Biodegradation of oil in uplifted basins prevented by deep-burial sterilization (vol 411, pg 1034, 2001)

Biodegradation of crude oil by bacterial activity-which has occurred in the majority of the Earth's oil reserves(1)-is known to reduce greatly the qua lity of petroleum in reservoirs(2). For economically successful prospecting for oil, it is therefore important to understand the processes and conditi ons in geological formations that lead to oil biodegradation. Although rece nt studies speculate that bacterial activity can potentially occur up to te mperatures as high as 150 degreesC (refs 3, 4), it is generally accepted th at effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degreesC (ref. 2). This app ears, however, to be at odds with the observation that non-degraded oils ca n still be found in reservoirs below this temperature. Here we compile data regarding the extent of oil biodegradation in several oil reservoirs, and rnd that the extensive occurrence of non-biodegraded oil in shallow, cool b asins is restricted to those that have been uplifted from deeper, hotter re gions of the Earth. We suggest that these petroleum reservoirs were sterili zed by heating to a temperature around 80-90 degreesC during deep burial, i nactivating hydrocarbon-degrading organisms that occur in the deep biospher e. Even when such reservoirs are subsequently uplifted to much cooler regio ns and filled with oil, degradation does not occur, implying that the steri lized sediments are not recolonized by hydrocarbon-degrading bacteria.