A new empirical approach for rapid quantification of organic and pyritic sulfur in sedimentary rocks using the Rock-Eval 7S

Abstract

Evaluation of individual sulfur (S) fractions in sedimentary rocks (e.g., pyrite, organic-S) provides essential information about paleo-environmental conditions of sediment deposition and petroleum systems characteristics. However, measuring organic- and pyritic-S concentrations involves tedious and time-consuming (days per sample) wet chemistry techniques. A more tractable approach uses gradual heating of the rock to separate the S-fractions according to their different thermal stabilities. Such an approach is implemented here using the newest Rock-Eval model (RE-7S, Vinci Technologies), which allows monitoring of the S products during gradual heating in sequential pyrolysis and combustion cycles. The rapid (∼1 h per sample) analysis differentiates between the peaks of pyrolyzed organic- and pyritic-S. However, the residual S of both organic and pyritic fractions is decomposed at the same temperature range during combustion, challenging their quantitative separation. Here we suggest an empirical approach to separate the S-fractions based on a linear correlation (R2 = 0.98, n = 9) between two new defined parameters: the relative amount of pyrolyzed organic-S (PyOS %) to the temperature at maximum organic-S elution during pyrolysis cycle (Tmax-S °C). The correlation was common to nine source rocks, including Types I, II, and II-S kerogens, varied S concentrations (0.8–8.7 wt%), and different mineral matrices (carbonates, siliceous). The average precision achieved by the approach was 6% relative standard deviation for replicate measurements. Accuracy was mostly within 10% compared with the values determined by the conventional wet-chemistry technique. This new empirical approach provides a rapid and robust method for quantifying organic and pyritic sulfur in sedimentary rocks.