| Popis: |
Two types of organic matter from the North Sea have been studied, one from the Kimmeridge clay shales of the Moray Firth basin (Type II organic matter) and the other from the Brent coals of the Viking Graben (Type III). Mineral matrices were removed by acid treatment and 50–200 mg of the immature organic matter sealed in a gold tube. A pressure of 80 MPa was applied, enhancing the establishment of a confined system where chemical reactions between all the present moieties are possible. The samples were isothermally heated at temperatures between 250 and 500°C, each time for about 45 h. After rapid quenching, the reaction products were analysed. At the same artificial maturation level, the productivity for all the fractions: methane, C2C5, C6C13, C14+ aliphatic and aromatic hydrocarbons and NSO-compounds (resins and asphaltenes) is always higher for the Kimmeridge clay sample than for the Brent coal sample. The ratio of the total yields of the Kimmeridge to the coal sample is greater than the ration of their Rock-Eval HIs. Qualitatively, the composition of the hydrocarbon fractions depend on the maturation level and are related to the type of organic matter. These differences are more noticeable in the early stages of generation. The beginning of the main generation of methane and to a lesser extent C2C5 compounds occurs as the Hydrogen Index of residual kerogen approaches zero. So, these light compounds are mainly the result of secondary cracking of the extractable compounds such as NSO-compounds and C13+ hydrocarbons. Direct generation of methane from kerogen by primary cracking is not important, even for the Type III organic matter (coal). Although these experiments do not perfectly reproduce natural evolution, the succession of the main events and differences of behavior observed between these two types of organic matter are comparable with what they would be under natural geological circumstances. |
