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
How do you do stratigraphy in a black shale?
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.
Modified from Comer (1991) Woodford Shale Consortium
Woodford Shale Consortium Black Mudstone Lithofacies RTC 12902
Woodford Shale Consortium Black Mudstone Lithofacies py py Radiolarian Dasycladacean algae Agglutinated foraminifera
Woodford Shale Consortium Laminated Carbonate Lithofacies RTC 12845
These are interpreted as turbidites.
Transgressive systems tract High stand systems tract Transgressive systems tract High stand systems tract Transgressive systems tract High stand systems tract
At least 10 depositional cycles are present in the RTC #1 core 20 million years Depositional cycles represent 3 rd order sea level cycles
Chert and phosphate more significant Black shale and carbonate predominate
357.9 ± 5.3 Ma Initial Os = 0.474 Chert and phosphate more significant 364 ± 13 Ma Initial Os = 0.69 Black shale and carbonate predominate 371.5 ± 5.8 Ma Initial Os = 0.404 Fr / Fm boundary 379.0 ± 7.1 Ma Initial Os = 0.291
organic mudstone with phosphate nodules Upper Woodford Middle Woodford
Chesapeake MBF Modified from Comer (1991)
bioturbated mudstone (± dolomite) novaculite organic mudstone organic mudstone with phosphate nodules organic mudstone Upper Woodford Middle Woodford
novaculite
bioturbated mudstone (± dolomite)
Alternating high and low TOCs Consistent high TOCs Abrupt alternations between high and low TOCs
Increase in terrestrial OM component
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)
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
Silled Basins Mo Mo Mo (ppm) %TOC Mo
Silled Basins Mo Mo Mo %TOC %TOC %TOC Mo (ppm) Mo (ppm) Mo (ppm)
12750 Mo/C 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 12800 12850 Mo / C ratios decrease systematically to the top of the formation depth (feet 12900 12950 Can this be a proxy for sea level? 13000 13050 13100 very restricted open
quartz clay
1 0.95 Different outcomes in the Mini-Frac/Injection tests performed in RTC #1, Woodford shale. Fracture gradient (psi/ft) 0.9 0.85 0.8 0.75 0.7 Lower Woodford Lower Woodford FG 0.65 0.6 0.55 Middle Woodford Upper part of the Middle Woodford Middle Woodford Lower part of the Middle Woodford 0.5 12800 12850 12900 12950 13000 13050 13100 13150 Depth (ft)
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+07 12750 12800 12850 Core depth (ft) 12900 12950 Core E Sonic log E Core v Sonic log v 13000 13050 13100 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Poisson's ratio, v (dimensionless)
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+07 12750 12800 12850 Core depth (ft) 12900 12950 Core E Sonic log E Core v Sonic log v 13000 13050 13100 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Poisson's ratio, v (dimensionless)
Harris et al., 2011, the Leading Edge
Cf, Slickwater Copyright B&A 2004
Cf, Hybrid with 30/50 ceramic Ceramic proppant increases the fracture conductivity by 2 to 3 times. Copyright B&A 2004
Whiting KCC 503 Modified from Comer (1991)
8250 8275 8300 8325 8350 8375 8400 8425 8450 8475 8500 8525 8550 lithology grain size???? gamma ray (GA 8100 0 100 200 300 400 500 600 700 800 8200 Upper Woodford 8300 depth (feet) 8400 8500 Middle Woodford 8600 8700 Lower Woodford 8800
grain size???? gamma ray (GA 8240 0 100 200 300 400 500 600 700 800 lithology 8250 8275 8290 8300 8325 8340 8350 8375 depth (feet) 8390 8400 8425 8440 8450 8475 8490 8500 8525 8540 8550
grain size???? Exotic Beds lithology 8250 8275 dolomite 8300 8325 8350 8375 8400 siliciclastic 8425 8450 8475 8500 chert 8525 8550
grain size???? 8250 lithology Cycles in Exotic Beds 8275 8300 Bundles of exotic beds, formed by sets of mm-scale laminae. 8325 8350 8375 - Scale of bundles is similar to that seen near carbonate margin. 8400 8425 8450 8475 8500 8525 8550
grain size???? 8250 lithology Brittle Beds 8275 8300 8325 8350 8375 8400 8425 8450 8475 8500 8525 8550 KCC 503 8330
0.35 0.3 Most brittle 1.40E+07 1.20E+07 0.25 1.00E+07 Poisson's Rat 0.2 0.15 8.00E+06 6.00E+06 Poisson's Ratio Young's Modulus 0.1 4.00E+06 0.05 2.00E+06 0 0.00E+00 8100 8200 8300 8400 8500 8600 8700 8800 depth (feet) 800 700 600 gamma ra 500 400 300 200 100 0 8100 8200 8300 8400 8500 8600 8700 8800 depth (feet)
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.
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.