Devonian Isopach and Structure Maps: Initial Results of the IEA Weyburn CO 2 Monitoring and Storage Project Area

Transcription

1 Devonian Isopach and Structure Maps: Initial Results of the IEA Weyburn CO 2 Monitoring and Storage Project Area L.K. Kreis, P.L. Thomas, R.B. Burke 1, and S.G. Whittaker Kreis, L.K., Thomas, P.L., Burke, R.B., and Whittaker, S.G. (2003): Devonian isopach and structure maps: Initial results of the IEA Weyburn CO 2 Monitoring and Storage Project area; in Summary of Investigations 2003, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep , CD-ROM, Paper A-5, 6p. Abstract Devonian mapping results from the International Energy Agency s (IEA) Weyburn CO 2 Monitoring and Storage Project have been jointly published by Saskatchewan Industry and Resources and the North Dakota Geological Survey as a test version CD entitled Devonian Isopach and Structure Maps, IEA Weyburn CO 2 Monitoring and Storage Project Area. This product contains computer-generated regional structural and isopach maps of the Devonian System in Saskatchewan, North Dakota, and Montana. These include the structural and isopach maps of ten formally recognized formations, and isopach maps of six halite (salt) sub-units and four groups. In addition, stratigraphic tops data, a description of the Saskatchewan Land Survey System, and a questionnaire are included. Copies of the CD are available upon request 2. Comments and suggestions from this questionnaire will be used to develop future subsurface mapping products from the Petroleum Geology Branch. Keywords: Devonian, subsurface mapping, isopach map, structure map, salt (halite), CO 2 storage, Weyburn, southeastern Saskatchewan. 1. Introduction The IEA Weyburn CO 2 Monitoring and Storage Project (the Weyburn Project) is a multi-agency investigation into the storage of anthropogenic CO 2 in the Mississippian Midale Beds of EnCana Corp. s Weyburn Pool, southeastern Saskatchewan. In this project, structure and isopach mapping of the Phanerozoic succession over a 200 km x 200 km area is centred on the Weyburn Pool and extends approximately 60 km into the United States of America (Figure 1). These stratigraphic data are used to form the geological framework around which other geoscience data are integrated to develop a 3D model of the geosphere enveloping the CO 2 injection target, EnCana s Weyburn Pool Midale reservoir. As part of the Weyburn Project, the Petroleum Geology Branch of Saskatchewan Industry and Resources and the North Dakota Geological Survey have jointly produced a test version of a CD entitled Devonian Isopach and Structure Maps, IEA Weyburn CO 2 Monitoring and Storage Project Area. The CD includes isopach and structure maps and stratigraphic data for major Devonian units within the Weyburn Project area presented in PDF format and viewable in Adobe Acrobat 4.0 (included on CD). The intent is to use this product as a prototype for future map products that will contain computer-generated maps for all major Phanerozoic units throughout the province accessible in ArcView GIS format and with an interactive attribute database. This report describes the data type, method of production, and map content. 2. Devonian Mapping Test Version CD a) Stratigraphic Overview The Devonian succession is subdivided into four groups: Elk Point, Manitoba, Saskatchewan, and Three Forks. The Elk Point Group comprises the Ashern, Winnipegosis, and Prairie (Evaporite) formations. The overlying Manitoba Group includes the Dawson Bay and Souris River formations; the Saskatchewan Group includes the Duperow and Birdbear formations. The Three Forks Group completes the succession and is made up of the Torquay, Big Valley, and Bakken formations (Figure 2). Some of these formations have been further subdivided and their geological tops are also included in an Excel spreadsheet, but were not mapped. 1 North Dakota Geological Survey, 600 East Boulevard Avenue, Bismarck, North Dakota, USA, Contact Kim Kreis, Petroleum Geology Branch, Saskatchewan Industry and Resources, 201 Dewdney Avenue East, Regina, SK, Canada S4N 4G3. Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1

2 Figure 1 - Location map showing the IEA Weyburn CO 2 Monitoring and Storage Project area (referred to on maps as Weyburn Project Area) in which Devonian mapping was performed. Figure 2 - Stratigraphic chart of the Devonian Stratigraphy in southeastern Saskatchewan. Note that on a given map sheet the colour-highlighted area indicates the stratigraphic interval mapped. Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 1

3 The regional distributions (isopach maps) of the ten formally recognized formations and four groups comprising the Devonian System in Saskatchewan are presented on the CD (Figure 2) along with the structural attitudes (structure maps) of the ten formations. In addition, isopach maps of six halite (salt) sub-units within in the study area are provided. A list of the thirty maps included in this CD is given in Table 1. b) Data Type All geological tops were interpreted from one or more geophysical well-log signatures for the given geological units, with depths presented as elevations (relative to Kelly Bushing). All stratigraphic data is presented within an Excel spreadsheet (Appendix A of CD) that contains information including unique well identifier (UWI), well location co-ordinates (eastings and northings; NAD 1983), Kelly Bushing (KB), geological tops, subsea depth (structure), and thickness (isopach) for given geological units. Some geological picks were not made in some wells due either to poor log quality or because the interval was not logged. All maps are also available for viewing and manipulation using Surfer 8. The Saskatchewan Land Survey System and instructions for assigning Well Identification Codes are also included. c) Method of Map Production All isopach and structure maps shown on the CD were generated using Surfer 8 mapping software. Kriging was used as the preferred gridding method after experimentation with other gridding methods. Kriging tends to express trends in the data rather than creating bull s-eyes. Although Surfer 8 allows for significant customization and enhancement of data anisotropy, no attempt was made to manipulate trends indicated in the Devonian data. All defaults given for the Kriging method in the construction of these maps were accepted except for the grid spacing. A typical grid spacing default for these maps was around 2900 spacing units or 2.9 km. Experimentation suggested that a grid spacing of 800 m produced a map that honoured the data more accurately; it has therefore been used in generating all maps shown on the CD. Surfer 8 applies an orthogonal grid to the map data using the most northward, southward, eastward, and westward data to define the limits of the maps. For the current set of maps it should be noted that the American data included one more range to the east and one township to the south beyond the defined Weyburn Project Area. The Canadian portion of the map did not include data for the range to the east and therefore contours in Range 1W2 are projections of trends. Table 1 - List of Devonian isopach and structure maps included on the CD-ROM. Isopach Maps Elk Point Group (Ashern, Winnipegosis, and Prairie formations) Ashern Formation Winnipegosis Formation Prairie Evaporite (Prairie Formation) Prairie Salt Manitoba Group (Dawson Bay and Souris River formations) Dawson Bay Formation Hubbard Salt Souris River Formation Upper Harris Salt Lower Harris Salt Davidson Salt Saskatchewan Group (Duperow and Birdbear formations) Duperow Formation Flat Lake Salt Birdbear Formation Three Forks Group (Torquay, Big Valley, and Bakken formations) Torquay Formation Big Valley Formation Bakken Formation Structure Maps Ashern Formation Winnipegosis Formation Prairie Evaporite Dawson Bay Formation Souris River Formation Duperow Formation Birdbear Formation Torquay Formation Big Valley Formation Bakken Formation Saskatchewan Geological Survey 3 Summary of Investigations 2003, Volume 1

4 As all gridding methods are geostatistical representations of data, some contour patterns, such as bull s-eyes and some thickness trends may be computer-generated artifacts, and do not necessarily reflect geological interpretation by the authors. No attempt has been made to edit these grids, nor efforts made to invoke geological knowledge the authors may have regarding isopach or structural anomalies. The only grid editing performed was to constrain the zero limits of some of the Devonian salt units mapped. The choice of contour intervals for each mapped geological unit depended upon the thickness of the unit. A contour interval of only 0.1 m was chosen for the Upper Harris Salt as it ranges in thickness from 0 to approximately 3 m whereas a 10 m contour interval was chosen for the Prairie Salt, which ranges in thickness from 0 to almost 200 m. A 30 m contour interval was chosen to show structural variations on top of all 10 Devonian formations mapped. d) Map Contents The geological unit featured on a given map sheet (Figure 3) is highlighted in yellow on the stratigraphic chart (Figure 2). A legend indicating contour interval, datum (for structure maps), grid file name (e.g. iso_prairieevaporite_kr_800m_unedited), date of production (e.g. April, 2003), number of wells used, and a plus sign symbol (+) for posted well locations is included for all map sheets. Some maps include an overlay of the Prairie salt zero edge that is shown as a red line in the legend (Figure 4). 3. References Kreis, L.K., Thomas, P.L., Burke, R.B., and Whittaker, S.G. (2003): Devonian Isopach and Structure Maps, IEA Weyburn CO 2 Monitoring and Storage Project Area (Test Version CD-ROM). Saskatchewan Geological Survey 4 Summary of Investigations 2003, Volume 1

5 Figure 3 - Example of map sheet showing colour contoured isopach map of Prairie Evaporite (Prairie Formation). Saskatchewan Geological Survey 5 Summary of Investigations 2003, Volume 1

6 Figure 4 - Example of map sheet showing colour contoured structure map on top of Prairie Evaporite (Prairie Formation). Saskatchewan Geological Survey 6 Summary of Investigations 2003, Volume 1