Water-soluble organic acids in sedimentary rocks: Compositions and influencing factors

Abstract

Water-soluble organic acids (WSOAs) are closely related to the formation of secondary pores, however, it's little-known for the distributions of WSOAs in sedimentary rocks at deep-ultradeep zone and high-over mature stage. In the present study, 22 sedimentary rocks, involving diverse sedimentary rocks, thermal maturities and buried depths. Using deionized water to extract sedimentary rocks selected was conducted, and then WSOAs in extracts were quantitatively characterized using 940 Ion Chromatography (Metrohm AG). Meanwhile, key factors affecting WSOA distributions in sedimentary rocks were also discussed. Results show that besides at shallow zone and before mature stage, sedimentary rocks at the deep-ultradeep zone and high-over mature stage involve a certain amount of WSOAs. WSOA distributions for diverse sedimentary rocks, on the whole, are similar, concentration is higher for monocarboxylate than dicarboxylate, moreover, monocarboxylate and dicarboxylate are dominated by formate and acetate, and oxalate, respectively. Formation pathway and occurrence state of WSOAs are the same on the basis of Pearson correlation coefficient among the concentrations of different types of WSOAs. TOC-normalized WSOA concentrations correlate positively with oxygen index (IO). With increasing buried depth, total WSOA concentrations tend to decrease. Total WSOA concentrations are greater at Tmax less than 435 °C for type-III organic matter (OM) such as coal due to greater oxygen content, while almost comparable for diverse OMs at Tmax more than 435 °C. Moreover, as thermal maturation rises, type-III OM possesses high proportional of formate while type-I and -II OMs involve great percentage of acetate.