Geochemical effects of asphaltene separation procedures: changes in sterane, terpane, and methylalkane distributions in maltenes and asphaltene co-precipitates

Asphaltene precipitation, either natural or artificial, may be an important cause of observed compositional differences in oils of identical source and maturity. We studied variations in both the bulk and molecular compositions of maltenes (and asphaltene co-precipitates) prepared from six different oils using different procedures. The first procedure used a single treatment with n-pentane (1PTC) to separate asphaltenes from maltenes. The second procedure applied four sequential n-pentane extractions of the 1PTC asphaltenes (four times-4PTC). Analyses of resulting fractions included thin layer chromatography (TLC), gas chromatography (GC), and gas chromatography–mass spectrometry (GC–MS). These asphaltene separation processes resulted in changes in geochemical parameters previously considered to be influenced solely by source or thermal maturity. Maltene geochemical parameters not significantly affected by multiple n-pentane extractions of the n-pentane-precipitated asphaltenes were the C15–C35 n-paraffin distributions (except for a small increase in higher n-alkanes), hopane and homohopane distributions, Tm/Ts ratios, and C27:C28:C29 sterane distributions. Maltene geochemical parameters which were affected by multiple n-pentane washes of the n-pentane-precipitated asphaltenes, were increases in the C21 and C22 sterane (pregnane) to C27, C28, C29, and C30 sterane and C20 to C24 tricyclic terpane to C29 to C35 hopane ratios. The n-pentane-precipitated asphaltene co-precipitates showed an increasing ratio of methylalkanes to n-alkanes in the range from C25 to C36 following multiple extractions with n-pentane. These results suggest mechanisms of molecular interactions between maltenes and asphaltenes.