Impact of formation water geochemistry and crude oil biodegradation on microbial methanogenesis. (August 2016)

• Record Type: Journal Article
• Authors: Shelton, Jenna L.; McIntosh, Jennifer C.; Warwick, Peter D.; McCray, John E.
• Is Part Of: Organic geochemistry. Volume 98(2016:Sep.)
• Page Start: 105
• Page End: 117
• Publication Date: 2016-08
• Subjects: Oil field methane -- Hydrogeochemical tracers -- Methanogenic crude oil biodegradation; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2016.05.008
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: Six Louisiana Paleocene–Eocene Wilcox Group oil fields sampled. Oil fields produced oil with varying extent of biodegradation. Microbial methanogenesis is likely occurring in only three fields. Extent of oil alteration and methanogenesis did not correlate across all fields. Salinity likely impacted oil alteration and methanogenic activity. Abstract: Converting non-producible crude oil to CH4 via methanogenic crude oil biodegradation in oil reservoirs could serve as one way to increase our energy profile. Yet, field data supporting the direct relationship between methanogenesis and crude oil biodegradation are sparse. Indicators of methanogenesis, based on the formation water and gas geochemistry (e.g. alkalinity, δ 13 C–CO2 ) were compared with indicators of crude oil biodegradation (e.g. pristane/phytane and n -alkane ratios) from wells in the Wilcox Group of Louisiana to determine if increases in extent of methanogenesis were related to increases in extent of crude oil biodegradation. Shallow wells (393–442 m depth) contained highly biodegraded oils associated with low extent of methanogenesis, while the deepest (> 1208 m) wells contained minimally degraded oils and produced fluids suggesting a low extent of methanogenesis. Mid-depth wells (666–857 m) in the central field had the highest indicators of methanogenesis and contained moderately biodegraded oils. Little correlation existed between extents of crude oil biodegradation and methanogenesis across the whole transect (avg. R 2 = 0.13). However, when wells with the greatest extent of crude oil biodegradation were eliminated (3 of 6 oilfields), better correlation between extent of methanogenesis and biodegradation (avg. R 2 = 0.53) was observed. The results suggest that oil quality and salinity impact methanogenic crude oil biodegradation. Reservoirs indicating moderate extent of crude oil biodegradation and high extent of methanogenesis, such as the central field, would be good candidates for attempting to enhance methanogenic crude oil biodegradation as a result of the observations from the study.