Geochemical characterization of solid residues, bitumen and expelled oil based on steam pyrolysis experiments from Irati oil shale, Brazil: A preliminary study

Steam pyrolysis experiments were performed on immature samples from the Irati oil shale, Parana Basin, Brazil, using a maximum temperature of 350 degrees C with up to 98 h exposure time at that temperature. The objectives were to study geochemical and petrographical changes in the source material during stepwise increase in maturity, in steam conditions, comparing the properties of expelled oil with the bitumen retained in the solid residue after experimentation. Petrographical and geochemical parameters such as vitrinite reflectance and T-max, indicated an increase in maturity related to the exposure time of the organic matter to the maximum temperature. However, biomarker ratios such as 22S/(22S + 22R) C-31 and C-32 homohopanes, 20S/(20S + 20R) and alpha beta beta/(alpha beta beta + alpha alpha alpha) C-29 sterane, which are considered to be indicators of organic matter maturity levels, did not reach their equilibrium values. Some biomarkers frequently used as indicators of specific sources and/or paleoenvironments of deposition such as hopane/sterane ratio, and the concentrations of C-27 and C-29 steranes showed significant variations related to the stage of maturity. Based on the evaluation of Rock-Eval parameters, the transformation ratios in steam pyrolysis conditions reached levels higher than 80% in samples having 9 and more hours of exposure time to maximum temperature. Bitumen was found to be enriched in components of heavier molecular weight (resins and asphaltenes), whereas the expelled oils contained higher quantities of aliphatic and aromatic components. At relatively low maturity levels the n-alkane distribution of expelled oils indicate a somewhat higher maturity level when compared to the n-alkane distribution of the bitumen retained in the source rock, whereas at higher maturity levels the n-alkane distribution for the expelled oil and for the bitumen is very similar. (C) 2009 Elsevier Ltd. All rights reserved.