Vitrinite reflectance of Cretaceous coals and thermal maturity of the

Vitrinite reflectance of Cretaceous coals and thermal maturity of the Niobrara Formation, DJ Basin, Colorado, USA
Dan Hallau, Ryan Sharma, Robert Cluff
RMS‐AAPG 2014, Denver, July 21, 2014
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Geology
Late Cretaceous paleogeography (~85Ma)
Study area
Modified from Blakey (2014)
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• Niobrara is an organic‐rich unit of interbedded chalks and marls
• It has emerged as a significant unconventional resource
• Self‐sourced
• Because migration of hydrocarbons in these shale oil systems can be minimal, maturation is crucial for exploration
• The “gold standard” for thermal maturation is vitrinite
reflectance
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Problems
• Vitrinite reflectance studies in the DJ suffer from small sample sizes
• In the Niobrara, the risk is high that measurements have been taken on oxidized/recycled macerals
– Difficult to identify tiny macerals
• Pseudo‐measurements and proxies add more uncertainty
• Well log‐based approximations, Tmax, etc. are less precise than VRo
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Solution
• Measure actual coal rather than dispersed OM in shale or marl
• Increase the sample density and area
Down section in the J‐sand there is coaly material.
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Stratigraphic column
~500’
• J‐sand: fluvial/deltaic with thin coaly laminations and “coffee grounds”
• Examined well over 300 J‐ & D‐sand cores
– About 70% appeared to have coaly material, so a sample was collected
• Ultimately we’d like to know maturity in the Niobrara
Modified from Tainter (1984)
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Collected sample
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Sample preparation
The sample preparation process is based on the recommended practices in ASTM D2797/D2797M‐09.
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Analysis
Leitz Orthoplan
microscopes fitted with Leitz MPV photometers
• 500x magnification under oil immersion
• 50+ measurements on randomly oriented vitrinite macerals
• Klein & Becker mono‐
crystalline standards
– Calibrations before and after analysis to ensure there was not drift
– Bracket the sample with two “nearby” standards
8/21
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Vitrinite examples
9/21
(Each image is roughly 500 microns across)
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Quality control
• Multiple petrographers analyzing the same samples
– 20+% redundancy
– Measuring on different days, different calibrations
– Excellent convergence (R2 = 0.95)
No. of measurements
Petrographer 1
Mean = 0.85%
Std Dev = 0.03
Mean = 0.85%
Std Dev = 0.03
%VRo
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Petrographer 2
%VRo
• Cross‐verification with a commercial lab
– 41 samples – Excellent correlation (R2 = 0.97)
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Data quality
• Typical Niobrara histogram
– Entire distribution:
• n = 33
• mean = 1.10
Mean = 1.10%
Std dev = 0.18
Data from USGS CRC
• Typical J‐sand histogram
11/21
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Data quality
• Typical Niobrara histogram
– Classed by inferred maceral
type:
• n = 10
• mean = 0.97%
Mean = 0.97%
Data from USGS CRC
• Typical J‐sand histogram
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Data quality
• Typical Niobrara histogram
– What the petrographer actually reported:
• n = 2
• mean = 0.88%
Mean = 0.88%
Data from USGS CRC
• Typical J‐sand histogram
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Data quality
• Typical Niobrara histogram
– n = 2
– mean = 0.88%
Mean = 0.88%
• Typical J‐sand histogram
Data from USGS CRC
Mean = 0.93%
– n = 50
– mean = 0.93%
– Std dev = 0.05
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Maturity transform
• Reflectance‐depth transform based on well in Tainter (1984) study
• Resulted in reduction in VRo by an average of 0.03%
Raw data from Tainter (1984)
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Burial history
J‐sand
Niobrara
Pierre
• Significant basin subsidence would not have occurred until after and during Kn deposition
• Therefore, maturation history for the Kj and Kn
would be functionally the same
Modified from Tainter (1984)
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Results
Resistivity as proxy
Modified from Smagala et al., 1984
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Vitrinite reflectance data
Vitrinite reflectance data
Discovery Group, 2014
Data from Higley et al., 1985
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Structure vs. %VRo
18/21
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Comparison with Tmax
Modified from Thul (2012)
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Conclusions
• Measuring vitrinite reflectance in the Niobrara is inherently problematic
• This can be overcome by measuring vitrinite
reflectance on coaly material from the stratigraphically deeper J‐ and D‐sands
• 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 previously‐
undocumented high and low temperature anomalies.
20/21
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References cited
ASTM, 2010a, ASTM D2797/D2797M‐09 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. 401–405
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 Open‐File Report 85‐384, 100 p.
Smagala, T. M., C. A. Brown, and G. L. Nydegger, 1984, Log‐derived 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. 355–363.
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.339‐354.
Thul, D.J., 2012, Niobrara source rock maturity in the Denver basin: a study of differential heating and tectonics on petroleum prospectivity
using programmed pyrolysis, Master’s thesis, Colorado School of Mines, Golden, CO, 137pp.
United States Geological Survey Core Research Center (USGS‐CRC) website, 2014, http://my.usgs.gov/crcwc/
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