Changes

One of the first and basic screening analyses for any source rock is organic richness, as measured by total organic carbon (TOC). The TOC is a measure of organic carbon present in a sediment sample, but it is not a measure of its generation potential alone, as that requires an assessment of hydrogen content or organic maceral percentages from chemical or visual kerogen assessments. As TOC values vary throughout a source rock because of organofacies differences and thermal maturity, and even depending on sample type, there has been a lengthy debate on what actual TOC values are needed to have a commercial source rock. All organic matter preserved in sediments will decompose into petroleum with sufficient temperature exposure; for EampP companies, it is a matter of the producibility and commerciality of such generation. In addition, the expulsion and retention of generated petroleum must be considered. However, original quantity (TOC) as well as source rock quality (type) of the source rock must be considered in combination to assess its petroleum generation potential.

One of the first and basic screening analyses for any source rock is organic richness, as measured by total organic carbon (TOC). The TOC is a measure of organic carbon present in a sediment sample, but it is not a measure of its generation potential alone, as that requires an assessment of hydrogen content or organic maceral percentages from chemical or visual kerogen assessments. As TOC values vary throughout a source rock because of organofacies differences and thermal maturity, and even depending on sample type, there has been a lengthy debate on what actual TOC values are needed to have a commercial source rock. All organic matter preserved in sediments will decompose into petroleum with sufficient temperature exposure; for EampP companies, it is a matter of the producibility and commerciality of such generation. In addition, the expulsion and retention of generated petroleum must be considered. However, original quantity (TOC) as well as source rock quality (type) of the source rock must be considered in combination to assess its petroleum generation potential.

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.<ref name=Li2010a>Li, P., M. E. Ratchford, and D. M. Jarvie, 2010a, Geochemistry and thermal maturity analysis of the Fayetteville Shale and Chattanooga Shale in the western Arkoma Basin of Arkansas: Arkansas Geological Survey, Information Circular 40, DFF-OG-FS-EAB/ME 012, 58 p.</ref> 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).<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>

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>

In South Africa, the Karoo Basin is being evaluated for its shale-gas potential by various companies<ref>Oil & Gas Journal, 2010b, [http://www.ogj.com/index/article-tools-template.articles.oil-gas-journal.exploration-development-2.2010.07.south-africa_karoo.html South Africa Karoo shale gas hunt growing].</ref> with various conventional and unconventional opportunities.<ref>Raseroka, L., and I. R. McLachlan, 2009, [http://www.searchanddiscovery.net/documents/2009/10196raseroka/index.htm?q=%2Btext%3Araseroka The petroleum potential of South Africa's onshore Karoo basins (abs.)]: AAPG International Conference and Exhibition, Cape Town, South Africa, October 26–29, 2008, Search and Discovery Article 10196, 3 p.</ref>

In South Africa, the Karoo Basin is being evaluated for its shale-gas potential by various companies<ref>Oil & Gas Journal, 2010b, [http://www.ogj.com/index/article-tools-template.articles.oil-gas-journal.exploration-development-2.2010.07.south-africa_karoo.html South Africa Karoo shale gas hunt growing].</ref> with various conventional and unconventional opportunities.<ref>Raseroka, L., and I. R. McLachlan, 2009, [http://www.searchanddiscovery.net/documents/2009/10196raseroka/index.htm?q=%2Btext%3Araseroka The petroleum potential of South Africa's onshore Karoo basins (abs.)]: AAPG International Conference and Exhibition, Cape Town, South Africa, October 26–29, 2008, Search and Discovery Article 10196, 3 p.</ref>

Saudi Aramco is also evaluating the potential for shale gas in the Kingdom of Saudi Arabia. Organic-rich and highly mature Silurian shales are being evaluated (Faqira et al., 2010). This further illustrates the value of shale-gas resource systems as a more economical energy source than higher priced oil. The goal in Saudi Arabia would seem to be using shale-gas resources internally while exporting oil.

Saudi Aramco is also evaluating the potential for shale gas in the Kingdom of Saudi Arabia. Organic-rich and highly mature Silurian shales are being evaluated.<ref>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.</ref> This further illustrates the value of shale-gas resource systems as a more economical energy source than higher priced oil. The goal in Saudi Arabia would seem to be using shale-gas resources internally while exporting oil.

Several companies including BP, Shell, ConocoPhillips, Newfield, and EOG Resources have signed deals to explore for shale resource systems in China, covering five different basins. In the Sichuan Basin, Late Ordovician and Early Silurian marine shales are being evaluated for their shale resource potential (Li et al., 2010b). The Upper Ordovician Wufeng and Lower Silurian Longmaxi shales average about 3.0% TOC, with thermal maturities from about 2.3 to 3.4% Ro being targeted (Li et al., 2010b). In the Junggar Basin, the Jurassic Badaowan Shale has TOC values from 2 to 8%, with thermal maturities from 0.60 to 2.0% Roe and is 50 to 80 m (164–262 ft) thick (Liu et al., 2009). An excellent source potential also exists in the Upper Permian Lacaogou Formation (M. Burnaman, 2010, personal communication). The Triassic shales of the Ordos Basin and Jurassic shales of the Sichuan Basin are speculated to be shale-oil resource plays (Zou et al., 2010).

Several companies including BP, Shell, ConocoPhillips, Newfield, and EOG Resources have signed deals to explore for shale resource systems in China, covering five different basins. In the Sichuan Basin, Late Ordovician and Early Silurian marine shales are being evaluated for their shale resource potential.<ref name=Li2010b /> The Upper Ordovician Wufeng and Lower Silurian Longmaxi shales average about 3.0% TOC, with thermal maturities from about 2.3 to 3.4% Ro being targeted.<ref name=Li2010b>Li, X., et al., 2010b, Upper Ordovician–Lower Silurian shale gas reservoirs in southern Sichuan Basin, China (abs.): Hedberg Research Conference on Shale Resource Plays, Austin, Texas, December 5–9, 2010, Book of Abstracts, p. 177–179.</ref> In the Junggar Basin, the Jurassic Badaowan Shale has TOC values from 2 to 8%, with thermal maturities from 0.60 to 2.0% Roe and is 50 to 80 m (164–262 ft) thick.<ref>Liu, H., H. Wang, R. Liu, Zhaoqun, and Y. Lin, 2009, Shale gas in China: New important role of energy in the 21st century: 2009 International Coalbed and Shale Gas Symposium, Tuscaloosa, Alabama, Paper 0922, 7 p.</ref> An excellent source potential also exists in the Upper Permian Lacaogou Formation (M. Burnaman, 2010, personal communication). The Triassic shales of the Ordos Basin and Jurassic shales of the Sichuan Basin are speculated to be shale-oil resource plays.<ref>Zou, C.-n., S.-z. Tao, X.-h. Gao, Y. Li, Z. Yang, Y.-j. Gong, D.-z. Dong, X.-j. Le, et al., 2010, [http://www.searchanddiscovery.net/abstracts/pdf/2010/annual/abstracts/ndx_caineng.pdf Basic contents, geological features and evaluation methods of continuous oil/gas plays in China]: AAPG Search and Discovery Article 90104, 7 p.</ref>

The Oil and Natural Gas Corp. of India (ONGC) is drilling its first shale resource system in the Damodar Basin, with an objective of a Permian shale with a thickness of about 700 m (sim2300 ft) (Natural Gas for Europe, 2010).

The Oil and Natural Gas Corp. of India (ONGC) is drilling its first shale resource system in the Damodar Basin, with an objective of a Permian shale with a thickness of about 700 m (sim2300 ft) (Natural Gas for Europe, 2010).

* 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.

* 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.

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* 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.

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* Jones, R. W., 1984, Comparison of carbonate and shale source rocks, in J. Palacas, ed., Petroleum geochemistry and source rock potential of carbonate rocks: AAPG Studies in Geology 18, p. 163–180.

* Jones, R. W., 1984, Comparison of carbonate and shale source rocks, in J. Palacas, ed., Petroleum geochemistry and source rock potential of carbonate rocks: AAPG Studies in Geology 18, p. 163–180.

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* Li, P., M. E. Ratchford, and D. M. Jarvie, 2010a, Geochemistry and thermal maturity analysis of the Fayetteville Shale and Chattanooga Shale in the western Arkoma Basin of Arkansas: Arkansas Geological Survey, Information Circular 40, DFF-OG-FS-EAB/ME 012, 58 p.

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* Li, X., et al., 2010b, Upper Ordovician–Lower Silurian shale gas reservoirs in southern Sichuan Basin, China (abs.): Hedberg Research Conference on Shale Resource Plays, Austin, Texas, December 5–9, 2010, Book of Abstracts, p. 177–179.

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* Liu, H., H. Wang, R. Liu, Zhaoqun, and Y. Lin, 2009, Shale gas in China: New important role of energy in the 21st century: 2009 International Coalbed and Shale Gas Symposium, Tuscaloosa, Alabama, Paper 0922, 7 p.

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* 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).

* Railroad Commission of Texas, 2011, [http://www.rrc.state.tx.us/meetings/ogpfd/RangePFD.PDF&sa=U&ei=WIAQT9DPA6Xu0gGBxlC_Aw&ved=0CAcQFjAB&client=internal-uds-cse&usg=AFQjCNHn-_WOT9BMMdwJJalA1JclwbfUfw Texas Railroad Commission hearing, 2011, Docket No. 7B-0268629] - Commission called hearing to consider whether operation of the Range Production Company Butler unit well no. 1H (RRC No. 253732) and the Teal unit well no. 1H (RRC No. 253779), Newark, East (Barnett Shale) Field, Hood County, Texas, are causing or contributing to contamination of certain domestic water wells in Parker County, Texas, v. 1, 123 p.

* Railroad Commission of Texas, 2011, [http://www.rrc.state.tx.us/meetings/ogpfd/RangePFD.PDF&sa=U&ei=WIAQT9DPA6Xu0gGBxlC_Aw&ved=0CAcQFjAB&client=internal-uds-cse&usg=AFQjCNHn-_WOT9BMMdwJJalA1JclwbfUfw Texas Railroad Commission hearing, 2011, Docket No. 7B-0268629] - Commission called hearing to consider whether operation of the Range Production Company Butler unit well no. 1H (RRC No. 253732) and the Teal unit well no. 1H (RRC No. 253779), Newark, East (Barnett Shale) Field, Hood County, Texas, are causing or contributing to contamination of certain domestic water wells in Parker County, Texas, v. 1, 123 p.

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* Zou, C.-n., S.-z. Tao, X.-h. Gao, Y. Li, Z. Yang, Y.-j. Gong, D.-z. Dong, X.-j. Le, et al., 2010, [http://www.searchanddiscovery.net/abstracts/pdf/2010/annual/abstracts/ndx_caineng.pdf Basic contents, geological features and evaluation methods of continuous oil/gas plays in China]: AAPG Search and Discovery Article 90104, 7 p.

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