Calculating Total Organic Carbon (TOC) using
the ‘Passey Method’
Chris Gent
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Overview
1. ‘Passey method’
2. Importance of TOC and maturity data.
3. Case Study: The Weald, UK.
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1. The Passey Method
A Practical Model for Organic Richness from Porosity and
Resistivity Logs. Passey et al 1990
• Using the change in deep resistivity and porosity curves to
highlight presence of organic-rich intervals in source rocks
• In ‘lean’ source rocks the curves overlay
• Organic-rich intervals are highlighted by curve separation
• The magnitude of the separation indicates amount of Total
Organic Carbon (TOC).
• Requires laboratory measured TOC values and maturity data.
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Sonic and Resistivity separation
‘Lean Shale’
Sonic highlights presence of kerogen
Resistivity highlights hydrocarbon fluids
Reservoir intervals need to be removed
Coals are highlighted but TOC values
inaccurate
Neutron and density curves can also be
used
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VR LOM
0.4 6
0.45 6.5
2. Calculating TOC and maturity 0.5
0.55
7.1
7.6
0.6 8.2
0.65 8.6
𝑇𝑇𝑇𝑇𝑇𝑇 = ∆𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 × 10(2.297−0.1688 × 𝐿𝐿𝐿𝐿𝐿𝐿) 0.7
0.75
9.2
9.6
0.8 9.9
0.85 10.1
0.9 10.3
• LOM = Level of Organic Maturity. A number 0.95 10.6
1 10.9
depicting maturity of a source rock based on 1.05 11.1
measured vitrinite reflectance (VR). - Hood, 1.1 11.2
1.15 11.3
Gutjahr and Heacock, 1975. 1.2 11.4
1.25 11.55
1.3 11.7
1.35 11.8
• Measured TOC values from core, sidewall core and 1.4 11.85
1.45 11.9
cuttings are used to calibrate the calculated curve 1.5 12
1.6 12.2
1.7 12.6
1.8 12.9
• Interactive Petrophysics™ inbuilt TOC module 1.9 13.2
2 13.5
software used for calculation 2.5 15.8
2.9 17.1
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Maturity Profiles
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Importance of good LOM control
Similar separation
Higher LOM Lower LOM
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3. The Weald, UK
41 wells initially
24 wells used
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The Weald: Stratigraphy
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Data preparation
• Checking log quality. Especially the caliper and density
correction curve.
• Prioritisation for wells with good geochemical data.
• Verification of digital logs against company composite
logs.
• Editing curves if necessary. Adding sections of density
or neutron calculation if sonic curve is faulty.
• Create a regional maturity profile based on
geochemical data and published reports.
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Statistics Output
• TOC statistics: Minimum, maximum and mean values
• Net/Gross (N/G): The amount of each formation that is
considered to be shale. Intervals with > 50% clay are
included as “Net”. Gross is the total formation thickness.
• Pay/Gross (P/G): The amount of each formation that could
be potentially prospective for shale gas/oil. Intervals where
TOC > 2wt% are included as “Pay”.
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Mature Intervals
Calculated TOC may be high, but is it mature?
Both curves deviate away Only the sonic curve deviates
= =
Presence of hydrocarbon fluid Presence of kerogen and no
and kerogen hydrocarbon
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Cut-off effects
Just above the 2wt% cut off
P/G 0.43
P/G 0.06
Just below the 2wt% cut off
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Other considerations
Minimum heights for calculations.
• Discriminators removal may give thin <2m sections of
calculated TOC, which approaches the resolution of
the loggings tools.
Accuracy of geochemical data. – measured TOC and VR
Coals should be removed from the calculation.
Cross plots of measured TOC vs calculated TOC can
help prove curve calibration
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Regional Variations
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Further Reading
• Shi, X., et al. (2016). Application of extreme learning machine
and neural networks in total organic carbon content prediction in
organic shale with wire line logs. Journal of Natural Gas Science
and Engineering 33: 687-702.
• Wang, P., et al. (2016). Revised models for determining TOC in
shale play: Example from Devonian Duvernay Shale, Western
Canada Sedimentary Basin. Marine and Petroleum Geology 70:
304-319.
• Zhao, P., et al. (2017). An improved model for estimating the
TOC in shale formations. Marine and Petroleum Geology 83:
174-183.
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