Experimental pyrolysis of Eocene lignite in the presence of water was conducted in a closed system for a heating duration of 120 min at temperatures of 400, 500, 600 and 700 degrees C and pressures of 1 to 3 GPa (equivalent to burial depths of about 30 to 100 km). The distribution of terpanes in the resulting pyrolysates was analyzed in order to investigate the effects of pressure and temperature on maturation of organic matter for this heating time at mantle depths. The results indicate that terpanes show more mature characteristics with increasing temperature at a given pressure. However, high pressures significantly retarded the thermal evolution of organic matter. Also, high pressures had a non-linear effect on maturation, resulting in anomalous variations of maturity parameters and ratios (tricyclics / hopanes, Ts/Tm, C3122S / (22S+22R), and beta a-moretane /a beta-hopanes). In addition, the isomerization of hopanes at the C-22 position did not reach typical end-point values, even at 700 degrees C. These results are a preliminary step towards a better understanding of organic matter maturation and oil and gas exploration in high temperature and pressure regimes.