Mathematical models of the distribution of geotracers during oil migration and accumulation

Compounds which partition into water and which are adsorbed by solid phases ('geotracers) are lost from petroleum along migration pathways, giving important clues about the nature and length of the route from source to reservoir. Many factors influence the distribution of petroleum geotracers, including migration distance, the inherent properties of the migration systems, the chemical properties of the tracers, the volume of reservoired oil and the filling sequence. This paper constructs the mathematical models that are required to describe adequately the occurrence of geotracers in migrated and reservoired oils. The models show that for commonly used geotracers (phenol and carbazole compounds): (1) adsorption to oil-wet mineral sites is a major process removing geotracers from oil; (2) adsorption onto mineral surfaces can be treated as an equilibrium process on a geological time-scale; (3) diffusion of tracers from a migrating oil slug to the surrounding sediments can be neglected; and (4) the tracer concentration in a reservoired oil is not related uniquely to migration distance but is negatively correlated to the ratio of relative migration distance divided by the volume of reservoired oil having travelled the migration pathway. The potential applications of the models in petroleum exploration include: assessment of the route and relative distance of oil migration (with implications for the identification of undrilled prospects); estimation of the volume of lost oil by spill from a reservoir; and differentiation of migration through fractures and capillary migration through fine-grained rocks.