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Progression of methanogenic degradation of crude oil in the subsurface

Barbara A. Bekins, Frances D. Hostettler, William N. Herkelrath, Geoffrey N. Delin, Ean Warren and Hedeff I. Essaid
Progression of methanogenic degradation of crude oil in the subsurface (in Environmental issues of petroleum exploration and production, Yousif K. Kharaka (prefacer) and Nancy S. Dorsey (prefacer))
Environmental Geosciences (June 2005) 12 (2): 139-152


Our results show that subsurface crude-oil degradation rates at a long-term research site were strongly influenced by small-scale variations in hydrologic conditions. The site is a shallow glacial outwash aquifer located near Bemidji in northern Minnesota that became contaminated when oil spilled from a broken pipeline in August 1979. In the study area, separate-phase oil forms a subsurface oil body extending from land surface to about 1 m (3.3 ft) below the 6-8-m (20-26 ft)-deep water table. Oil saturation in the sediments ranges from 10-20% in the vadose zone to 30-70% near the water table. At depths below 2 m (6.6 ft), degradation of the separate-phase crude oil occurs under methanogenic conditions. The sequence of methanogenic alkane degradation depletes the longer chain n-alkanes before the shorter chain n-alkanes, which is opposite to the better known aerobic sequence. The rates of degradation vary significantly with location in the subsurface. Oil-coated soils within 1.5 m (5 ft) of land surface have experienced little degradation where soil water saturation is less than 20%. Oil located 2-8 m (6.6-26 ft) below land surface in areas of higher recharge has been substantially degraded. The best explanation for the association between recharge and enhanced degradation seems to be increased downward transport of microbial growth nutrients to the oil body. This is supported by observations of greater microbial numbers at higher elevations in the oil body and significant decreases with depth in nutrient concentrations, especially phosphorus. Our results suggest that environmental effects may cause widely diverging degradation rates in the same spill, calling into question dating methods based on degradation state.

ISSN: 1075-9565
EISSN: 1526-0984
Serial Title: Environmental Geosciences
Serial Volume: 12
Serial Issue: 2
Title: Progression of methanogenic degradation of crude oil in the subsurface
Title: Environmental issues of petroleum exploration and production
Author(s): Bekins, Barbara A.Hostettler, Frances D.Herkelrath, William N.Delin, Geoffrey N.Warren, EanEssaid, Hedeff I.
Author(s): Kharaka, Yousif K.prefacer
Author(s): Dorsey, Nancy S.prefacer
Affiliation: U. S. Geological Survey, Menlo Park, CA, United States
Affiliation: U. S. Geological Survey, Menlo Park, CA, United States
Pages: 139-152
Published: 200506
Text Language: English
Publisher: American Association of Petroleum Geologists (AAPG), Division of Environmental Geosciences, Tulsa, OK, United States
Meeting name: AAPG-SEPM annual meeting, symposium on Exploration and production environmental issues and best management practices; impacts on water, soils, and ecosystems
Meeting location: Dallas, TX, USA, United States
Meeting date: 20040416April 16-24, 2004
References: 38
Accession Number: 2005-043168
Categories: Environmental geology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., 2 tables, sketch maps
N47°34'23" - N47°34'23", W95°05'33" - W95°05'33"
Secondary Affiliation: U. S. Environmental Protection Agency, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by American Association of Petroleum Geologists, Tulsa, OK, United States
Update Code: 200526
Program Name: USGSOPNon-USGS publications with USGS authors
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