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

Abstract

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.

A novel molecular index for secondary oil migration distance

“…The properties and types of pathways were studied by Yang et al (2005)4. To investigate the source input influence, we focus here on a uniform migration system, in which the properties of the system, including porosity, density of solids, sorption coefficients, migration velocity of oil and oil saturation, are constant.…”

“…When large polar compounds such as carbazoles are selected for a secondary migration study, molecular diffusion is insignificant42 and can be safely neglected4. Lateral migration is very slow, especially in cratonic basins such as the Ordos Basin.…”

“…Nitrogen-, sulfur- and oxygen-containing compounds exhibit strong sorption on minerals and/or high solubility in water due to their polarities, and thus variations in the distribution of these molecules are used to study oil migration processes34. However, those compounds with high solubility in water such as alkylphenols can be easily affected by water saturation, water washing and injection for enhancement of oil production, and are sensitive to any oil-water interactions in the subsurface environment5.…”

“…However, recent studies show that these empirical indicators do not solely reflect migration-related fractionations and thus do not actually correlate with migration distance, because their concentrations and ratios can also be affected by variations in organic facies (such as marine, lacustrine, or terrigenous organics; anoxic or suboxic depositional environment; carbonate or shale lithology) and thermal maturation of source rocks as well as biodegradation of oils8101617181920. In addition, it appears that properties of migration systems, such as porosity, sorption coefficients, oil saturation and oil volume, may also influence the utility of these tracers4.…”

Controls on alkylphenol occurrence and distribution in oils from lacustrine rift basins in East China

“…[6] proposed the molecular migration index (MMI), based on ratios of o-cresol/phenol, which increases with petroleum migration distance. More recently, Yang et al [7] constructed the mathematical models to describe the occurrence of geotracers in migrated and reservoired oils. Similar abundances of isomers across the C 0 C 3 alkylphenol distributions in oils derived from a wide range of source rock types led Taylor et al [8] and Ioppolo et al [9] to suggest that the source-organic matter type of an oil is not an important factor in controlling the alkylphenol distributions.…”