Insights on TOC Modeling and Anoxia in Wolfcamp Formation
Explore the modeling of Total Organic Carbon (TOC) and anoxia in the Wolfcamp Formation based on elemental data analysis. Discover the significance of proxies like Nickel (Ni) and the impact of lithology and redox controls on TOC burial in shale plays. Gain valuable insights into the depositional settings and mineralogy variations affecting TOC distribution.
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Presentation Transcript
Modelling TOC and Anoxia From Elemental Data in the Wolfcamp Fm: A Reality Check Milly Wright, Eliza Mathia, Ken Ratcliffe Chemostrat Inc.
Why Model TOC ? TOC in shale plays is critical Gathering elemental data has become commonplace with the trace elements U or Mo widely used as a proxy for TOC The Wolfcamp is different ! 200 14 5 180 4.5 12 160 4 10 140 3.5 Mo (ppm) Ni (ppm) U (ppm) 120 3 8 100 2.5 6 80 2 60 1.5 4 40 1 2 20 0.5 0 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 TOC (wt.%) TOC (wt.%) TOC (wt.%) Why some proxies don t work? What does this tell us about the depositional setting of the Wolfcamp ?
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 ? At least partial restriction Fluctuating chemocline associated with carbonate deposition Periodic disoxia-anoxia during shale deposition Modelling TOC from trace elements Nickel (Ni) is the only trace element that can be used as a proxy for TOC
Lithology (Facies ?) and TOC Mn Ca 15 Al Si 3 TOC 16 Si/Al EF Mo Fe/Al 0.01 0 0.02 10 0.03 0.04 0.05 0.06 60 20 30 40 50 0 0 5 10 10 20 15 30 20 40 0 1 2 4 5 6 7 8 4 10 22 28 34 0 1 2 3 4 5 6 0 5 10 15 20 25 30 35 5 25 35 -9220 -9220 -9220 -9220 -9220 -9220 -9220 -9220 40 ft -9260 -9260 -9260 -9260 -9260 -9260 -9260 -9260 -9300 -9300 -9300 -9300 -9300 -9300 -9300 -9300 -9340 -9340 -9340 -9340 -9340 -9340 -9340 -9340 -9380 -9380 -9380 -9380 -9380 -9380 -9380 -9380 -9420 -9420 -9420 -9420 -9420 -9420 -9420 -9420 -9460 -9460 -9460 -9460 -9460 -9460 -9460 -9460 -9500 -9500 -9500 -9500 -9500 -9500 -9500 -9500 -9540 -9540 -9540 -9540 -9540 -9540 -9540 -9540 -9580 -9580 -9580 -9580 -9580 -9580 -9580 -9580 -9620 -9620 -9620 -9620 -9620 -9620 -9620 -9620
Lithology (Facies ?) and TOC TOC < 2% TOC > 2%
Mineralogy variation reveals 3 distinct trends Qtz+Fd Qtz+Fd : Clays variation Carbonate trend along the Si:Al illite line Carbonate trend with the Si: Al ratio > illite Clays Carbonates How lithology controls TOC?
Facies assignment helps to predict TOC values 12 Qtz+Fd 10 No. of samples 20 8 No. of samples 6 15 4 10 2 0 5 0 TOC (%) 20 TOC (%) No. of samples 15 10 5 0 TOC (%) Clays Carbonates
Authigenic enrichment Mo auth U auth 6 Clays Carbonates 3 TOC Qtz+Fd 0 3 9 12 15 0 20 40 60 80 100 0 10 20 30 40 50 0 1 2 4 5 6 7 8 0 20 40 60 80 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -9220 -9220 -9220 -9220 -9220 -9220 -9260 -9260 -9260 -9260 -9260 -9260 -9300 -9300 -9300 -9300 -9300 -9300 -9340 -9340 -9340 -9340 -9340 -9340 -9380 -9380 -9380 -9380 -9380 -9380 -9420 -9420 -9420 -9420 -9420 -9420 -9460 -9460 -9460 -9460 -9460 -9460 -9500 -9500 -9500 -9500 -9500 -9500 -9540 -9540 -9540 -9540 -9540 -9540 -9580 -9580 -9580 -9580 -9580 -9580 -9620 -9620 -9620 -9620 -9620 -9620 How redox controls TOC?
TOC: Redox or Dilution? Facies Carbonate mud. / Limestone Siliceous mudstone Mixed mudstone Argillaceous
TOC: Redox or Dilution? Facies Carbonate mud. / Limestone Siliceous mudstone Mixed mudstone Argillaceous TOC < 2% TOC > 2%
Summary so far 60 80 70 50 Al2O3 and SiO2 (%) 60 Lithology control 40 CaO (%) 50 40 30 30 20 20 10 Redox control 10 0 0 1 5 7 3 0 2 4 6 0 1 2 3 4 5 6 7 TOC (wt.%) TOC (wt.%)
What elements are suitable as TOC proxies? 400 200 200 a = 38.5 180 180 350 a = 27.9 Pearson s r = 0.85 Pearson s r = 0.97 160 160 300 140 140 Cu (ppm) Cr (ppm) Ni (ppm) 250 120 120 a = 8.8 Pearson s r = 0.76 200 100 100 TM 80 80 150 W TM 60 60 TM W 100 W 40 40 a = 68.5 50 20 20 a = 9.0 Pearson s r = 0.83 Pearson s r = 0.51 0 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 TOC (wt.%) TOC (wt.%) TOC (wt.%) 10 20 400 9 18 350 8 16 300 7 14 Mo (ppm) a = 24.6 Pearson s r = 0.69 U (ppm) V (ppm) 250 6 12 5 10 200 a = 0.3 a = 0.4 Pearson s r = 0.24 4 8 Pearson s r = 0.58 150 TM a = 1.6 3 a = -0.02 6 W Pearson s r = 0.34 100 Pearson s r = -0.04 2 4 W W 50 2 1 TM a = 58.1 TM Pearson s r = 0.82 0 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 TOC (wt.%) TOC (wt.%) TOC (wt.%)
Enrichment Factors (EF) & Interpretation 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. Algeo et al., 2009
Enrichment Factors (EF) & Interpretation H2S/HS- MoS4 The Particulate Shuttle (PS) requires free hydrogen sulfide. EFMo This is easier to accomplish in clay poor environments, because there is less Fe to scavenge sulfur compounds. EFU Algeo et al., 2009
Enrichment Factors (EF) & Interpretation Eagle Ford Shale EFMo EFMo EFU EFU Lwr Eagle Ford samples show strong evidence of persistent Euxinic conditions and with particulate shuttle controlling high Mo values in these sequences. Algeo et al., 2009
Redox conditions & water chemistry 0.3 x SW 3 x SW 1 x SW 1000 0.1 x SW 100 EFMo 10 1 0.1 0.1 1 10 100 1000 EFU
Redox conditions & water chemistry 0.3 x SW 3 x SW 1 x SW 1000 0.1 x SW 3 redox states identified: At least intermittent H2S associated with the carbonate deposition and Fe-Mn cycling Low TOC Fe-Mn cycling 100 EFMo 10 HighTOC Anoxic Suboxic with little OM burial and EFU > EFMo 1 Anoxic with enhanced OM burial and higher Mo accumulation TOC 2% Suboxic-anoxic 0.1 0.1 1 10 100 1000 EFU
Residence time in seawater of redox elements 400 200 200 a = 38.5 180 180 350 a = 27.9 Pearson s r = 0.85 Pearson s r = 0.97 160 160 300 140 140 Cu (ppm) Cr (ppm) Ni (ppm) 250 120 120 a = 8.8 Pearson s r = 0.76 200 100 100 TM 80 80 150 W TM 60 60 TM W 100 W 40 40 a = 68.5 50 6kyr 8kyr 5kyr 20 20 a = 9.0 Pearson s r = 0.83 Pearson s r = 0.51 0 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 TOC (wt.%) TOC (wt.%) TOC (wt.%) 10 20 400 9 18 350 8 16 300 7 14 Mo (ppm) a = 24.6 Pearson s r = 0.69 U (ppm) V (ppm) 250 6 12 5 10 200 a = 0.3 a = 0.4 Pearson s r = 0.24 4 8 Pearson s r = 0.58 150 TM a = 1.6 3 a = -0.02 6 W Pearson s r = 0.34 100 Pearson s r = -0.04 2 4 W W 50 400 kyr 800 kyr 50 kyr 2 1 TM a = 58.1 TM Pearson s r = 0.82 0 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 TOC (wt.%) TOC (wt.%) TOC (wt.%)
Optimum proxy for the TOC abundance Ni is the best proxy for TOC abundance in the investigated Wolfcamp strata: 7 y = 1.0403x - 0.1068 R = 0.9361 p < 0.001 6 It met the following conditions: Predicted TOC (wt.%) 5 Similarly to TOC, it is affected by the carbonate matrix 4 3 Ni burial controlled by redox with no effect of Fe-Mn cycling 2 1 Short residence time in seawater 0 0 1 2 Measured TOC (wt.%) 3 4 5 6 7
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 H2S 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?
