Lithological variation of Middle Bakken reservoirs in SE Saskatchewan: implications for optimizing multi-stage hydraulic fracturing

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1 Lithological variation of Middle Bakken reservoirs in SE Saskatchewan: implications for optimizing multi-stage hydraulic fracturing Hairuo Qing, Guoxiang Chi, Adam Sturiala Dept of Geology University of Regina

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6 Bakken black gold Bruce Johnstone, Leader-Post Published: Monday, December 10, 2007 Saskatchewan could be sitting on 25 billion to 100 billion barrels of sweet, light crude oil in the Bakken formation in the SE part of the province, according to industry and government estimates. By comparison, the heavy oil resource in west-central SK, is estimated to be 25 billion barrels of oil in place. We're excited about it,'' said Ed Dancsok of SER. "It's probably the biggest oil find in Saskatchewan since the 1950s. "It's fair to say, the Bakken play is the hottest play in North America,'' Mr. Smith, VP of Petrobank Resources, a company that has a large land position in southeastern Saskatchewan, said. What has government and industry observers so excited is the sheer magnitude of the Bakken formation, which is found in the Williston Basin underlying much of North Dakota, eastern Montana, SE SK and SW Manitoba. Bakken wells tend to be highly productive (200 barrels a day or more), producing sweet, light crude oil with 41- degree gravity, basically the highest grade of crude oil you can find anywhere. According to the U.S. Geological Survey (USGS), the Bakken formation could contain a mind-boggling 413 billion barrels of oil in place.

7 Department of Geology on.aspx

8 From Yang 2009

9 345 Ma ago Early Miss.

10 Bakken lithology from some recent publications (After Angulo et al. 2008)

11 Department of Geology Bakken Producing Wells

12 Core examined in this study Department of Geology

13 Dolomitic Siltstone w w m Department of Geology

14 Sandy-Silty Grainstone w m w m Department of Geology

15 Silty Dolostone w m w m Department of Geology

16 Dolomitic siltstone Dolomitic sandstone 100m 100m Silty dolostone Calcitic sandstone 100m m 100m m Silty dolostone Sandy limestone 100m 100m

17 Well# W2

18 A A A C B A A B C

19 Average mineral composition Mineral composition Quartz 14-64% (Av.40%) % Feldspar 1-3% (Av. 2%) Lithics 0-2% (Av. 1%) Mica 0-2% (Av 1%) Carbonates 20-80% (Av.51%) (Cal +Dol) Pyrite <1-8% (Av.2%) % 2% 1% 1% 2% Quartz Feldspar Lithics Mica Carbonates Pyrite High grain to matrix ratio ::: Texturally mature sandstone

20 % Rock types - Percentages : Total samples analysed = 93 36% % 13% Dol. Silty dolostone Cal-dol sandsdtone Sand-silt limestone

21 Texture Grain size: F-vf san d Silt 27% 73% Grain sorting: Mostly well sorted, few rocks moderately sorted Angularity/Roundness (clastic components) : Mostly subangular to subrounded, few rocks with rounded grain dominating Porosity: % 30 25% % 1% or less >1% -5% >5% -15% Thin section porosity

22 Quartz cement and pressure solution Sample# 4, well w2 Sample# 4, well w2 Sample# 5, well w2 Sample# 40, well w2

23 Calcite: Grains Bioclasts/Peloids Post depositional Cement Sample# 5, well w2 Sample# 5, well w2

24 Calcite SEM-EDS analyses Non-ferroan calcite cement Sample# 9, Well W2

25 Dolomite: Post depositional replacement Detrital Sample# 11 Well W2 Sample# 35 Well W2

26 Silty dolostone Sample# 1, Well W2

27 Dolomite Back Scattered SEM Image-EDS Sample #2, Well W2 Sample #2, Well W2

28 Sample # 2 Well: W2 Dolomite Back Scattered SEM Image-EDS

29 Anhydrite: A minor cement May be locally abundant Sample# 4, well w2 Sample# 18, well w2

30 Pyrite cement Sample# 44, well w2 Sample# 44, well w2 Sample# 28, well w2

31 Dissolution pore after partial dissolution of calcite and dolomite cement silisclastic grains Sample# 43, well W2 Sample# 6, well w2 Sample# 6, well w2

32 Sample# 24, well w2 Fracture pore: Open Filled Sample# 42, well w2 Sample# 43, well w2

33 Preliminary conclusions Our preliminary research indicates that Bakken reservoirs are characterized by a variety of lithologies with different amount of quartz grains, carbonate grains, and dolomite. The primary porosity was commonly filled by calcite, dolomite and quartz, with minor anhydrite cement and pyrite. Both calcite and dolomite extensively replace detrital grains. Local dissolution of carbonate and quartz grains resulted in additional secondary porosity. Minor fractures might improve permeability locally. The dolomite reservoirs seems to have a better porosity and permeability

34 The results of detailed characterization of lithology of Bakken reservoirs will benefit to: selection of effective EOR strategies; optimizing the pressure used for multi-stage frac.

35 Future work Silty dolostone Calcitic sandstone The different types of lithologies will result in reservoirs characterized by different types of petrophysical properties (e.g. porosity, permeability, and porethroat), which are important data for production design to optimize its production. 100m Petrophysical characterization of different types of Bakken reservoirs: The porosity, permeability, and pore throat will be analyzed for selected core samples (15-20) from different lithofacies. 100m m

36 Silty dolostone Calcitic sandstone Future work The different types of lithologies will result in reservoirs characterized by different types of petrophysical properties (e.g. porosity, permeability, and porethroat), which are important data for production design to optimize its production. Petrophysical characterization of different types of Bakken reservoirs: The porosity, permeability, and pore throat will be analyzed for selected core samples (15-20) from different lithofacies. 100m m

37 Future work The different lithologies (e.g. quartz, calcite, dolomite) would react differently to the pressures that were selected for multi-stage hydraulic fracturing. In addition, the reservoirs with different lithologies also have different capability to retain the fractures open during the subsequent production. Representative samples from reservoirs with different lithologies (e.g. quartz, calcite, dolomite) will be selected to investigate the nature of the fractures during pressure test and capability to maintain the fractures open during the subsequent production. Silty dolostone Calcitic sandstone 100m m

38 This project is supported by PTRC-STEPS and Sask MER Thank you!