Changes

Multiple ways to derive an original TOC (TOCo) value exist, two of which are (1) from a database or analysis of immature samples, thereby allowing the percentage of kerogen conversion to be estimated; and (2) by computation from visual kerogen assessments and related HI assumptions.<ref name=Jrv2007 /> However, it is difficult to assign an original HI (HI_o) to any source rock system in the absence of a collection of immature source rocks from various locations or even by measuring maceral percentages. For example, to assume all lacustrine shales such as the Green River Oil Shale have an HI_o of 700 or higher, or that all are equivalent to the Mahogany zone (950 mg HC/g TOC), is inconsistent with measured values that range from about 50 to 950 mg/g, with an average of only 534 mg HC/g TOC.<ref name=Jrvie2006 /> Thus, our previous selection of 700 mg HC/g TOC for type I kerogen is likely overstated,<ref name=Jrv2007 /> and a comparable issue exists for organic matter categorized as a type II marine shale.

Multiple ways to derive an original TOC (TOCo) value exist, two of which are (1) from a database or analysis of immature samples, thereby allowing the percentage of kerogen conversion to be estimated; and (2) by computation from visual kerogen assessments and related HI assumptions.<ref name=Jrv2007 /> However, it is difficult to assign an original HI (HI_o) to any source rock system in the absence of a collection of immature source rocks from various locations or even by measuring maceral percentages. For example, to assume all lacustrine shales such as the Green River Oil Shale have an HI_o of 700 or higher, or that all are equivalent to the Mahogany zone (950 mg HC/g TOC), is inconsistent with measured values that range from about 50 to 950 mg/g, with an average of only 534 mg HC/g TOC.<ref name=Jrvie2006 /> Thus, our previous selection of 700 mg HC/g TOC for type I kerogen is likely overstated,<ref name=Jrv2007 /> and a comparable issue exists for organic matter categorized as a type II marine shale.

As most shale-gas resource plays to date have been marine shales, comparison of HI_o values for a worldwide collection of marine source rocks provides a means to assess the range of expected values. Using a database of immature marine source rocks, the predominant distribution of HI_o values is between 300 and 700 mg HC/g TOC, although the population of samples yield a range from about 250 to 800 mg HC/g TOC ([[:File:M97FG3.jpg|Figure 3]]). This is similar to, but broader than, the range of values suggested by Peters and Caasa (1994) for type II kerogens of 300 to 600 mg HC/g TOC and slightly broader than the range of values suggested by Jones<ref>Jones, R. W., 1984, [http://archives.datapages.com/data/specpubs/geochem1/data/a030/a030/0001/0150/0163.htm 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.</ref> of 300 to 700 mg HC/g TOC. The important point is that these are primarily marine shales with oil-prone kerogen with variable hydrogen contents. Lacustrine source rocks are not ruled out as potential shale-gas resource systems, but they likely require a much higher thermal maturity to crack their dominantly paraffin composition to gas; as of this date, no such systems have been commercially produced.

As most shale-gas resource plays to date have been marine shales, comparison of HI_o values for a worldwide collection of marine source rocks provides a means to assess the range of expected values. Using a database of immature marine source rocks, the predominant distribution of HI_o values is between 300 and 700 mg HC/g TOC, although the population of samples yield a range from about 250 to 800 mg HC/g TOC ([[:File:M97FG3.jpg|Figure 3]]). This is similar to, but broader than, the range of values suggested by Peters and Caasa<ref>Peters, K. E., and M. R. Caasa, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0050/0093.htm Applied source rock geochemistry], in L. B. Magoon and W. G. Dow, eds., The petroleum system: From source to trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 93–117.</ref> for type II kerogens of 300 to 600 mg HC/g TOC and slightly broader than the range of values suggested by Jones<ref>Jones, R. W., 1984, [http://archives.datapages.com/data/specpubs/geochem1/data/a030/a030/0001/0150/0163.htm 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.</ref> of 300 to 700 mg HC/g TOC. The important point is that these are primarily marine shales with oil-prone kerogen with variable hydrogen contents. Lacustrine source rocks are not ruled out as potential shale-gas resource systems, but they likely require a much higher thermal maturity to crack their dominantly paraffin composition to gas; as of this date, no such systems have been commercially produced.

Using these same data, an indication of this population average HI_o is given by the slope of a trend line established by a plot of TOCo versus the present-day generation potential (i.e., in this case, also original Rock-Eval measured kerogen yields [S2 or S2o])<ref name=L&B-V1990 /> ([[:File:M97FG4.jpg|Figure 4]]). This graphic suggests an average HI_o of 533 mg HC/g TOC for this population of marine kerogens, assuming fit through the origin. However, using an average value is not entirely satisfactory either because these marine shales show considerable variation in HI_o, as shown by a distribution plot ([[:File:M97FG5.jpg|Figure 5]]). Using this distribution, the likelihood of a given marine kerogen exceeding a certain HI_o value can be assessed, that is, application of P90, P50, and P10 factors. This distribution indicates that 90% of these marine shales exceed an HI_o of 340, 50% exceed 475, and only 10% exceed 645 mg HC/g TOC (Table 1):

Using these same data, an indication of this population average HI_o is given by the slope of a trend line established by a plot of TOCo versus the present-day generation potential (i.e., in this case, also original Rock-Eval measured kerogen yields [S2 or S2o])<ref name=L&B-V1990 /> ([[:File:M97FG4.jpg|Figure 4]]). This graphic suggests an average HI_o of 533 mg HC/g TOC for this population of marine kerogens, assuming fit through the origin. However, using an average value is not entirely satisfactory either because these marine shales show considerable variation in HI_o, as shown by a distribution plot ([[:File:M97FG5.jpg|Figure 5]]). Using this distribution, the likelihood of a given marine kerogen exceeding a certain HI_o value can be assessed, that is, application of P90, P50, and P10 factors. This distribution indicates that 90% of these marine shales exceed an HI_o of 340, 50% exceed 475, and only 10% exceed 645 mg HC/g TOC (Table 1):

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* Peters, K. E., and M. R. Caasa, 1994, Applied source rock geochemistry, in L. B. Magoon and W. G. Dow, eds., The petroleum system: From source to trap: AAPG Memoir 60, p. 93–117.

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* 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.