Steroid biomarker-clay mineral adsorption free energies: Implications to petroleum migration indices

Abstract

Enrichments of 14β,17β steranes over 5α,20R steranes in petroleums relative to shale source rocks have bee discussed inthe literature and ascribed to possible migrational geochromatographic processes involving mineral surfaces. Liquid−solid chromatographic (LSC) effects arise from adsorption free energy (AFE) differences between various chemical compounds and an active surface. If nonmaturational concentration differences of 14β,17β/5α,20R sterane isomers in petroleum arise frommigrational geochromatographic effects, then those differences must be reflected by sterane-mineral AFE differences. Accordingly, AFE values for 23 selected steroid biomarkers, including saturated-isomerized, mono-,di-, and triaromatized and partially cracked structures were measured on montmorillonite clay using a chromatographic technique. Those data conform to LSC theory: laboratory migration rates for biomarkers on montmorillonite of increasing water content can be calculated from AFE values. Biomarker AFE values are shown to depend on specific structural chemical features, i.e. aromatic carbon number and stereochemistry, in a systematic way. Quantitative contributions of those features to AFE values are calculated and rules are developed which permit the prediction of relative chromatographic migration rates for related biomarker compounds. Finally, AFE valuess are used to calculate 253 possible biomarker pair concentration ratio variations (measurable by GC-MS) expected for clay mineral LSC-type geochromatographic effects.