A New Method to Improve the Accuracy of Total Organic Carbon Measurement in Carbonate Rocks

The methods for determining total organic carbon (TOC) in carbonate rocks are mainly the conventional TOC method and the montmorillonite thickening method. However, both methods suffer from organic matter loss during the measurement process and inaccuracy in determining low TOC concentrations (TOC ≤ 0.5%). In this study, carbonate rock samples from the Ordovician of the Ordos Basin and synthetic standard samples were investigated using the conventional TOC method, the montmorillonite thickening method, and a newly developed elemental analysis method based on sodium polyacrylate thickening after acid digestion. The results show that gypsum-facies carbonate rocks contain organic acid salts, whereas shoal-facies and argillaceous carbonate rocks do not develop organic acid salts. For synthetic samples spiked with small-molecule organic acid salts, the relative error of the conventional TOC method exceeds 98%; for large-molecule organic acid salts, the relative error is relatively small, below 16%. The TOC measurement error of the sodium polyacrylate thickening method is lower than that of the montmorillonite thickening method. Compared with the conventional and montmorillonite methods, when carbonate rock TOC ≤ 0.5%, the sodium polyacrylate thickening method yields TOC errors mostly ≤10%, with better stability and lower error rates. The new TOC method for carbonate rocks exhibits higher accuracy and stability in determining organic carbon content, and can more objectively reflect the total organic carbon (TOC) of samples.