TOC and Rock-Eval

Application

The purpose of TOC and Rock-Eval pyrolysis is to screen for the presence of source rocks, and to determine their hydrocarbon generation potential, the type of organic matter present and an indication of its thermal maturity (Espitalié et al. 1977). The presence of migrated hydrocarbons in suspected reservoir intervals can also be monitored.

The parameters most frequently reported are TOC, S1, S2, S3 and Tmax, together with a number of derived parameters (HI, OI, PP and PI), as listed in the following table.

Parameters from TOC and Rock-Eval analysis

ParameterDefinitionMeasurementApplication
TOCtotal organic carbonwt% of Corg/rockrichness
S1free bitumenwt‰ of HC/rockoil or gas shows
S2hydrocarbon potentialwt‰ of HC/rockrichness/quality
S3CO2 potentialwt‰ CO2/rockquality
Tmaxtemp of maximum generation rate (i.e. S2)°Cmaturity
HIhydrogen index100*S2/TOCkerogen quality/typing
OIoxygen index100*S3/TOCkerogen typing
PPpetroleum potentialS1+S2oil and gas potential
PIproduction indexS1/(S1+S2)maturity/transformation ratio

Rock-Eval can also be operated in a ‘shale play’ mode for unconventional sources. It involves two heating stages to obtain free and sorbed hydrocarbons (Sh0 and Sh1, corresponding to traditional S1), as well as a residual hydrocarbon potential measurement (Sh2, corresponding to S2):

Parameters from Rock-Eval analysis in shale play mode

ParameterDefinitionMeasurementApplication
Sh0 and Sh1free/sorbed hydrocarbonswt‰ of HC/rockoil or gas shows
Sh2hydrocarbon potentialwt‰ of HC/rockrichness/quality
Tmaxtemp at max of Sh2°Cmaturity
HIhydrogen index100*Sh2/TOCkerogen quality/typing
HCcontHC content indexwt‰ of HC/rockfree hydrocarbon content
HQIHC quality index100*Sh0/(Sh0+Sh1)light : heavy free hydrocarbons
PIShaleproduction index100*(Sh0+Sh1)/(Sh0+Sh1+Sh2)maturity/transformation ratio

Sample requirements

For both types of analysis, finely milled rock samples are required. They can be cuttings, chips or plugs from core or side-wall core. In the case of cuttings, the 1−4 mm size fraction should preferably be used, and picking of selected lithologies may be required.

Sampling frequency should aim to capture quality variations in source rock intervals. It may also be used to aid selection of both source rock and reservoir samples for more detail analysis of extracts by GC and GC-MS.

Washed and dried samples are crushed and milled. 20−30 g of unwashed cuttings, or 2−3 g of washed cuttings or core material normally suffices for TOC and Rock-Eval analyses, although a minimum of ~0.5 g of washed material may be adequate if limited sample is available.

Often washing does not remove all traces of organic mud components, which contribute to TOC and adversely influence Rock-Eval measurements on source rock samples. More complete removal requires pre-extraction with an organic solvent. Although this removes any free bitumen (affecting S1 and derived Rock-Eval parameters), it provides reliable TOC, S2 and Tmax values.

Even so, there may be signs of contamination in the Rock-Eval results from samples exhibiting the poorest hydrocarbon potential, so thermal extraction-gas chromatography (TE-GC) plus pyrolysis-gas chromatography (Py-GC) may be useful (the analyses are performed sequentially upon the same sample aliquot). Such pre-extraction methods are unsuitable for reservoir rock samples because the material of interest would be removed.

Analytical procedure

TOC
Rock-Eval basic and shale play modes (after Romero-Sarmiento et al. 2016)

Prior to TOC analysis, the powdered rock sample is treated with dilute HCl to remove carbonate, which would otherwise overestimate the amount of organic C present. A Leco (SC-632) elemental analyser is normally used, which combusts the sample and the amount of carbon is measured as carbon dioxide by an infrared (IR) detector. Although TOC may also be determined on Rock-Eval 6 instruments, it is considered less reliable than the Leco instrument.

Rock-Eval 6

During basic mode operation of Rock-Eval, the powdered sample of rock is initially heated to 300°C in an inert atmosphere in order to liberate all the free bitumen present, without causing significant decomposition of the kerogen. The amount of free hydrocarbons obtained by this thermal extraction is measured by a flame ionisation detector (FID) to give the S1 parameter.

The temperature is then increased steadily to 850°C on Rock-Eval 6 instruments (previous versions used lower maximum of 550−600°C), during which all the kerogen capable of generating petroleum is converted into hydrocarbons, which are quantified by the FID to give the S2 parameter. The temperature at which the maximum rate of generation of S2 hydrocarbons occurs is recorded as Tmax.

During the S2 pyrolysis the oxygen content of the kerogen is assessed by measuring the amount of carbon dioxide evolved using a thermal conductivity detector (TCD), yielding the S3 parameter. Consequently, decarbonisation of samples is not required.

For unconventional sources, the shale play mode may be more appropriate. The lightest vaporisable hydrocarbons (Sh0) are determined over a temperature range of 100–200°C, at a heating rate of 25°C/min and an isothermal period of 3 min at the upper limit. Heavier free/sorbed hydrocarbons (Sh1) are then determined by heating at 25°C/min to 350°C, with another 3 min hiatus.

Finally, the temperature is increased to 650°C at 25°C/min in order to perform the usual pyrolysis of residual kerogen (Sh2), and the corresponding Tmax value is recorded at the maximum generation rate.

HAWK

APT also have a HAWK instrument, which offers certain advantages over Rock-Eval 6 in terms of efficiency of heating, particularly with regard to kinetic analyses. In addition to the normal pyrolysis, HAWK can perform a TOC determination via a separate oxidation step. The separation between organic C and mineral C in the oxidation cycle is set automatically.

As long as the CO signal from the IR detector is greater than zero, the source of CO2 is defined as organic. When the signal drops to zero, all remaining CO2 is defined as deriving from a mineral source. Normally, this division occurs between 600 and 650°C. APT normally recommends Leco TOC determination.

For pyrolysis alone, the temperature programme incorporates a 5 min purge before 3 min heating at 300°C for the S1 measurement and then 25°C/min to 650°C for S2, S3 and Tmax.

The oxidation cycle for TOC involves the same 3 min at 300°C followed by heating at 25°C/min to 850°C, where the temperature is held for 5 min.

As for Rock-Eval 6, Jet-Rock 1 is analysed as every tenth sample and its results checked against the acceptable range given in NIGOGA4.

Potential problems

The presence of organic-additives in drilling mud can adversely affect both TOC and Rock-Eval data. Although washing can remove the bulk of the additives, it is not uncommon for residues of varying proportions to remain in some samples, so caution should be exercised during interpretation. Although TOC is likely to be affected to an extent, although the greatest influence of mud residues is usually observed in Rock-Eval results.

Oil-based mud (OBM) generally augments the S1 measurement, but polymeric organic material based on glycol and similar additives is less volatile and tends to produce a precursor to the S2 peak. If sufficiently great, this precursor becomes indistinguishable from the genuine S2 signal and a single broad peak results, with a depressed Tmax value. Inspection of pyrograms is therefore always recommended. Oxygen-containing organic mud additives such as polyglycols also contribute to the S3 signal, so abnormally high OI values can result.

If oil or other organic mud additives are known to have been used, this contamination of source rocks is best removed by solvent extraction prior to the TOC and Rock-Eval analyses, so that more reliable TOC, S2, S3, HI, OI and Tmax values can be obtained. The S1, PI and PP values will be of no use because any indigenous bitumen or migrated hydrocarbons will be removed together with the OBM contaminant. This is not too detrimental where the prime purpose is to evaluate kerogen quality, but clearly cannot be performed for reservoir analysis.

Coring and core plugging can involve the use of oil-based lubricants. Whereas nothing can be done about coring procedures during exploration, samples are best taken away from core margins or the sites of core plugging to minimize potential contamination. Core plugs for the purposes of organic geochemical analysis should be drilled using water as a lubricant (but not if the purpose is inorganic analysis of reservoir sections, such as Sr isotope RSA).

Cavings can be problematic for cuttings, particularly if insufficient of what is believed to be the indigenous lithology can be picked. Worn conventional bits can produce mostly flour, which is difficult to recover during washing and cannot be picked effectively. Turbo-drilling can result in localised heating to the extent that the resulting fine cuttings exhibit signs of artificial maturation in data obtained from organic analyses. Casing and bit change depths can aid identification of problem intervals.

References

Espitalié J.J., Laporte L., Madec M., Marquis F., Leplat P., Paulet J., Boutefeu A. (1977) Méthode rapide de caractérisation des roche mères, de leur potential pétrolier et de leur degré d'evolution. Revue Institut Français du Pétrole 32, 23−45.

Peters K.E. (1986) Guidelines for evaluating petroleum source rock using programmed pyrolysis. AAPG Bulletin 70, 318−329.

Romero-Sarmiento M.-F., Pillot D., Letort G., Lamoureux-Var V., Beaumont V., Huc A.-Y., Garcia B. (2016) New Rock-Eval method for characterization of unconventional shale resource systems. Oil & Gas Science and Technology – Revue IFP Energies nouvelles 71, 37.