A suite of common-source crude oils and tar sands from Murray and Garvin counties, Oklahoma, were analyzed in detail to define the physical and chemical effects of progressive biodegradation on a crude oil.

Biodegradation decreases the gravity from 32° to 4°API and increases the sulfur from 0.6 to 1.6 wt % and metals content from 4 to 293 ppm. The changes in chemical class compositions are a decrease in saturates from 55 to 20 wt %, an increase in polars from 21 to 41 wt %, and an increase in asphaltenes from 2 to 21 wt %.

The chemical fossil composition is affected. The n-alkanes, isoprenoids, light aromatics, and light thiophenes are completely removed, with a resultant concentration of naphthenes and heavy or highly branched aromatics and thiophenes. In the most severely degraded samples, steranes are partially removed. Diasteranes and triaromatic steranes, however, appear to be resistant to bacterial attack.