calculating total organic carbon using the ‘passey...

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Calculating Total Organic Carbon (TOC) using the ‘Passey Method’

Chris Gent


Overview

1. ‘Passey method’

2. Importance of TOC and maturity data.

3. Case Study: The Weald, UK.


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.


Sonic and Resistivity separation

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

‘Lean Shale’


2. Calculating TOC and maturity 𝑇𝑇𝑇𝑇𝑇𝑇 = ∆𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 × 10(2.297−0.1688 × 𝐿𝐿𝐿𝐿𝐿𝐿)

• LOM = Level of Organic Maturity. A number depicting maturity of a source rock based on measured vitrinite reflectance (VR). - Hood, Gutjahr and Heacock, 1975.

• Measured TOC values from core, sidewall core and cuttings are used to calibrate the calculated curve

• Interactive Petrophysics™ inbuilt TOC module software used for calculation

VR LOM0.4 6

0.45 6.50.5 7.1

0.55 7.60.6 8.2

0.65 8.60.7 9.2

0.75 9.60.8 9.9

0.85 10.10.9 10.3

0.95 10.61 10.9

1.05 11.11.1 11.2

1.15 11.31.2 11.4

1.25 11.551.3 11.7

1.35 11.81.4 11.85

1.45 11.91.5 121.6 12.21.7 12.61.8 12.91.9 13.2

2 13.52.5 15.82.9 17.1


Maturity Profiles


Importance of good LOM control

Higher LOM Lower LOM

Similar separation


3. The Weald, UK

41 wells initially 24 wells used


The Weald: Stratigraphy


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.


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”.


Mature Intervals

Both curves deviate away =

Presence of hydrocarbon fluid and kerogen

Only the sonic curve deviates =

Presence of kerogen and no hydrocarbon

Calculated TOC may be high, but is it mature?


Cut-off effects

Just above the 2wt% cut off

Just below the 2wt% cut off

P/G 0.06

P/G 0.43


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


Regional Variations


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.

calculating total organic carbon using the ‘passey...

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