Effects of clay minerals and carbonate content on hydrogen and oxygen indices of source rock- a case study of Huai Hin Lat formation

Abstract

The study of petroleum geochemistry of Huai Hin Lat Formation cutting samples found that Huai Hin Lat Formation had high hydrogen index and low oxygen index ( Chamchoy, 2014) . Therefore, this research aims to study whether clay minerals and carbonate content affect the hydrogen and oxygen indices. Petrographic thin sections and X-Ray Diffractometer were used to identify the type and amount of minerals in the samples especially carbonate and clay minerals. Moreover, the comparison between hydrogen and oxygen indices of the original and decarbonated samples could evaluate the effect of carbonate minerals on oxygen index values. The study area is Tat Yai waterfall located on the boundary between Amphoe Phu Pha Man, Changwat Khon Kaen and Amphoe Nam Nao, Changwat Phetchabun. Twenty-five samples of Dat Fa Member laminated black calcareous mudstone interbedded with argillaceous limestone were collected for this study. The results revealed that Tat Yai samples contain carbonate minerals: dolomite with an average content of 30.76 wt% and calcite with an average content of 7.96 wt% and clay minerals, illite with an average content of 4.29 wt%. The samples have low hydrocarbon generation potential (average S2 0.11 mg HC/ g rock). The regression lines of S2 vs. TOC diagram were used to determine mineral matrix effect on S2 values. True average hydrogen index calculated from slope of the regression lines is 11.89 mg HC/g TOC which falls into kerogen type IV range. Y-intercept which is an amount of hydrocarbons retained by mineral matrix during pyrolysis is low (0.1257 mg HC/ g rock in average). Low content of illite (which is the most active minerals that could retain hydrocarbons on its surface) , low S2 values, high coke from pyrolysis and dead carbons ( 1.0386 wt%) and high maturation level, all suggest that kerogen might have already cracked to hydrocarbons and only dead carbons are left which cannot generate hydrocarbons. Therefore, hydrocarbon retention during pyrolysis slightly affects the hydrogen index values. From Modified Van-Krevelen diagram, the decarbonated samples have slightly lower oxygen indices than the original samples and have less scatter data showing little effect of carbonate content on oxygen indices. Tat Yai samples have low - high TOC content (0.47 - 3.55 wt%). Most samples have good source rock potential and are in post-mature stage based on Tmax values. Keywords: Huai Hin Lat Formation, Rock-Eval pyrolysis, Hydrogen index, Oxygen index, Mineral matrix and carbonate content effect, Hydrocarbon retention