Products and Depth Limits of Microbial Activity in Petroliferous Subsurface Settings

Abstract

Most microbes live in near-surface or shallow subsurface diagenetic settings where temperatures and pressures are relatively low and nutrients are abundant. With increasing depth, temperature and pressure increase and nutrients become scarce, which leads to a general decrease in the rates of metabolic activity and eventually to the death of all microbes. The major objective of this chapter is to characterize microbial activity and products in petroliferous subsurface settings, with an attempt to place absolute depth limits on the geologically significant groups of microbes. Four groups of microbes are known to be geologically significant in petroliferous subsurface settings, i.e., the aerobic respiratory bacteria, and three anaerobic groups that commonly live in consortia/communities: fermentative, sulfate-reducing, and methanogenic bacteria. These microbes form a number of economically important products and by-products in subsurface settings, including napthenic crude oils and tar in the form of tar sand deposits (aerobic biodegraders), H2S, metal sulfides and elemental sulfur (sulfate-reducing bacteria), and dry methane gas (methanogens). The role of fermentative microbes is mainly in the partial breakdown of organic molecules that then serve as nutrients for the sulfate reducers and the methanogens. In all cases, carbonate cements with distinctive isotopic compositions may be formed as byproducts. In addition, nanobacteria might be important in clay mineral diagenesis in buried sandstones. Various types of these microbes can be used for microbially enhanced recovery of oil. Ultramicrobacteria constitute a special class, as they are injected in a dormant state and then resuscitated in situ to form biobarriers. With increasing subsurface depth, temperature appears to be the principal factor limiting microbial metabolism (and life), besides the availability of suitable nutrients. The activity of all four groups of bacteria is commonly limited by a maximum subsurface depth of about 2000 m, except for rare cases where aerobic biodegradation or sulfate reduction may occur as deep as about 3000 m. Sulfate-reducing bacteria appear to be able to tolerate the highest temperatures (up to at least 110 ° C) and, consequently, they appear to be vegetative at the relative greatest depths. However, sulfate reducers are usually limited to about the same depths (less than 2000–2200 m) as aerobic biodegraders whose waste products they may use as nutrients. The lower limit of the biosphere probably is marked by other types of bacteria at much greater depths.