The Pristane Formation Index, a new molecular maturity parameter. A simple method to assess maturity by pyrolysis/evaporation-gas chromatography of unextracted samples

Abstract

A set of eleven samples from the Handil borehole, Mahakam delta, Indonesia, has been analyzed to test the hypothesis that the Pristane Formation Index (PFI = [pristane]/{[pristane]+ [pristenes]}) can be used as a molecular maturation parameter. By pyrolysis/evaporation-gas chromatography (py/ev-GC) of unextracted samples the amounts of the product (pristane) and of the precursor(s) (measured as pristenes) of the pristane formation reaction(s) were measured simultaneously. PFI shows very high correlations with maturity parameters such as vitrinite reflectance, Carbon Preference Index (CPI), T-max and with burial depth. Calculation of the pseudo-kinetic parameters of the Arrhenius equation k = A exp(− E RT ) , using published values for effective time of burial ( t eff ) and temperature gradient, yields values of 59 kJ mol −1 and2.2∗10 −7 s −1 for E and A, respectively. The value of E is not very sensitive to variations in the assumed values of t eff and temperature gradient; the value of A is extremely low in all cases. The values of the pseudo-kinetic parameters are much lower than those of the isomerization and aromatization reactions and thus offer interesting possibilities for the reconstruction of thermal histories of basins.