Crude oils from Venezuela, Oklahoma, and New Mexico were water washed in the laboratory at temperatures from room temperature to 80°C, and with water salinities from 0 to 300,000 mg/L in experiments lasting from 7 to 338 hours. The effects of water washing were determined by gas chromatographic and gas chromatography-mass spectrometric analysis of the residual oil. These experiments show that water washing is particularly effective for the C_15- fraction and hydrocarbons are removed in the sequence aromatics, then n-alkanes, then naphthenes. In the C₁₅₊ fraction, no loss of pristane, phytane, steranes, or terpanes occurs, but some aromatics and sulfur compounds, especially dibenzothiophene, are depleted. Although water washing reduces API gravity water washing is unlikely to produce tar layers.

The effects of water washing were simulated with a numerical model based on the equation of diffusion. Predicted oil-water ratios were in good agreement with the experimental values for the light ends, and the model suggests that water washing is very fast in the subsurface. Comparison of oils that appeared to have been water washed in nature with the equivalent unwashed oils showed the expected compositional trends.

Water washing is probably the dominant process affecting crude oil composition in the subsurface when water flows past oils under conditions where bacterial degradation is precluded by temperature (> 80°C) or by lack of dissolved oxygen, and the temperatures are too low for thermal cracking.