1. Modelling TOC and Anoxia From Elemental Data in the Wolfcamp Fm: A Reality Check Milly Wright, Eliza Mathia, Ken Ratcliffe Chemostrat Inc.

2. Why “Model” TOC ? TOC in “shale” plays is critical Why some proxies don’t work? What does this tell us about the depositional setting of the Wolfcamp ? The Wolfcamp is different ! Gathering elemental data has become commonplace with the trace elements U or Mo widely used as a proxy for TOC

3. Presentation Outline Lithology Controls on TOC Burial How do we define lithology from elemental data Do the facies provide a means to understand TOC burial Carbonates dilute TOC “Redox” Control on TOC Burial Once Carbonate Dilution is discarded, Redox is the primary control Only Nickel (Ni) shows a consistent positive linear association to TOC………what does that tell us about depositional environments in the Wolfcamp ? Modelling TOC from trace elements Nickel (Ni) is the only trace element that can be used as a proxy for TOC At least partial restriction Fluctuating chemocline associated with carbonate deposition Periodic disoxia-anoxia during shale deposition

4. Lithology (Facies ?) and TOC 40 ft

5. Lithology (Facies ?) and TOC

6. Mineralogy variation reveals 3 distinct trends Qtz+Fd : Clays variation Carbonate trend along the Si:Al illite line Carbonate trend with the Si: Al ratio > illite How lithology controls TOC?

7. Facies assignment helps to predict TOC values

8. Authigenic enrichment How redox controls TOC?

9. TOC: Redox or Dilution?

11. Summary so far… Lithology control Redox control

12. What elements are suitable as TOC proxies?

13. Enrichment Factors (EF) & Interpretation Algeo et al., 2009 EF Element = Element Measured Aluminum Measured Element Standard Aluminum Standard ( ( ) ) The standard often used is Post Archian Australian Shale (PAAS). Care must be used, if Al is very low the calculation can be misleading.

14. Enrichment Factors (EF) & Interpretation Algeo et al., 2009 H 2 S/HS -  MoS 4 The Particulate Shuttle (PS) requires free hydrogen sulfide. This is easier to accomplish in clay poor environments, because there is less Fe to scavenge sulfur compounds. EFMo EFU

15. Enrichment Factors (EF) & Interpretation Eagle Ford Shale Algeo et al., 2009 EFMo EFU EFMo EFU Lwr Eagle Ford samples show strong evidence of persistent Euxinic conditions and with particulate shuttle controlling high Mo values in these sequences.

16. Redox conditions & water chemistry

17. 3 redox states identified: At least intermittent H 2 S associated with the carbonate deposition and Fe-Mn cycling Suboxic with little OM burial and EFU > EFMo Anoxic with enhanced OM burial and higher Mo accumulation

18. Residence time in seawater of redox elements 6kyr 5kyr 8kyr 50 kyr 400 kyr800 kyr

19. Optimum proxy for the TOC abundance Ni is the best proxy for TOC abundance in the investigated Wolfcamp strata: It met the following conditions: Similarly to TOC, it is affected by the carbonate matrix Ni burial controlled by redox with no effect of Fe-Mn cycling Short residence time in seawater

20. Summary Carbonate content (dilution) is a first-order proxy for the TOC abundance in lithologies with carbonate content > 50% In the carbonate-poor shale (< 10%), TOC varies primarily as a function of palaeo-redox conditions The concentrations of redox-sensitive elements were affected by the water mass restriction, with elements of high residence time in seawater having the lowest abundances High enrichment in Mo, Cu, and Fe in the carbonate lithology suggests operation of the Fe-Mn cycling and at least intermittent H 2 S despite very low TOC concentrations (high dilution) Understanding mechanisms operating during deposition of shales (water chemistry, basin restriction, redox) is essential for establishing correct relationships between TOC, elemental data, mineralogy and rock facies. Why do we care? & how can we use this information?

21. Thank You for your attention