The Illinois basin contains many active oil seeps in varying stages of degradation. This study documents the organic geochemistry of these degraded seep oils and compares them to crude oils from the same basin that have been subjected to controlled field degradation. To document the progress of biodegradation, duplicate limestone slabs doped with nondegraded crude oils have been buried in a flat meadow in central Indiana under homogeneous soil conditions and allowed to degrade for up to one year. At intervals of weeks to months, representative samples of each crude oil have been retrieved and the saturated and aromatic hydrocarbon fractions have been analyzed using gas chromatography (GC), GC-mass spectrometry, and GC-isotopic ratio mass spectrometry. Geochemical analyses show that seep oils exhibit varying stages of biodegradation ranging from mild (loss of <n-C₁₅) to heavy (steranes removed, diasteranes apparently unaffected). Controlled field degradation of the crude oils over the winter months has resulted in slight increases in the pristane/n-C₁₇ and phytane/n-C₁₈ ratios as well as decreases in the abundance of n-alkanes less than C₁₅, indicating the occurrence of minor biodegradation. As seasonal temperatures have increased, biodegradation rates have accelerated. Results of this study are important in identifying and tracking the progress of biodegradation of hydrocarbons in temperate, mid-continental settings, especially at hydrocarbon-contaminated field sites where the initial amount of biodegradation is small.