Secondary Microbial Methane From Very Slight Biodegradation of Undersaturated Oil in a Deep Hot Reservoir: Geochemical Evidence

Milkov, Alexei V.¹, Leon Dzou¹ (1) BP America, Houston, TX

Quantitative importance of secondary microbial gas in petroleum reservoirs has been recently suggested, but little indisputable evidence of that process exists. A finding of early mature undersaturated oils with low gas/oil ratios enables us to document secondary microbial methane generation during very slight biodegradation in a deep, hot reservoir in the ultra-deep-water Gulf of Mexico. In three studied gas samples, methane is enriched in ¹³C (d¹³C is from -63‰ to -64‰) relative to pure thermogenic methane (estimated d¹³C is from -71‰ to -67‰) and pure primary microbial methane (d¹³C is -67‰). Carbon dioxide in gases has d¹³C values which negatively correlate with d¹³C values of pure thermogenic methane. Methane is unusually enriched in ²H relative to associated ethane. Some extracted oils are depleted in long-chain alkyl aromatics. These lines of geochemical evidence suggest anaerobic microbial degradation of oil and subsequent reduction of resulting CO₂ to methane. Biodegradation in the studied reservoir is not extensive likely because suppressed microbial activity at relatively high temperatures (~80-115°C during reservoir filling by petroleum in the last 5 Ma) cannot efficiently metabolize large and continuous supply of fresh petroleum in this reservoir. Our results suggest that biodegradation, albeit very slight and insufficient to affect bulk properties and commercial value of oils, may occur at temperatures higher than currently accepted “biodegradation floor” of 80°C. Although specific geo-biological details of secondary microbial methane generation are yet unclear, this process may be partially responsible for charging some of the largest gas and gas hydrate fields in the world.