Experimental investigation of the compositional variation of petroleum during primary migration

Anhydrous non-isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from Permian through Miocene age. The objectives of this study were to investigate molecular and compound class fractionation during petroleum generation and expulsion and to highlight causes of compositional variations due to primary migration from source rocks. Molecular compositions of n-alkanes, triterpanes, methylphenanthrenes and fatty acids of the original samples (initial bitumen), the samples after pyrolysis (residual bitumen) and the expelled products were compared. Acyclic alkanes were preferentially expelled with compositions depending on source kerogen and partial evaporative loss from the open experimental system expulsion. No substantial molecular fractionation within various compound classes or preferential primary migration of n-alkanes relative to acyclic isoprenoids occurred. Cyclic triterpanes and aromatic hydrocarbons were found only in small concentrations in the expelled bitumen indicating preferential retention relative to acyclic alkanes. Palmitic (C-16) and stearic (C-18) acids occurred as dominant compounds in the initial and residual bitumen. In the expelled bitumen they constitute virtually the only fatty acid species, indicating that expulsion efficiency is strongly dependent on concentration and generation intensity. (C) 2007 Elsevier Ltd. All rights reserved.