Biodegradation of oil in uplifted basins prevented by deep-burial sterilization

Abstract

Biodegradation of crude oil by bacterial activity--which has occurred in the majority of the Earth's oil reserves--is known to reduce greatly the quality of petroleum in reservoirs. For economically successful prospecting for oil, it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation. Although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees C (refs 3, 4), it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees C (ref. 2). This appears, however, to be at odds with the observation that non-degraded oils can still be found in reservoirs below this temperature. Here we compile data regarding the extent of oil biodegradation in several oil reservoirs, and find that the extensive occurrence of non-biodegraded oil in shallow, cool basins is restricted to those that have been uplifted from deeper, hotter regions of the Earth. We suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80-90 degrees C during deep burial, inactivating hydrocarbon-degrading organisms that occur in the deep biosphere. Even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil, degradation does not occur, implying that the sterilized sediments are not recolonized by hydrocarbon-degrading bacteria.

Thermodynamic constraints on methanogenic crude oil biodegradation

“…The geothermal history of petroleum basins has been shown to exert a broad control on the occurrence of biodegraded petroleum reservoirs and has led to the concept of palaeopasteurization or palaeosterilization, which prevents the biodegradation of crude oil in subterranean formations (Wilhelms et al, 2001). The limited availability of nutrients in aquifers associated with petroleum reservoirs and geological constraints on the physical interaction of water and oil legs have also been proposed as factors which may limit inreservoir oil biodegradation Larter et al, 2006).…”

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

“…Biodegradation of LMWHC is considered to occur at temperatures below approx. 80°C (Wilhelms et al, 2001), which in the Batumi subsurface might prevail not deeper than about 2.4 km bsf.…”

Evidence for extensive gene flow and Thermotoga subpopulations in subsurface and marine environments

“…2812B; Figure 5) suggests that the Thermotoga population has entered the Tuna reservoir recently from a marine environment. The temperature of the reservoir is reported to be 105 1C (Sutcliffe et al, 2013), which is considerably higher than the proposed 80-90 1C threshold temperature for sustaining life in oil reservoirs (Stetter and Huber, 1999;Wilhelms et al, 2001), suggesting that Thermotoga sp. A7A may be inhabiting the petroleum pipelines, where the temperature is lower.…”

“…As for subsurface sediments in general, two alternative hypotheses explaining where these microorganisms originate from have been proposed. In the first, 'burial and isolation' hypothesis (a corollary from the paleosterilization model of oil biodegradation), the microorganisms buried with the sediments adapt to the new environment as the oil reservoir is formed (Wilhelms et al, 2001). Under this hypothesis, the microbial community within an oil reservoir is isolated, and there is no subsequent migration of other microorganisms into the reservoir from adjacent areas.…”

“…The support for this hypothesis comes from examination of cool reservoirs with no biodegraded oil. During their geological history, the reservoir sediments were buried at depths where the temperature exceeds the survival limit of hydrocarbon-degrading microorganisms, resulting in 'paleosterilization' (Wilhelms et al, 2001). The conjecture is that new organisms have not colonized such oil reservoirs after their subsequent uplift and cooling, and that is why no oil biodegradation is presently observed.…”