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Helge Løseth, StatoilHydro (Norway)
Lars Wensaas, StatoilHydro (Norway)
Børge Arntsen, StatoilHydro (Norway)
Marita Gading, StatoilHydro (Norway)
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In an active petroleum system the amount of trapped hydrocarbons is the difference between the charge volume and the leakage or otherwise destroyed volume. Here we study how the leakage processes above a hydrocarbon filled trap is expressed on seismic data.
A variety of seismic anomalies related to hydrocarbon leakage like gas chimney, pipe structures, pockmarks, brights and dim zones have been studied. These anomalies can sometime easily be observed on seismic data but what rock structures and hydrocarbon saturations they represent are less obvious. Gas chimneys, for example, are often described but seldom explained. On seismic data it is basically seen as a noise zone. Wells drilled inside gas chimneys typically have:
• higher pore fluid pressure
• higher mud gas readings
• higher mud gas wetness
• more hydrocarbon shows
• lower velocities
• higher temperatures
than wells drilled outside gas chimneys. We argue that a fractured cap rock that is partly filled with gas best explain the seismic and well observations. By combining seismic observations with outcrop data, seismic modelling and knowledge of leakage process we have tried to explain several of the above mentioned anomalies.
During a typical cap rock leakage study we have found it useful to first observe, describe and map all anomalies related to hydrocarbon leakage. Secondly, the individual anomalies need be interpreted. Last, but not least, we have found it useful to group leakage anomalies from each trap into a leakage zone. A leakage zone has a root where the leakage from the reservoir initiates, a body or the zone itself where vertical movements of hydrocarbons occur and a top where the leakage terminates. This approach may help to discriminate high-rate leakage zones from low-rate leakage zones.
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