Middle and large oil/gas fields are largely distributed in the deep fault depressions along the two sides of NNE Sunwu-shuangliao lithosphere fault crossing the central part of Songliao basin.These fault depressions has favorable geological conditions for hydrocarbon reservoiring.First

very thick source rocks were deposited in the inherited large abyssal-hypabyssal lakes caused by constant and quick subsidence of the fault depression and the surrounding depressions'blockade of the large proveniences on basin edge. Secondly

two types of reservoir rocks-volcanics and sandstone-were formed when a severe strike slip extensional movement happened in early-middle Yanshan period and caused the development of lacustrine facies mudstone with volcanic rocks and detrital rocks interbedded.Thirdly

after the formation of fault depression

the continuous quick subsidence

together with the lacustrine facies mudstone and shale sequences with abnormal pressure in drape depression

forced the source rocks go through thermal evolution and hydrocarbon generation and shaped the cap rocks into favorable sealing material for natural gas in the depressions.Fourthly

as the depressions experienced stable subsidence with no severe lifting in the later period

well-preserved regional cap rocks provide favorable preservation condition for gas reservoirs.Finally

much less activities of basal deep faults in the Late Cretaceous induced less destruction to the gas reservoirs. A study on the accumulation pattern in deep fault depressions shows that middle and large oil/gas fields are mainly distributed in the inherited latitudinally trending nose-like structure

stratigraphic overlapping pinch out traps

and complex lithological traps

among them the uplift zone between two deep fault depressions is where the largest oil/gas fields are distributed.