Using Geochemical Method to Distinguish Lateral Migration and Vertical Migration in Rifted Basin:A Case Study from Eastern Lujiapu Depression in the Kailu Basin,NE China

Migration fractionation diagnosis is complicated in rifted basins where migration distance is generally short and lateral migration in sandy beds and vertical migration along faults are co-existed. Quantitative data from GC-MS analysis makes it possible to distinguish lateral and vertical migration effects.Oils discovered from the Jiaolige oilfield,eastern Lujiapu Depression are derived from single source rock with similar maturity,which is an ideal case to study the migration fractionation effects. Compositional differences among oils are largely caused by the migration fractionation either laterally in sand beds or vertically along the faults.Subtle maturity differences are assessed by the classic saturated hydrocarbon parameters which have certain influence on nitrogen compounds.In a certain maturity range,the ratios of shield and semi-shield isomers to the exposed isomers of aikylcarbazoles change with maturity in an opposite direction with migration fractionation,which may conceal the migration influence.However,migration and maturation have the same effects on absolute concentrations of alkylated carbazoles and benzocarbazole[a]/([a] [c])ratios,which provides an ideal tool for migration direction assessment.Continuous variations among different samples reflect increased migration distance in sandy beds,while abrupt changes may indicate the change of migration conduit systems.Integrated both geochemical interpretation and geological constrains,not only migration direction can be determined,but also the conduit systems through the sandy beds or the faults can be recognized.