Oil is trapped in strata. On top of the formation are the gases, in the middle is the oil and below is the water.
When the perforation procedes through this oil trap, the trapped gases can be collected. This method of beneficiation is called "gas channeling". The North Sea oil field pumps this gas to Norway, Denmark, Sweeden and the English Isles where it is purified, distributed and sold.
The extracted oil, Brent Type, is sold apart.

http://www.onepetro.org/mslib/servlet/onepetropreview?id=SPE...
However, reservoir simulation surprisingly demonstrated that when gas channeling from the upper gas layer is taken into account, oil production rates ...

http://cat.inist.fr/?aModele=afficheN&cpsidt=6278951
Consecuently, the drilling of horizontal wells is a potential option to minimize gas channeling and maximize the drainage area, increasing crude production,

http://pubs.acs.org/cgi-bin/jtext?iecred/39/i08/abs/ie990818...
Viscous fingering and gas channeling through higher permeability strata can result in significant gas-handling problems at production wells without the ...