Sequence Stratigraphy of a Black Shale: How to Do It, and Why It Matters

Transcription

1 Sequence Stratigraphy of a Black Shale: How to Do It, and Why It Matters Nicholas B. Harris Earth & Atmospheric Sciences University of Alberta nharris@ualberta.ca

2 How do you do stratigraphy in a black shale?

3 How do you do stratigraphy in a black shale? Sea-level cycles will be expressed even in the middle of a shale basin These cycles will be expressed in lithofacies, mineralogy, geochemistry and well logs. Significance: Connect the basin margin to the basin center. The cycles will be important for shale gas, impacting gas generation, storage and fracture development.

4 Modified from Comer (1991) Woodford Shale Consortium

5 Woodford Shale Consortium Black Mudstone Lithofacies RTC 12902

6 Woodford Shale Consortium Black Mudstone Lithofacies py py Radiolarian Dasycladacean algae Agglutinated foraminifera

7 Woodford Shale Consortium Laminated Carbonate Lithofacies RTC 12845

8 These are interpreted as turbidites.

9 Transgressive systems tract High stand systems tract Transgressive systems tract High stand systems tract Transgressive systems tract High stand systems tract

10 At least 10 depositional cycles are present in the RTC #1 core 20 million years Depositional cycles represent 3 rd order sea level cycles

11 Chert and phosphate more significant Black shale and carbonate predominate

12 357.9 ± 5.3 Ma Initial Os = Chert and phosphate more significant 364 ± 13 Ma Initial Os = 0.69 Black shale and carbonate predominate ± 5.8 Ma Initial Os = Fr / Fm boundary ± 7.1 Ma Initial Os = 0.291

13 organic mudstone with phosphate nodules Upper Woodford Middle Woodford

14 Chesapeake MBF Modified from Comer (1991)

15 bioturbated mudstone (± dolomite) novaculite organic mudstone organic mudstone with phosphate nodules organic mudstone Upper Woodford Middle Woodford

16 novaculite

17 bioturbated mudstone (± dolomite)

18 Alternating high and low TOCs Consistent high TOCs Abrupt alternations between high and low TOCs

19 Increase in terrestrial OM component

20 2 nd and 3 rd order sea level cycles Woodford deposition coincides with overall sea level fall. 3 rd order cycles superimposed on a longterm trend of falling sea level. Modified from Haq and Schutter (2008)

21 Silled Basins Mo Mo behavior Mo Mo recharge from global ocean varies with degree of restriction across barrier. Mo concentration should vary with sea level AND redox conditions. Mo

22 Silled Basins Mo Mo Mo (ppm) %TOC Mo

23 Silled Basins Mo Mo Mo %TOC %TOC %TOC Mo (ppm) Mo (ppm) Mo (ppm)

24 12750 Mo/C Mo / C ratios decrease systematically to the top of the formation depth (feet Can this be a proxy for sea level? very restricted open

25 quartz clay

26 Different outcomes in the Mini-Frac/Injection tests performed in RTC #1, Woodford shale. Fracture gradient (psi/ft) Lower Woodford Lower Woodford FG Middle Woodford Upper part of the Middle Woodford Middle Woodford Lower part of the Middle Woodford Depth (ft)

27 Young's modulus, E (psi) 0.E+00 1.E+07 2.E+07 3.E+07 4.E+07 5.E+07 6.E+07 7.E+07 8.E+07 9.E Core depth (ft) Core E Sonic log E Core v Sonic log v Poisson's ratio, v (dimensionless)

28 Decrease in Poisson s Ratio indicates increasing brittleness upsection Young's modulus, E (psi) 0.E+00 1.E+07 2.E+07 3.E+07 4.E+07 5.E+07 6.E+07 7.E+07 8.E+07 9.E Core depth (ft) Core E Sonic log E Core v Sonic log v Poisson's ratio, v (dimensionless)

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30 Harris et al., 2011, the Leading Edge

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32 Cf, Slickwater Copyright B&A 2004

33 Cf, Hybrid with 30/50 ceramic Ceramic proppant increases the fracture conductivity by 2 to 3 times. Copyright B&A 2004

34 Whiting KCC 503 Modified from Comer (1991)

35 lithology grain size???? gamma ray (GA Upper Woodford 8300 depth (feet) Middle Woodford Lower Woodford 8800

36 grain size???? gamma ray (GA lithology depth (feet)

37 grain size???? Exotic Beds lithology dolomite siliciclastic chert

38 grain size???? 8250 lithology Cycles in Exotic Beds Bundles of exotic beds, formed by sets of mm-scale laminae Scale of bundles is similar to that seen near carbonate margin

39 grain size???? 8250 lithology Brittle Beds KCC

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41 Most brittle 1.40E E E+07 Poisson's Rat E E+06 Poisson's Ratio Young's Modulus E E E depth (feet) gamma ra depth (feet)

42 CONCLUSIONS - 1 Sequence stratigraphic analysis can be done on black shales; it requires an integrated, multidisciplinary approach. 3 rd order stratigraphic cycles are indicated by the repetition of exotic beds. The composition of these beds will vary regionally. 2 nd order cycles are represented by (1) variation in SiO 2 and clay, (2) Mo/C; (3) organic assemblage, and (4) TOC. Cycles affect gas generation capacity and rock properties.

43 CONCLUSIONS - 2 Redox events, expressed by geochemical ratios, may be a tool for chronostratigraphic correlation. Clay content will have a eustatic and a local control. Chert beds may be significant for gas production these may have a eustatic and a local control.