www.DiscoveryâGroup.com
Dan Hallau, Ryan Sharma, Robert Cluff
Vitrinite reflectance of Cretaceous coals and thermal maturity of the
Niobrara Formation, DJ Basin, Colorado, USA
SIPES Luncheon, April 27, 2017
⢠Building a vitrinite reflectance lab from scratch⢠Vitrinite reflectance of the J sandstone / maturity of the
Niobrara
Outline
⢠A maceral is a building block of an organic rockâ Mineral: rock as maceral: coal
â Main types:⢠Vitrinite
â Cells and cell walls of woody material⢠Liptinite
â Spores, waxy material, resins⢠Inertinite
â Fusinite, semiâfusiniteÂť Oxidized material
What is a maceral?
⢠Relationship between âgraynessâ of vitrinite macerals and thermal stressâ Developed in the coal industry to assess rank
⢠In a controlled environment, we measure the amount of light reflected off the surface of a vitrinite maceral as a proxy for thermal stressâ Need to be able to identify vitrinite and distinguish it from other
maceral types
What is vitrinite reflectance?
⢠Commercial reflectance workâ Heavy backlogâ Expenseâ Inconsistency between service companies, between petrographersâ A need to independently verify their work
⢠Vitrinite reflectance studies from marine shales:â The risk is high that measurements have been taken on
oxidized/recycled maceralsâ Difficult to identify tiny maceralsâ Small sample sizesâ Well logâbased approximations, Tmax, etc. are less precise than VRo
Problems
⢠Bring reflectance work in houseâ âHow hard could it be?â
⢠Measure actual coal rather than dispersed OM in shaleâ In the case of the Niobrara, letâs measure coaly material in the J
sandstone rather than âvitriniteâ in the Niobrara
Solutions
Letâs buy a complete vitrinite reflectance setâupâŚ
Then again, maybe notâŚ
⢠In the late 1970s Bob Cluff and others at the Illinois State Geological Survey were studying thermal maturity of the New Albany shale
D.I.Y.
⢠On their desks were Leitzmicroscopes complete with photometers and analog photometer controllers
⢠âI bet we can reâcreate that.â
The treasure hunt beginsâŚ
⢠Leitz Orthoplan base and stage (~$1500â3000)â Common in silicon wafer inspection,
biology
âThis is gonna be easy!â
⢠Leitz Orthoplan base and stage (~$1500â3000)â Common in silicon wafer inspection,
biology⢠Illumination (~$800â1200)
â Lamp housing, DC power supply, mirror house
âThis is gonna be easy!â
⢠Leitz Orthoplan base and stage (~$1500â3000)â Common in silicon wafer inspection,
biology⢠Illumination (~$800â1200)
â Lamp housing, DC power supply, mirror house
⢠Trinoc head (~$0â600)
âThis is gonna be easy!â
⢠Leitz Orthoplan base and stage (~$1500â3000)â Common in silicon wafer inspection,
biology⢠Illumination (~$800â1200)
â Lamp housing, DC power supply, mirror house
⢠Trinoc head (~$0â600)⢠Vertical illuminator (~$100â1200)
â Single objective, clutchâmount preferred
âGosh, some of these things are hard to findâŚâ
⢠Leitz Orthoplan base and stage (~$1500â3000)â Common in silicon wafer inspection,
biology⢠Illumination (~$800â1200)
â Lamp housing, DC power supply, mirror house
⢠Trinoc head (~$0â600)⢠Vertical illuminator (~$100â1200)
â Single objective, clutchâmount preferred⢠Oil immersion objective (~$600â1200)
â âUnobtainiumâ
âGosh, some of these things are hard to findâŚâ
⢠Leitz Orthoplan base and stage (~$1500â3000)â Common in silicon wafer inspection,
biology⢠Illumination (~$800â1200)
â Lamp housing, DC power supply, mirror house
⢠Trinoc head (~$0â600)⢠Vertical illuminator (~$100â1200)
â Single objective, clutchâmount preferred⢠Oil immersion objective (~$600â1200)
â âUnobtainiumâ⢠Photometer and controller (~$3000)
â The hardest part
âPatience is a virtueâŚâ
⢠Reflectance standards (~$850 each)⢠546 nm green interference filter⢠Desiccator cabinets (~$200â300 each)⢠Grinding/polishing wheel⢠Pelican foamâlined shipping case (~$250)⢠Tons of lab supplies⢠Reference books
Lots of other stuff required
⢠Vitrinite
Macerals
Macerals (contâd)
⢠Several maceral types coâoccuring
⢠Fusinite
Macerals (contâd)
⢠Semifusinite
Macerals (contâd)
Name that maceral
Search through coreâŚ
USGS CRC (2017)
Collect the good stuffâŚ
Prepare the plugsâŚ
The sample preparation process is based on the recommended practices in ASTM D2797/D2797Mâ09.
Finally, we can get started!
LeitzOrthoplanmicroscopes fitted with Leitz MPV photometers
⢠500x magnification under oil immersion
⢠50+ measurements on randomly oriented vitrinite macerals
⢠Klein & Becker monoâcrystalline standards
⢠Calibrations before and after analysis to ensure there was not drift
⢠Bracket the sample with two ânearbyâ standards
(Each image is roughly 500 microns across)
Analysis
Typical histograms
Marine shale example: Mel Bickling 42â21
Coaly material example Horst 1
Geology
Modified from Blakey (2014)
Late Cretaceous paleogeography (~85Ma) ⢠Niobrara is an organicârich unit of interbedded chalks and marls
⢠It has emerged as a significant unconventional resource
⢠Selfâsourced⢠Because migration of
hydrocarbons in these shale oil systems can be minimal, maturation is crucial for exploration
Study area
Modified from Blakey (2014)
Stratigraphic column
⢠Niobrara: marine unit, poor in terrestrial organic material
⢠Jâsand: fluvial/deltaic with thin coaly laminations and âcoffee groundsâ
⢠Examined well over 300 Jâ & Dâsand coresâ About 70% appeared to have
coaly material, so a sample was collected
⢠Ultimately weâd like to know maturity in the Niobrara
modified from Higley and Cox (2007)
Subsidence profile
Modified from Tainter (1984)
Significant basin subsidence would not have occurred until after and during Kndeposition
Therefore, maturation history for the Kj and Kn would be functionally the same
Quality control⢠Multiple petrographers analyzing the same
samplesâ 20+% redundancyâ Measuring on different days, different
calibrationsâ Excellent convergence (R2 = 0.95)
Petrographer 1 Petrographer 2
Mean = 0.85%Std Dev = 0.03
Mean = 0.85%Std Dev = 0.03
No. of m
easuremen
ts
%VRo %VRo
Quality control (contâd)
⢠Crossâverification with a commercial labâ 41 samples â Excellent correlation
(R2 = 0.97)
Maturity transform
⢠Reflectanceâdepth transform based on well in Tainter (1984) study
Raw data from Tainter (1984)
Correction
⢠Resulted in reduction in VR by an average of 0.03%
VRo contour map
⢠Over 180 new data points
Comparison to other studies
⢠Measuring vitrinite reflectance in the Niobrara is inherently problematic
⢠This can be overcome by measuring vitrinite reflectance on coaly material from the stratigraphically deeper Jâ and Dâsands
⢠A researchâquality vitrinite reflectance microscope can be built from parts on ebay for less than $10,oooâ With patience and perseverance it can probably be done for
considerably less⢠Over 180 new vitrinite reflectance data points have helped to
both expand and refine the maturity profile of the DJ Basin.⢠The new data have revealed localized previouslyâ
undocumented high and low temperature anomalies.
Conclusions
ASTM, 2010a, ASTM D2797/D2797Mâ09 Standard practice for preparing coal samples for microscopical analysis by reflected light, Annual book of ASTM standards: Petroleum products, lubricants, and fossil fuels; Gaseous fuels; coal and coke, sec. 5, v. 5.06, ASTM International, West Conshohocken, PA (2010), pp. 401â405
Blakey, R. C., 2014, Paleogeography of North America website, http://jan.ucc.nau.edu/rcb7/nam.html.
Higley, D.K., Pawlewicz, M.J., and Gautier, D.L., 1985, Isoreflectance map of the J sandstone in the Denver basin of Colorado: U.S. Geological Survey OpenâFile Report 85â384, 100 p.
Smagala, T. M., C. A. Brown, and G. L. Nydegger, 1984, Logâderived indicator of thermal maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming, in J. Woodward, F. F. Meissner, and J. L. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Rocky Mountain Association of Geologists, p. 355â363.
Tainter, P. A., 1984, Stratigraphic and paleostructural controls on hydrocarbon migration in Cretaceous "D" and "J" Sandstones of the Denver basin, in Woodward, J., Meissner, F.F. Meissner, and, J.L. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Rocky Mountain Association of Geologists, p.339â354.
References cited