Changes

From a qualitative point of view, part of this issue includes the assessment of variations in quantitative TOC values that are altered by, for example, thermal maturity, sample collection technique, sample type (cuttings versus core chips), sample quality (e.g., fines only, cavings, contamination), and any high grading of core or cuttings samples. Documented variations in cuttings through the Fayetteville and Chattanooga shales illustrate variations due to sample type and quality as cuttings commonly have mixing effects. An overlying organic-lean sediment will dilute an organic-rich sample often for 10 to 40 ft (3 to 12 m). This is evident in some Fayetteville and Chattanooga wells with cuttings analysis, where the uppermost parts of the organic-rich shales have TOC values suggesting the shale to be organic lean. However, TOC values increase with deeper penetration into the organic-rich shale, to and through the base of the shale, but then also continuing into underlying organic-lean sediments, until finally decreasing to low values (Li et al., 2010a). This is a function of mixing of cuttings while drilling. The same issue in Barnett Shale wells was reported by MEDC,<ref name=St2007 /> who also reported lower vitrinite reflectance values for cuttings than core (sim0.15% Ro lower). The big problem with this mixing effect is that it does not always occur and picking of cuttings does not typically solve the problem in shale-gas resource systems, although it may work in less mature systems. One solution is to minimize the quantitation of the uppermost sections (sim9 m [sim30 ft]) of a shale of interest when cuttings are used for analysis. The inverse of this situation is often identifiable in known organic-lean sediments below an organic-rich shale or coal. This latter effect is more obvious below coaly intervals, where TOC values will be high unless picked free of coal.

From a qualitative point of view, part of this issue includes the assessment of variations in quantitative TOC values that are altered by, for example, thermal maturity, sample collection technique, sample type (cuttings versus core chips), sample quality (e.g., fines only, cavings, contamination), and any high grading of core or cuttings samples. Documented variations in cuttings through the Fayetteville and Chattanooga shales illustrate variations due to sample type and quality as cuttings commonly have mixing effects. An overlying organic-lean sediment will dilute an organic-rich sample often for 10 to 40 ft (3 to 12 m). This is evident in some Fayetteville and Chattanooga wells with cuttings analysis, where the uppermost parts of the organic-rich shales have TOC values suggesting the shale to be organic lean. However, TOC values increase with deeper penetration into the organic-rich shale, to and through the base of the shale, but then also continuing into underlying organic-lean sediments, until finally decreasing to low values (Li et al., 2010a). This is a function of mixing of cuttings while drilling. The same issue in Barnett Shale wells was reported by MEDC,<ref name=St2007 /> who also reported lower vitrinite reflectance values for cuttings than core (sim0.15% Ro lower). The big problem with this mixing effect is that it does not always occur and picking of cuttings does not typically solve the problem in shale-gas resource systems, although it may work in less mature systems. One solution is to minimize the quantitation of the uppermost sections (sim9 m [sim30 ft]) of a shale of interest when cuttings are used for analysis. The inverse of this situation is often identifiable in known organic-lean sediments below an organic-rich shale or coal. This latter effect is more obvious below coaly intervals, where TOC values will be high unless picked free of coal.

In any case, what is measured in any geochemical laboratory is strictly present-day TOC (TOCpd), which is dependent on all previously mentioned factors. In the absence of other factors, the decrease in original TOC (TOCo) is a function of thermal maturity due to the conversion of organic matter to petroleum and a carbonaceous char. The TOC measurements may include organic in oil or bitumen, which may not be completely removed during the typical decarbonation step before the LECO TOC analysis. Bitumen and oil-free TOC is described in various ways but always having two components whose distribution is dependent on the originally deposited and preserved biomass: generative organic carbon (GOC) and nongenerative organic carbon (NGOC) fractions. These have been referred to by various names without specifying bitumen and/or oil free (e.g., reactive and inert carbon; Cooles et al., 1986). As such, the GOC fraction has sufficient hydrogen to generate hydrocarbons, whereas the NGOC fraction does not yield substantial amounts of hydrocarbons. Decomposition of the GOC also creates organic porosity, which is directly proportional to the GOC fraction and its extent of conversion. The NGOC fraction accounts for adsorbed gas storage and some organic porosity development due to restructuring of the organic matrix. The creation of such organic porosity in a reducing environment creates sites for possible catalytic activity by carbonaceous char (Fuhrmann et al., 2003; Alexander et al., 2009) or other catalytic materials, for example, low valence transition metals (Mango, 1992, 1996).

In any case, what is measured in any geochemical laboratory is strictly present-day TOC (TOCpd), which is dependent on all previously mentioned factors. In the absence of other factors, the decrease in original TOC (TOCo) is a function of thermal maturity due to the conversion of organic matter to petroleum and a carbonaceous char. The TOC measurements may include organic in oil or bitumen, which may not be completely removed during the typical decarbonation step before the LECO TOC analysis. Bitumen and oil-free TOC is described in various ways but always having two components whose distribution is dependent on the originally deposited and preserved biomass: generative organic carbon (GOC) and nongenerative organic carbon (NGOC) fractions. These have been referred to by various names without specifying bitumen and/or oil free (e.g., reactive and inert carbon).<ref>Cooles, G. P., A. S. Mackenzie, and T. M. Quigley, 1986, Calculation of petroleum masses generated and expelled from source rocks: Organic Geochemistry, v. 10, p. 235–245.</ref> As such, the GOC fraction has sufficient hydrogen to generate hydrocarbons, whereas the NGOC fraction does not yield substantial amounts of hydrocarbons. Decomposition of the GOC also creates organic porosity, which is directly proportional to the GOC fraction and its extent of conversion. The NGOC fraction accounts for adsorbed gas storage and some organic porosity development due to restructuring of the organic matrix. The creation of such organic porosity in a reducing environment creates sites for possible catalytic activity by carbonaceous char<ref>Fuhrmann, A., K. F. M. Thompson, R. di Primio, and V. Dieckmann, 2003, Insight into petroleum composition based on thermal and catalytic cracking: 21st International Meeting on Organic Geochemistry (IMOG), Krakow, Poland, September 8–12, 2003, Book of Abstracts, part I, p. 321–322.</ref><ref>Alexander, R., D. Dawson, K. Pierce, and A. Murray, 2009, Carbon catalyzed hydrogen exchange in petroleum source rocks: Organic Geochemistry, v. 40, p. 951–955, doi:10.1016/j.orggeochem.2009.06.003.</ref> or other catalytic materials, for example, low valence transition metals.<ref>Mango, F. D., 1992, Transition metal catalysis in the generation of petroleum: A genetic anomaly in Ordovician oils: Geochimica et Cosmochimica Acta, v. 56, p. 3851–3854, doi:10.1016/0016-7037(92)90177-K.</ref><ref>Mango, F. D., 1996, Transition metal catalysis in the generation of natural gas: Organic Geochemistry, v. 24, no. 10–11, p. 977–984, doi:10.1016/S0146-6380(96)00092-7.</ref>

A slight increase in NGOC occurs as organic matter decomposes and uses the limited amounts of hydrogen in GOC (maximum of sim1.8 hydrogen to carbon [H-to-C] in the very best source rocks and about 2.0 H-to-C in bitumen and/or oil). Most shale-gas resource systems at a high thermal maturity have only small amounts or no GOC remaining and are dominated by the enhanced NGOC fraction. The decomposition of GOC generates all the petroleum, creates organic storage porosity, and both GOC and NGOC function in retention of generated petroleum that ultimately is cracked to gas in high-thermal-maturity shale-gas resource systems.

A slight increase in NGOC occurs as organic matter decomposes and uses the limited amounts of hydrogen in GOC (maximum of sim1.8 hydrogen to carbon [H-to-C] in the very best source rocks and about 2.0 H-to-C in bitumen and/or oil). Most shale-gas resource systems at a high thermal maturity have only small amounts or no GOC remaining and are dominated by the enhanced NGOC fraction. The decomposition of GOC generates all the petroleum, creates organic storage porosity, and both GOC and NGOC function in retention of generated petroleum that ultimately is cracked to gas in high-thermal-maturity shale-gas resource systems.

==References==

==References==

{{reflist}}

{{reflist}}

* Alexander, R., D. Dawson, K. Pierce, and A. Murray, 2009, Carbon catalyzed hydrogen exchange in petroleum source rocks: Organic Geochemistry, v. 40, p. 951–955, doi:10.1016/j.orggeochem.2009.06.003.

*  

* Bharati, S., R. L. Patience, S. R. Larter, G. Standen, and I. J. F. Poplett, 1995, Elucidation of the Alum Shale kerogen structure using a multidisciplinary approach: Organic Geochemistry, v. 23, no. 11–12, p. 1043–1058, doi:10.1016/0146-6380(95)00089-5.

* Bharati, S., R. L. Patience, S. R. Larter, G. Standen, and I. J. F. Poplett, 1995, Elucidation of the Alum Shale kerogen structure using a multidisciplinary approach: Organic Geochemistry, v. 23, no. 11–12, p. 1043–1058, doi:10.1016/0146-6380(95)00089-5.

* Buchardt, B., A. Thorshoj Nielsen, and N. Hemmingsen Schovsbo, 1997, Alun Skiferen i Skandinavien, Dansk Geologisk Forenings Nyheds: OG Informationsskirft, 32 p.

* Buchardt, B., A. Thorshoj Nielsen, and N. Hemmingsen Schovsbo, 1997, Alun Skiferen i Skandinavien, Dansk Geologisk Forenings Nyheds: OG Informationsskirft, 32 p.

* Espitalie, J., M. Madec, and B. Tissot, 1984, Geochemical logging, in K. J. Voorhees, ed., Analytical pyrolysis: Techniques and applications: Boston, Massachusetts, Butterworth, p. 276–304.

* Espitalie, J., M. Madec, and B. Tissot, 1984, Geochemical logging, in K. J. Voorhees, ed., Analytical pyrolysis: Techniques and applications: Boston, Massachusetts, Butterworth, p. 276–304.

* Faqira, M., A. Bhullar, and A. Ahmed, 2010, Silurian Qusaiba Shale Play: Distribution and characteristics (abs.): Hedberg Research Conference on Shale Resource Plays, December 5–9, 2010, Austin, Texas, Book of Abstracts, p. 133–134.

* Faqira, M., A. Bhullar, and A. Ahmed, 2010, Silurian Qusaiba Shale Play: Distribution and characteristics (abs.): Hedberg Research Conference on Shale Resource Plays, December 5–9, 2010, Austin, Texas, Book of Abstracts, p. 133–134.

* Fuhrmann, A., K. F. M. Thompson, R. di Primio, and V. Dieckmann, 2003, Insight into petroleum composition based on thermal and catalytic cracking: 21st International Meeting on Organic Geochemistry (IMOG), Krakow, Poland, September 8–12, 2003, Book of Abstracts, part I, p. 321–322.

*  

*  

*  

* Horsfield, B., S. Bharati, S. R. Larter, F. Leistner, R. Littke, H. J. Schenk, and H. Dypvik, 1992, On the atypical petroleum-generating characteristics of alginate in the Cambrian Alum Shale, in M. Schidlowski, S. Golubic, M. M. Kimerly, and P. A. Trudinger, eds., Early organic evolution: Implications for mineral and energy resources: Berlin, Springer-Verlag, p. 257–266.

* Horsfield, B., S. Bharati, S. R. Larter, F. Leistner, R. Littke, H. J. Schenk, and H. Dypvik, 1992, On the atypical petroleum-generating characteristics of alginate in the Cambrian Alum Shale, in M. Schidlowski, S. Golubic, M. M. Kimerly, and P. A. Trudinger, eds., Early organic evolution: Implications for mineral and energy resources: Berlin, Springer-Verlag, p. 257–266.

*  

*  

*  

*  

* Mango, F. D., 1992, Transition metal catalysis in the generation of petroleum: A genetic anomaly in Ordovician oils: Geochimica et Cosmochimica Acta, v. 56, p. 3851–3854, doi:10.1016/0016-7037(92)90177-K.

*  

* Montgomery, C. T., and M. B. Smith, 2010, Hydraulic fracturing: History of an enduring technology: http://www.jptonline.org/index.php?id=481 (accessed January 10, 2011).

* Montgomery, C. T., and M. B. Smith, 2010, Hydraulic fracturing: History of an enduring technology: http://www.jptonline.org/index.php?id=481 (accessed January 10, 2011).

* Montgomery, S. L., D. M. Jarvie, K. A. Bowker, and R. M. Pollastro, 2005, Mississippian Barnett Shale, Forth Worth Basin, north-central Texas: Gas-shale play with multi-tcf potential: AAPG Bulletin, v. 89, no. 2, p. 155–175.

* Montgomery, S. L., D. M. Jarvie, K. A. Bowker, and R. M. Pollastro, 2005, Mississippian Barnett Shale, Forth Worth Basin, north-central Texas: Gas-shale play with multi-tcf potential: AAPG Bulletin, v. 89, no. 2, p. 155–175.