Calculating Total Organic Carbon (TOC) using the Passey Method

Transcription

1 Calculating Total Organic Carbon (TOC) using the Passey Method Chris Gent

2 Overview 1. Passey method 2. Importance of TOC and maturity data. 3. Case Study: The Weald, UK.

3 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.

4 Lean Shale 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

5 2. Calculating TOC and maturity TTTTTT = llllllll 10 ( LLLLLL) LOM = Level of Organic Maturity. A number depicting maturity of a source rock based on measured vitrinite reflectance (VR). - Hood, Gutjahr and Heacock, 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 LOM

6 Maturity Profiles

7 Importance of good LOM control Similar separation Higher LOM Lower LOM

8 3. The Weald, UK 41 wells initially 24 wells used

9 The Weald: Stratigraphy

10 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.

11

12 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.

13 Mature Intervals Calculated TOC may be high, but is it mature? Both curves deviate away = Presence of hydrocarbon fluid and kerogen Only the sonic curve deviates = Presence of kerogen and no hydrocarbon

14 Cut-off effects Just above the 2wt% cut off P/G 0.43 P/G 0.06 Just below the 2wt% cut off

15 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

16 Regional Variations

17 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: 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: Zhao, P., et al. (2017). An improved model for estimating the TOC in shale formations. Marine and Petroleum Geology 83: