Asset retirement obligation (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Asset retirement obligation, January 1	$ 2,702	$ 2,573
Acquisition of oil and gas properties	0	0
Revisions in the estimated cash flows	0	0
Liability incurred upon acquiring and drilling wells	0	0
Liability settled upon plugging and abandoning wells	0	0
Accretion of discount expense	68	129
Asset retirement obligation, December 31	$ 2,770	$ 2,702