Occurrence and origin of olefins in crude oils. A critical review

Unsaturated aliphatic hydrocarbons, or olefins, occur in many crude oils and condensates from numerous basins worldwide. Various types of olefins have been identified in the oils and condensates of North America, Africa, Europe and Asia. These compounds occur as homologous series and as characteristic compounds. This report compiles prior observations and conclusions, and presents new data on their identification, occurrence and origins in oils and condensates. We recognize one primary and three secondary sources for olefins in crude oils and condensates. As primary input, olefins may migrate with other soluble organic compounds directly from the source rock. One secondary origin occurs through the process of migration-contamination, wherein light oils act as solvents for syndepositional olefins that occur along the migration route or within the reservoir section. The other two sources for olefins in oil involve abiogenic alteration of the crude after it has entered the trap. The first such source involves "cold" radiolytic dehydrogenation of saturated hydrocarbons, introduced as a byproduct of decay of uranium, thorium and other radioactive elements among the reservoir minerals. The second source is pyrolysis due to thermal impact from igneous intrusions that occur close to the reservoired oil. From an exploration perspective, primary olefins (i.e., olefinic biomarkers) are useful as oil-oil correlation tools and depositional indicators. Secondary olefins may be used as supporting evidence in reservoir geochemical studies, particularly those involving reservoir continuity determinations. The most intriguing application of secondary radiogenic olefins is their potential as indicators of the time an oil has spent in its present trap. (C) 1998 Elsevier Science Ltd. All rights reserved.