Abstract:

The deep zone of Songliao basin is made up of more than 30 separated fault depressions where volcanic gas reservoir is the main exploration target. Starting from the analysis of the formation characteristics of deep fault depression and analyzing the factors of controlled source and controlled gas reservoir, the conditions of giant volcanic gas field in fault depressions are summarized for guiding the exploration. The comprehensive research suggested that the deep fault depressions are controlled by two faults system that are NNE-NE and NNW-NW strikes, along the strike direction of faults controlled fault depressions. The typical asymmetrical model of volcanic eruption pattern controlled by faults is characterized by one smallscale side on the higher part of faults and by the other large-scale side in the lower part of faults, as molten magma flowed from the top down, with zonal distribution along the strike direction of faults controlled fault depression. Generally, each fault depression is composed by one or more fault troughs which control the distribution of source rock, so each fault depression has its own independent petroleum system and the key factor of gas accumulation is favorable spatial configuration between fault troughs and volcanic rocks. The deep volcanic gas reservoirs are characterized by nearby distribution surrounding source rocks and main troughs. The faults controlled the formation and distribution of fault depressions, fault troughs and volcanic rocks, and improved the channel conditions for oil-gas migration and accumulation, which indicates fault structural zone close to hydrocarbon bearing troughs is gas enrichment region. In short, deep faults controlled regional distribution of giant volcanic gas field. Further exploration practice prove that: for oil and gas exploration of fault basins, main fault troughs which controlled hydrocarbon generation is the key unit for the evaluation and exploration. As long as the high quality hydrocarbon source rock are developed in the fault troughs, with good space configuration with volcanic rocks, small and medium sized fault depressions with area smaller than 3,000 km² also can form giant volcanic gas field (proven geological reserves of more than 300×108 m³), which have presented as exploration strategy changed from searching for big lake basins or fault depressions to main hydrocarbon bearing troughs which need to be proved.

Key words: Songliao basin, deep fault depressions, faults controlled fault depression, conditions of giant volcanic gas field, exploration practice