Whole oil gas chromatography shows that petroleum inclusions trapped in 1.05 Ga veins in the Midcontinent rift experienced variable water washing but no significant biodegradation or phase separation. By comparing our results with published experimental data, we estimate that water:petroleum ratios varied from 200:1 to much greater than 9000:1. Extreme water washing with water:petroleum ratios very much greater than 9000:1 likely produced pyrobitumen (solid hydrocarbon) that was locally trapped as primary inclusions in vein calcite. Hydrothermal ore deposits, with their extremely high water:rock ratios, are one of the best places to search for end-member water-washed petroleum. Sedimentary basins, however, with their variable water:rock ratios, also have a wide range of water:petroleum ratios, which significantly affect the composition of crude oils along the flow path between source rock and trap. Water:petroleum ratios may be a useful concept to help evaluate crude oil compositions in a variety of settings.

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