatically; it is interpreted that this thickening reflects syn-depositional subsidence associated with evaporite dissolution during the earliest phases of Mannville Group sedimentation. Hydrocarbon expulsion and migration are interpreted to have occurred during the Tertiary. The absence of any bitumen whatsoever in much of the eastern portions of the study area where McMurray sandstones are thickest suggests that post-depositional structural movements, again consistent with evaporite dissolution, created barriers to the up-dip migration of bitumen and its predecessor materials. Thus, evaporite dissolution processes may have extended into the Tertiary (at least). Lastly, the presence of relatively fresh water within the McMurray Formation today, combined with the interpreted presence of topographically-driven groundwater flow cells, suggests that evaporite dissolution is continuing today in northeastern Alberta.

Methodology

The regional study covers Townships 69-97 and Ranges 4-12 W4M. The nine specific SAGD projects that have been evaluated. In general, the assessments of specific projects cover an area of one or two townships that surround and include the primary development site(s).

Within the study area, about 6500 wells were available that penetrate the McMurray Formation at a depth greater than 100 metres and these wells were used in the evaluation. Of the approximately 8000 wells used in the evaluation, about 1500 occur within the individual SAGD sites.

Petrel Robertson has reviewed over 15 kilometres of core within the study area and the assessments of representative samples have been included.

The technical approaches employed in the regional evaluation include the following:

1. The region was subdivided into two sub-areas; a southern one that covers Townships 69-84 and a northern one that covers Townships 84-97. Township 84 was made common to both map sheets in order to minimize breaks in map trends located at artificial boundaries.

2. Cross-section grids were constructed for each map sheet and examples are provided as illustrations.

3. Each well was subdivided on the basis of reservoir quality.

4. Each well was assessed on the basis of its content of bituminous strata with a bitumen saturation of 75%, or more. In the southern map sheet a 40 ohm-m deep resistivity cut-off consistently approximates 75% bitumen saturation (cf. Gulf, 2001). In the north map sheet, there is a large variability in Rw which makes log cut-offs imprecise. As a consequence, more emphasis was placed on core analysis data in the determination of the 75% minimum bitumen saturation.

5. Gas and water zones were identified in each well. The recognition of water includes bottom water, top water, and free water zones found within bitumen columns; the latter situation is relatively common within the northern map sheet.

6. From the assessment of each individual log, a suite of maps was created that includes structure, isopach, isolith (i.e. reservoir), bitumen column thickness and the distributions of gas and water. In addition, slice maps of reservoir and bitumen distributions were created for the entire map area in order to assess the three-dimensional distributions of rocks and fluids.