Page 126 - The Petroleum System From Source to Trap

120 Peters and Cassa

APPENDIX C:
Methods for Measuring TOC

Direct Combustion Indirect (by Difference)
Weighed, pulverized rock (1-2 g) is treated with Two weighed aliquots of a pulverized, homogeneous
hydrochloric acid (6 N HCI) in Leco filtering crucibles. Spent sample are treated separately. One aliquot is mixed with accel­
acid and wash water are removed by vacuum filtration. The erator and combusted to yield total carbon (TC = organic plus
dried residue (100°(, 30 min. is mixed with metal accelerator carbonate carbon). Another aliquot is treated dropwise with
(elemental iron and copper), combusted using a high-frequency HCI and the evolved C02 is measured. Total organic carbon is
induction furnace (1200°C), and measured as carbon dioxide. the difference between TC and carbonate carbon. Indirect TOC
Direct combustion is the most commonly used method for is generally more accurate for organic-poor (<0.5 wt. % TOC),
TOC. However, certain samples that contain immature oil­ carbonate-rich samples than direct TOC. However, for samples
prone organic matter can lose from 10% (Peters and Simoneit, with TOC values over about 0.5 wt. %, cumulative errors
1982) to 44% (Roberts et a!., 1973) TOC as hydrolyzate with the resulting from measurements for two aliquots make this
acid filtrate prior to combustion. approach less satisfactory than other methods.

Modified (Nonfilterin g ) Direct Combustion Pyrolysis plus Combustion Products
Pulverized rock is weighed into Leco nonfiltering crucibles Some pyrolysis systems allow determination of TOC by
and treated dropwise with HCI until all C� evolution ceases, summing the carbon in the pyrolyzate with that obtained by
followed by reaction overnight. Spent acid and wash water are oxidizing the residual organic matter at 600°C. For small
decanted, the residue is dried, mixed with accelerator, and samples (100 mg), this method provides more reliable TOC
analyzed as described for direct combustion. Although data than conventional combustion methods, which require
prohibiting loss of hydrolyzate, this method is laborious, and about 1-2 g of ground rock. However, mature samples, in
difficulties in removing all acid can result in corrosion of the which vitrinite reflectance (Rc,) exceeds about 1%, yield poor
drying oven. TOC data because the temperature is insufficient for complete
combustion.

CHAPTER APPENDIX D:
Key Rock-Eval Pyrolysis and TOC Parameters

51 measures hydrocarbon shows as the amount of free kerogen and thus indicate the potential of the rock to generate
hydrocarbons that can be volatilized out of the rock without oil. High hydrogen indices indicate greater potential to
cracking the kerogen (mg HC /g rock). S1 increases at the generate oil. Although HI versus OI plots are generally reliable
expense of Sz with maturity. indicators of kerogen type, gas-prone coals and coaly rocks can
S2 measures the hydrocarbon yield from cracking of give anomalously high HI values that must be confirmed by
kerogen (mg HC/g rock) and heavy hydrocarbons and repre­ elemental analysis (Peters, 1986). The average HI in a rock
sents the existing potential of a rock to generate petroleum. Sz is interval is best determined from the slope of a regression line
a more realistic measure of source rock potential than TOC on a graph of S2 versus TOC (Langford and Blanc-Valleron,
because TOC includes "dead carbon" incapable of generating 1990).
petroleum. Oxy g en index [01 = (S3/TOC) x 100, mg COz/g TOC] is
51 + � i s a measure of genetic potential (Tissot and Welte, related to the amount of oxygen in the kerogen. In general, the
1984) or the total ammmt of petroleum that might be generated 5] measurement is not as reliable as other Rock-Eva! parame­
from a rock. ters, partially because of interference of carbonate minerals or
Production or productivity index [PI = St/(Sl+Sz)) grad­ kerogen oxidation resulting from pulverizing the sample.
ually increases with depth for fine-grained rocks as thermally When � results are suspected to be unreliable, HI versus T max
labile components in the kerogen (Sz) are converted to free (Espitalie et al., 1984) can be substituted for the HI versus 01
hydrocarbons (�). Reservoir rocks show anomalously high PI plot.
T max measures thermal maturity and corresponds to the
values compared to adjacent fine-grained rocks. For T max
values of <435°C and Tmax in the range 435,-445° C, PI values Rock-Eva! pyrolysis oven temperature (0C) at maximum S2
exceeding 0.2 and 0.3, respectively, are considered anomalous. generation. Tmax should not be confus with geologic temper­
Hydro g en index [HI = (Sz/TOC) x 100, mg HC/g TOC] atures. T max is partly determined by the type of organic matter
and 52/53 are proportional to the amount of hydrogen in the (Figure 5.9) (Peters, 1986).