Improving the Total Organic Carbon Estimation of the Eagle Ford Shale with Density Logs by Considering the Effect of Pyrite
Abstract
:1. Introduction
2. Experimental Methods
3. Pyrolysis Results of the Eagle Ford Shale Samples
4. Relationship between TOC and Pyrite in the Eagle Ford
5. Petrophysical Model Considering Pyrite and Organic Porosity
6. Discussion
7. Conclusions
- Based on the Rock-Eval experimental results, the Eagle Ford samples in this study were in the post-mature zone. The samples were very good to excellent source rocks with fair to good potential for oil and gas generation.
- There were cyclic changes in Fe and S concentrations, as well as in the pyrite content, corresponding to the trend of gamma-ray log and reflecting changes in degrees of anoxia. A positive linear relationship between pyrite and TOC in the Eagle Ford Shale was identified.
- In the updated model for estimating TOC, pyrite content and organic porosity were taken into consideration. The shale rock was divided into five constituent parts, including organic pores, solid organic matter, pyrite, inorganic pores, and rock matrix without pyrite.
- Comparison between the TOC results calculated from the two models showed that the updated model had a better estimation performance than Schmoker’s model, as reflected by reduced RMSE.
Acknowledgments
Author Contributions
Conflicts of Interest
Nomenclature
R | ratio of weight of organic matter to weight of organic carbon, dimensionless |
Vk | volume fraction of organic matter in rock sample, dimensionless |
Vnk | volume fraction of inorganic parts without pyrite in rock sample, dimensionless |
Vpy | volume fraction of pyrite in rock sample, dimensionless |
Wpy | weight percent of pyrite in rock sample, dimensionless |
Greek Terms
φk | volume fraction of organic pores in organic matter, dimensionless |
φnk | volume fraction of inorganic pores in inorganic rock without pyrite, dimensionless |
ρb | bulk density, g/cm3 |
ρpy | density of pyrite, g/cm3 |
ρnk | density of inorganic rock matrix without pyrite, g/cm3 |
ρk | density of solid organic matter, g/cm3 |
ρhc | density of hydrocarbon, g/cm3 |
ρw | density of water, g/cm3 |
Appendix A
Formation | Depth (m) | S1 (mg/g) | S2 (mg/g) | Tmax | HI | OI | TOC (%) | Mineral Carbon |
---|---|---|---|---|---|---|---|---|
Upper Eagle Ford Shale | 13,612 | 1.96 | 1.3 | 454 | 48 | 8 | 2.71 | 7.26 |
13,613.33 | 2.31 | 1.51 | 463 | 56 | 7 | 2.71 | 9.1 | |
13,614.42 | 2.75 | 0.99 | 467 | 40 | 10 | 2.45 | 9.05 | |
13,615.67 | 2.09 | 2.19 | 472 | 57 | 6 | 3.81 | 7.21 | |
13,616.75 | 3.39 | 2.56 | 476 | 65 | 6 | 3.95 | 7.29 | |
Lower Eagle Ford Shale | 13,792.08 | 6.11 | 3.1 | 487 | 54 | 3 | 5.69 | 7.93 |
13,792.79 | 6.12 | 2.72 | 474 | 43 | 4 | 6.27 | 5.81 | |
13,793.63 | 8.36 | 3.32 | 477 | 48 | 5 | 6.97 | 5.75 | |
13,794.33 | 6.69 | 3.29 | 482 | 50 | 4 | 6.55 | 7.21 | |
13,795 | 6.41 | 3.62 | 484 | 50 | 3 | 7.18 | 6.13 | |
13,795.46 | 5.79 | 2.88 | 477 | 41 | 4 | 7.03 | 5.73 | |
13,796 | 6.25 | 3.67 | 484 | 51 | 3 | 7.23 | 6.23 | |
13,796.5 | 7.27 | 3.61 | 477 | 48 | 5 | 7.5 | 6.31 | |
13,797.38 | 6.85 | 3.04 | 474 | 42 | 4 | 7.23 | 5.8 | |
13,798.58 | 6.79 | 3.52 | 479 | 46 | 4 | 7.6 | 6.32 | |
13,799.33 | 5.09 | 2.43 | 471 | 62 | 4 | 3.9 | 2.81 | |
13,800.17 | 6.53 | 1.75 | 473 | 46 | 6 | 3.79 | 8.96 | |
13,810.17 | 5.86 | 3.06 | 477 | 39 | 2 | 7.83 | 5.77 | |
13,812.58 | 7.04 | 3.5 | 478 | 43 | 3 | 8.14 | 6.58 | |
13,813 | 5.62 | 2.58 | 481 | 60 | 6 | 4.27 | 9.75 | |
13,813.67 | 7.22 | 2.52 | 477 | 51 | 3 | 4.91 | 8.9 | |
13,813.92 | 8.55 | 2.25 | 478 | 50 | 6 | 4.52 | 9.7 | |
13,815.5 | 6.66 | 2.19 | 475 | 45 | 5 | 4.86 | 9.01 | |
13,816.42 | 7.59 | 3.18 | 480 | 55 | 5 | 5.83 | 7.6 | |
13,817.33 | 7.52 | 3.22 | 480 | 47 | 4 | 6.9 | 6.94 | |
13,817.83 | 6.51 | 3.47 | 481 | 46 | 4 | 7.55 | 6.01 | |
13,818.67 | 6.27 | 3.13 | 475 | 42 | 4 | 7.48 | 5.2 | |
13,819 | 7.42 | 3.33 | 478 | 47 | 4 | 7.06 | 5.66 | |
13,819.75 | 6.63 | 3 | 448 | 56 | 5 | 5.32 | 7.59 | |
13,820.25 | 5.39 | 1.9 | 469 | 43 | 4 | 4.43 | 8.28 | |
13,821.25 | 3.98 | 1.89 | 464 | 46 | 7 | 4.07 | 8.2 | |
Buda Limestone Formation | 13,907.25 | 1.01 | 0.92 | 457 | 34 | 4 | 2.74 | 2.23 |
13,908.25 | 3.5 | 1.56 | 479 | 33 | 4 | 4.75 | 5.25 | |
13,910 | 0.43 | 0.26 | 435 | 57 | 63 | 0.46 | 10.35 | |
13,915.83 | 0.24 | 0.17 | 426 | 44 | 56 | 0.39 | 11.16 | |
13,917 | 0.25 | 0.16 | 422 | 55 | 121 | 0.29 | 10.68 | |
13,918 | 0.78 | 0.74 | 446 | 103 | 60 | 0.72 | 9.84 |
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Categories | Method | Explanations | References |
---|---|---|---|
Single-well log methods | (1) Natural Gamma-Ray Log | This is the earliest way to identify source rocks from well logs. Quantification of TOC using only the gamma-ray log leads to high levels of uncertainty. | [2,3] |
(2) Spectral Gamma-Ray Log | This reflects the amounts of uranium and potassium in the rock. The relationship between spectral gamma-ray and TOC can be inconsistent. | [4,5,6] | |
(3) Density Log | This method involves the development of petrophysical models of shale formations and associated equations relating TOC and bulk density. | [7,8,9,10,11] | |
Multi-well logs methods | (4) Clay Indicator | This method overlays the scaled clay indicator curve (difference of neutron and density porosities) on the gamma-ray log. | [12] |
(5) ΔlogR and Revised ΔlogR Method | This method is widely used in shale formation evaluation. It combines the porosity log with resistivity log data and takes maturation into consideration. | [13,14,15] | |
(6) Multivariate Fitting | In this method, linear relationships between TOC and various petrophysical log data are identified. Although generally accurate for the formation of interest, the results are not transferable to other shale formations. | [16,17] | |
(7) Artificial Intelligence Technique | This method examines nonlinear relationships between TOC and well log data. This technique requires a large database and heavy computational work. | [18] |
Formation | Schmoker’s Model | Updated Model |
---|---|---|
Upper Eagle Ford | 1.620 | 0.762 |
Lower Eagle Ford | 3.015 | 1.098 |
Eagle Ford | 2.572 | 0.983 |
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Jiang, S.; Mokhtari, M.; Borrok, D.; Lee, J. Improving the Total Organic Carbon Estimation of the Eagle Ford Shale with Density Logs by Considering the Effect of Pyrite. Minerals 2018, 8, 154. https://doi.org/10.3390/min8040154
Jiang S, Mokhtari M, Borrok D, Lee J. Improving the Total Organic Carbon Estimation of the Eagle Ford Shale with Density Logs by Considering the Effect of Pyrite. Minerals. 2018; 8(4):154. https://doi.org/10.3390/min8040154
Chicago/Turabian StyleJiang, Shuxian, Mehdi Mokhtari, David Borrok, and Jim Lee. 2018. "Improving the Total Organic Carbon Estimation of the Eagle Ford Shale with Density Logs by Considering the Effect of Pyrite" Minerals 8, no. 4: 154. https://doi.org/10.3390/min8040154