Effects of biodegradation on oil and gas field PVT properties and the origin of oil rimmed gas accumulations

Dry gas overlying biodegraded oil rims is a common occurrence in the North Sea oil rimmed gas accumulations (ORGAs) and elsewhere in the world. Wet gas biodegradation occurs in Troll, Frigg, Balder and other ORGAs, with propane preferentially degraded over ethane, resulting in gases becoming dryer and oils heavier. We apply several models of biodegradation and compositional change to examine the effect of biodegradation on the physical properties and phase behaviour of degraded oils and compare the predicted results with data from real degraded oils. Several models, including gas biodegradation, gas cap compression and later interaction of undersaturated oil with dry gas caps can account for the ca. 10 m oil rim presence. Wet gas and oil degradation alone and divergent oil and gas physical properties during biodegradation can also account for the oil legs and dry gas caps. All the models rely on biodegradation of both oil and gas, which not only affects the chemical composition but also dramatically affects reservoir engineering properties such as saturation pressure, GOR and Bo. ORGAs seem to represent altered oil or gas accumulations where the heavy oil and dry gas are in equilibrium and are related. Classical admixed biogenic gas or high maturity origin for the dry gas are not needed to account for these accumulations though methane addition to the accumulations during oil biodegradation may sometimes be a dominant process.