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 (Jarvie et al., 2007). However, it is difficult to assign an original HI (HIo) 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 HIo 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 (Jarvie et al., 2006). Thus, our previous selection of 700 mg HC/g TOC for type I kerogen is likely overstated (Jarvie et al. 2007), 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 (Jarvie et al., 2007). However, it is difficult to assign an original HI (HIo) 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 HIo 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 (Jarvie et al., 2006). Thus, our previous selection of 700 mg HC/g TOC for type I kerogen is likely overstated (Jarvie et al. 2007), 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 HIo 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 HIo 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 (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 (1984) 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 HIo 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 HIo 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 (1984) 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.

[[File:M97FG3.jpg|thumb|300px|FIGURE 3. Modified Espitalie et al. (1984) kerogen type and thermal maturity plot. A worldwide collection of immature marine shales shows a range of original hydrogen index (HIo) values from approximately 250 to 800 mg HC/g TOC, with the majority plotting in the 300 to 700 mg HC/g TOC range. The key points are the range of values, and that all generate more oil than gas from primary cracking of kerogen. TOC = total organic carbon.]]

[[File:M97FG3.jpg|thumb|400px|{{figure number|3}}Modified Espitalie et al. (1984) kerogen type and thermal maturity plot. A worldwide collection of immature marine shales shows a range of original hydrogen index (HIo) values from approximately 250 to 800 mg HC/g TOC, with the majority plotting in the 300 to 700 mg HC/g TOC range. The key points are the range of values, and that all generate more oil than gas from primary cracking of kerogen. TOC = total organic carbon.]]

Using these same data, an indication of this population average HIo 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]) (Langford and Blanc-Valleron, 1990) (Figure 4). This graphic suggests an average HIo 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 HIo, as shown by a distribution plot (Figure 5). Using this distribution, the likelihood of a given marine kerogen exceeding a certain HIo value can be assessed, that is, application of P90, P50, and P10 factors. This distribution indicates that 90% of these marine shales exceed an HIo 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 HIo 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]) (Langford and Blanc-Valleron, 1990) (Figure 4). This graphic suggests an average HIo 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 HIo, as shown by a distribution plot (Figure 5). Using this distribution, the likelihood of a given marine kerogen exceeding a certain HIo value can be assessed, that is, application of P90, P50, and P10 factors. This distribution indicates that 90% of these marine shales exceed an HIo of 340, 50% exceed 475, and only 10% exceed 645 mg HC/g TOC (Table 1).

[[File:M97FG4.jpg|thumb|300px|FIGURE 4. Organofacies plot of original total organic carbon (TOCo) and original generation potential (S2o). These data show the high degree of correlation of the worldwide collection of marine shale source rocks. The slope of the correlation line is inferred to indicate the initial original hydrogen index (HIo) value (533 mg HC/g TOC) for the entire group of source rocks with a y-intercept forced through the origin (Langford and Blanc-Valleron, 1990). R2 = linear correlation coefficient.]]

[[File:M97FG4.jpg|thumb|400px|{{figure number|4}}Organofacies plot of original total organic carbon (TOCo) and original generation potential (S2o). These data show the high degree of correlation of the worldwide collection of marine shale source rocks. The slope of the correlation line is inferred to indicate the initial original hydrogen index (HIo) value (533 mg HC/g TOC) for the entire group of source rocks with a y-intercept forced through the origin (Langford and Blanc-Valleron, 1990). R2 = linear correlation coefficient.]]

[[File:M97FG5.jpg|thumb|300px|FIGURE 5. Distribution of original hydrogen index (HIo) values for a marine shale database containing immature samples. The highest percentage of HIo values are in the 400 to 499 mg HC/g TOC range. Delimiting P90, P50, and P10 values from this distribution yields a P90 of 340, a P50 of 475, and a P10 of 645 mg HC/g TOC. TOC = total organic carbon.]]

[[File:M97FG5.jpg|thumb|400px|{{figure number|5}}Distribution of original hydrogen index (HIo) values for a marine shale database containing immature samples. The highest percentage of HIo values are in the 400 to 499 mg HC/g TOC range. Delimiting P90, P50, and P10 values from this distribution yields a P90 of 340, a P50 of 475, and a P10 of 645 mg HC/g TOC. TOC = total organic carbon.]]

If HIo is known or taken as an average value or P50 value, the percent GOC in TOCo can readily be determined. Assuming that a source rock generates hydrocarbons that are approximately 85% carbon, the maximum HIo can be estimated by its reciprocal, that is, 1/0.085 or 1177 mg HC/g TOC. The values for organic carbon content in hydrocarbons can certainly vary depending on the class of hydrocarbons and can range from about 82 to 88% (which would yield maximum HIo values of 1220 and 1136 mg/g, respectively; the most commonly reported value in publications is 1200 mg HC/g TOC; Espitalie et al., 1984). However, from rock extract and oil fractionation data of marine shales or their sourced oils, the value of 85% appears sound with a plusmn3% variance.

If HIo is known or taken as an average value or P50 value, the percent GOC in TOCo can readily be determined. Assuming that a source rock generates hydrocarbons that are approximately 85% carbon, the maximum HIo can be estimated by its reciprocal, that is, 1/0.085 or 1177 mg HC/g TOC. The values for organic carbon content in hydrocarbons can certainly vary depending on the class of hydrocarbons and can range from about 82 to 88% (which would yield maximum HIo values of 1220 and 1136 mg/g, respectively; the most commonly reported value in publications is 1200 mg HC/g TOC; Espitalie et al., 1984). However, from rock extract and oil fractionation data of marine shales or their sourced oils, the value of 85% appears sound with a plusmn3% variance.

In lieu of these computations, a simple graphic can be used and is readily constructed in a spreadsheet. An HIo isoline can be constructed for any HIo using TOCo and S2o values. A nomograph is illustrated for every 20 mg/g of HIo in the marine shale range of values in Figure 6A. Using the fact that the GOC is a function of HIo/1177, the slopes for each 100 mg HC/g TOC value have isodecomposition lines that represent bitumen oil-free TOC and NGOC corrected for increased char formation by a simple function of 0.0004 times HIo subtracted from base TOC values. Bitumen- and/or oil- and kerogen-free TOC is simply the subtraction of carbon in S1 and S2 from TOC, that is, {TOCpdminus (0.085 times (S1pd + S2pd))}. Regardless of HIo or kerogen type, these isodecomposition lines are always parallel when 85% carbon in hydrocarbons is assumed.

In lieu of these computations, a simple graphic can be used and is readily constructed in a spreadsheet. An HIo isoline can be constructed for any HIo using TOCo and S2o values. A nomograph is illustrated for every 20 mg/g of HIo in the marine shale range of values in Figure 6A. Using the fact that the GOC is a function of HIo/1177, the slopes for each 100 mg HC/g TOC value have isodecomposition lines that represent bitumen oil-free TOC and NGOC corrected for increased char formation by a simple function of 0.0004 times HIo subtracted from base TOC values. Bitumen- and/or oil- and kerogen-free TOC is simply the subtraction of carbon in S1 and S2 from TOC, that is, {TOCpdminus (0.085 times (S1pd + S2pd))}. Regardless of HIo or kerogen type, these isodecomposition lines are always parallel when 85% carbon in hydrocarbons is assumed.

[[File:M97FG6A.jpg|thumb|300px|FIGURE 6A. (A-B) Iso-original hydrogen index (HIo) (solid lines) and isodecomposition (dashed lines) on an original total organic carbon (TOCo) versus original S2 (S2o) nomograph. (A) Iso-HIo lines from 100 to 900 mg HC/g TOC with isodecomposition lines illustrates the change in TOCo and S2o caused by kerogen conversion for the selected end point values.]]

[[File:M97FG6A.jpg|thumb|400px|{{figure number|6A}}(A-B) Iso-original hydrogen index (HIo) (solid lines) and isodecomposition (dashed lines) on an original total organic carbon (TOCo) versus original S2 (S2o) nomograph. (A) Iso-HIo lines from 100 to 900 mg HC/g TOC with isodecomposition lines illustrates the change in TOCo and S2o caused by kerogen conversion for the selected end point values.]]

[[File:M97FG6B.jpg|thumb|300px|FIGURE 6B. Once the adjusted present-day TOC (TOCadj-pd) corrected for carbon in kerogen and bitumen and/or oil (see Table 2) is determined, the TOCo is derived by tracing the decomposition line to the HIo intercept and dropping a perpendicular to the x-axis. S2 = Rock-Eval measured kerogen yields.]]

[[File:M97FG6B.jpg|thumb|400px|{{{{figure number|6B}}(A-B) Iso-original hydrogen index (HIo) (solid lines) and isodecomposition (dashed lines) on an original total organic carbon (TOCo) versus original S2 (S2o) nomograph. (B) Once the adjusted present-day TOC (TOCadj-pd) corrected for carbon in kerogen and bitumen and/or oil (see Table 2) is determined, the TOCo is derived by tracing the decomposition line to the HIo intercept and dropping a perpendicular to the x-axis. S2 = Rock-Eval measured kerogen yields.]]

Use of this nomograph is illustrated using data from the Barnett Shale (Figure 6B). Using a measured present-day TOC of 4.48%, with correction for bitumen and/or oil and kerogen in the rock and any increase in NGOC caused by hydrogen shortage, an original TOC of 6.27% is calculated. This means that the original generation potential (S2o) was 27.19 mg HC/g rock or, when converted to barrels of oil equivalent, 7.67 times 10minus2 m3/m3 (595 bbl/ac-ft). Data for this calculation are summarized in Table 2.

Use of this nomograph is illustrated using data from the Barnett Shale (Figure 6B). Using a measured present-day TOC of 4.48%, with correction for bitumen and/or oil and kerogen in the rock and any increase in NGOC caused by hydrogen shortage, an original TOC of 6.27% is calculated. This means that the original generation potential (S2o) was 27.19 mg HC/g rock or, when converted to barrels of oil equivalent, 7.67 times 10minus2 m3/m3 (595 bbl/ac-ft). Data for this calculation are summarized in Table 2.