TOC calculation of organic matter rich sediments using Rock-Eval pyrolysis: Critical consideration and insights

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The Total Organic Carbon (TOC) is one of the most important Rock-Eval pyrolysis derived parameters. The present study documents the suppression and underestimation of Rock-Eval TOC as a function of sample amount being used for analysis. For the purpose of study, a manually isolated vitrain band, carbonaceous shale and two shales were analyzed at different weights using a Rock-Eval pyrolyzer. For the vitrain band and the carbonaceous shale samples, a drop in TOC content was observed with increasing sample amount. In each of the case (vitrain and carbonaceous shale), although the S2 pyrograms were fairly similar at successive higher weight fractions, it was observed that increasing portions of the CO2 released during Rock-Eval oxidation and represented by S4 oxidation graphics, was undercounted with increasing sample charge, thereby underestimating the residual carbon (RC) and TOC content. The results indicate that at lower sample weights the entire organic matter is oxidized below 650°C (S4–S5 boundary). However, at higher weights the organic matter is oxidized through a wider temperature range (beyond 650°C) and consequently resulting in release of CO2 through a wider temperature range. The insights developed in the paper indicate that sample amount should be around 5–10mg and 35mg for type III–IV organic matter-bearing coals and carbonaceous shales, respectively. However, when the grain size was kept finer (<63μm), it was observed that sample amounts can be kept around 15mg for type III–IV organic matter-bearing coals. No change in TOC with increasing/decreasing sample charge was observed in case of low TOC shales.