Thermal maturation, mineral catalysis, and gas generation kinetics of carbonate source rock

Abstract

Source-rock evaluation must start by first characterizing thermal maturity. The concept of thermal maturation or thermal decomposition is a well-defined quantitative modelling of kinetic parameters to explain the generation and expulsion of liquid and gaseous hydrocarbons from sedimentary source rocks. However, unlike coal and shale source rocks, carbonate source rocks are emerging source rocks and no analyses of thermal maturation kinetics have been reported in the literature. Apparently, this study is one of the most recent and first laboratory study of thermal maturation kinetics of carbonate source rock. In this paper, we simulated this process by conducting controlled laboratory experiments under high temperature from 40 °C to 450 °C using advanced integrated analytical techniques. The dataset is modeled by the solid state kinetics of first order reactions. In addition, the poor knowledge of how catalytic minerals stimulate thermal maturation is investigated; a unique and significant contribution that sets this work apart from previous thermal maturation studies. The result from kinetic modelling showed that the distribution of activation energy in the range of 99–325 kJ/mol is bimodal, reflecting possible peak shift in the thermal decomposition process, reaction mechanism and maturation prediction. The model further suggests that the maturation process of carbonate-rich source rock is very likely to be different from that of clay-rich source rock. Elemental and mineralogical analyses of investigated sample revealed the presences of catalytic elements such as iron, gypsum, pyrite and kaolinite. The analysis of the generated gases constrained to only three gases showed that the volume content of carbon dioxide, methane and water are 2%, 46% and 52% respectively. The generation of carbon dioxide is constant whereas methane increases and water decreases respectively with thermal maturation. Although, the investigated sample has a high calcite content of 91.9%, the methane content of 46% gives a strong indication of high natural gas generative potential as contrasted with the 2% of carbon dioxide generated. The findings of this study can help to better understand the coupled mechanism controlling thermal maturation of carbonate source rock and also provide useful reference dataset for basin modelling of carbonate source rocks for field studies.