The Dynamic Interplay of Oil Charge, Basin Dynamics, Caprock Leakage and Gas Generating Biodegradation Produces Heavy and Super-Heavy Oil Fields
Worldwide, giant heavy and super heavy oil (bitumen) resources are found near surface (< 1 km depth) in reservoirs that have experienced temperatures less than 80°C since filling. Heavy oil and bitumen resources develop by extensive in-reservoir oil biodegradation resulting in a wide range of oil properties reflecting a complex interplay of oil charge rate and composition, biodegradation in oil-water transition zones, geologically controlled in-reservoir diffusive mixing and biogenic gas leakage or removal over geological time. Worldwide, observed compositional and viscosity gradients are maintained by mixing fresh oil charge near the top of reservoirs and heavy oils derived from biodegradation near the base of the reservoirs at rates comparable to charge rates of oil fields. These compositional variations can be used to assess well placements and reservoir compartmentalization. Across the Alberta oil sands, elevated CO2, high CH4 and low C2+ gas contents, steep oil compositional and viscosity gradients, high aqueous bicarbonate concentrations and isotopic values in equilibrium with enriched δ13CCO2 gas signatures are indicative of present day biodegradation. The dominant reaction pathway of subsurface hydrocarbon biodegradation as methanogenic alkane degradation by CO2 reduction, which produces large volumes of isotopically light methane and heavy CO2 in solution gas (up to 6 times reservoir volumes). This generated biogenic gas should have displaced oils from the traps; however, thin or absent gas caps in shallow heavy oil reservoirs and degraded oil in many oilfield caprocks suggest seal leakage is common. The paucity of large gas caps, evidence of methane-rich and sometimes oil charged cap rocks, anomalously high formation water alkalinity and enriched aqueous δ13Ccarbonate values in shallow Alberta biodegraded oil reservoirs point to leaky reservoir top seals and dissolution of biogenic CO2 into the water phase. Indeed we consider top seal leakage of biogenic gas required to produce heavy oil and super heavy oil fields to maintain active biodegradation at the oil-water contact by transport of nutrients through the water phase. This would otherwise be curtailed by petroleum completely filling the reservoir to the underseal. The abundance of heavy oil and super heavy oil in shallow reservoirs reflects leaky reservoir seals at these depths and systematic removal of large volumes of gas generated by petroleum biodegradation and sometimes spill of oil.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009