Abstract: In a section of an oil field a first and only bore hole (5) is drilled through the strata (3) carrying heavy oil until the underlying stratum (4) void of heavy oil and enlarged to a cavern-like collection chamber (9). Around this first bore hole (5) a plurality of second bore holes (6) are drilled through the strata (3) carrying heavy oil to the stratum (4) in a pattern comprising several circles. A further pattern of third bore holes (7) is drilled also in a circle-like pattern through the strata (5) carrying heavy oil, which third bore holes (7) are followed by drainage channels (8), which extend to the collecting chamber (9). By means of electrodes arranged in the second bore holes (6) the heavy oil strata are preheated and thereafter steam is injected through the same bore holes (6) in order to drive flowable heavy oil against the third bore holes (7).