A review of the occurrences and causes of migration-contamination in crude oil

Migration-contamination occurs when a migrating or trapped crude oil functions as a solvent, dissolving molecular components from the syndepositional organic matter of the migration conduit and/or the reservoir rock. This review documents the causes and worldwide occurrences of migration-contamination, and suggests methods of identifying the signatures of this phenomenon within the molecular and isotopic composition of crude oils. Instances of migration-contamination have been identified in oils of Australia, Asia, Africa and North America, and are best identified by the presence of molecular and/or isotopic imbalances in the oil. Such imbalances are most apparent when co-occurring molecular suites are incompatible with one another, particularly in terms of thermal maturity. Occurrences of migration-contamination in oils of the US Gulf of Mexico - documented by molecular maturity imbalances such as the presence of unsaturated hydrocarbons known to be unstable at petroleum generation temperatures - provide the best examples of this phenomenon. An attempt is made to quantify the extent and impact of this process in the Vermilion 14 field, offshore Louisiana in the US Gulf of Mexico. Here, molecular maturity markers in the migrated fluid and in the syndepositional organic matter of the reservoir rocks are compared, and the proportion of contamination in each reservoired fluid is estimated. Examples such as this may assist in explaining maturity anomalies observed in several oils worldwide, particularly in Tertiary-reservoired oils that have migrated through, and are trapped in, thermally immature sediments. It is suggested that the low molecular maturity ratios often observed in Tertiary-reservoired oils and attributed to unusually high heating rates may be caused, at least in part, by migration-contamination. (C) 2002 Elsevier Science Ltd. All rights reserved.