Impact of siderite on Rock-Eval S3 and oxygen index

• Published in: Marine and petroleum geology 2022-09, Vol.143, p.105804, Article 105804
• Main Authors: Hazra, Bodhisatwa, Katz, Barry J., Singh, Deependra Pratap, Singh, Pradeep K.
• Format: Article
• Language: English
• Subjects: Oxygen index; Rock-Eval; S3 peak; Siderite; Source rock
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2022.105804

Over the past four decades Rock-Eval pyrolysis has become a common means to type organic matter. The modified van Krevelen diagram, using the hydrogen and oxygen indices, has become a commonly used tool for organic matter classification. It assumes that the components used to calculate the two indices are representative of the indigenous organic matter, which is especially the case for the oxygen index. This study provides a series of experimental results that further characterize the impact of siderite on the hydrogen and oxygen index values, which decrease and increase, respectively, with increasing mineral content. This work suggests that the changes in hydrogen index values are a result of the interaction between the mineral phase and the generated hydrocarbons, where hydrocarbons may be adsorbed on to the mineral surfaces and not fully released. The increase in the oxygen index appears to be a result of mineral decomposition and the release of CO2. The oxygen index was found to be particularly sensitive to the presence of siderite in the sample. Siderite decomposition was found to occur at temperatures below often published decomposition temperatures and at or below the S3 peak trapping temperature (390 °C). Thus, the impact of organic carbon content and mineral interaction need to be taken into consideration when characterizing organic matter, especially when siderite and to a lesser extent other carbonate minerals are present. At lower organic carbon levels, organic matter will appear more gas-prone and would suggest that the organic matter has been strongly oxidized. Although corrections have been proposed in the literature they reduce the value of the Rock-Eval “quick” screen by adding additional analyses or data treatment. It is further suggested that the effect may be sensitive to the early diagenetic environment as well as cation substitution. •Importance of Rock-Eval S3 and S3′ curves to the correct kerogen classification.•Stronger influence of siderite on the S3 peak and OI values than originally suggested.•OI was found to significantly increase with increasing siderite, and to a lesser extent with limestone content.•An associated reduction in the HI appears due to interaction with the minerals.