Simulation experiments on secondary thermal stress of biodegraded bituminous sandstones

Secondary variation of reservoir is a hot problem in petroleum geochemistry field. Several kinds of secondary variations have taken place after the formation of bituminous sandstones in the Tarim Basin including biodegra-dation, washing, dissipation and secondary thermal stress. Biodegraded bituminous sandstones were used in the experiment. Pyrolysis experiment has been performed in a closed system, simulating secondary thermal stress with continual burial of Silurian bituminous sandstones which may cause the changes in molecular compositions, carbon isotope and physical properties of bituminous sandstones. The results are as follows: (i) Gases are mainly product during the experiment and carbon isotope of gas is lighter. (ii) Yield of C6+ hydrocarbon is relative smaller, and yielded oil is mainly light oil. (iii) Chromatogram character of biodegraded oil-sand is similar to that of saturated hydrocarbon in extracts from present-day bituminous sandstones, (iv) Porosity of bituminous sandstones gradually increases with increasing temperature. Fluorescence color of bituminous sandstones gradually become darker. Reflected light color of bitumen in bituminous sandstones becomes darker with increasing temperature and reflectivity of bitumen increases with increase in temperature. It is speculated that secondary thermal stress has great effect on molecular composition and structure of bituminous sandstones.