Abstract:

This paper discusses the essential petroleum geological conditions of gas accumulations in the Lower Cretaceous in Southern Songliao basin based on the achievements got in exploration and development in recent years. Source rocks are lacustrine dark mudstone and coal measure strata,reservoir rocks are mainly sandbodies of subaqueous fan delta and delta facies,and local and interlayer cap rocks constitute effective seals of gas accumulations in the Lower Cretaceous. Vertically, there exist three reservoir combinations(lower, middle and upper),of which the middle and upper reservoir combinations are the result of modification and remigration of hydrocarbons in the lower combination. Hydrocarbon source, structure, trap and preservation conditions are the main factors controlling gas accumulation in the Lower Cretaceous. Laterally,gas accumulations in the Lower Cretaceous are separately distributed in fault depressions and become several gas-bearing units.Hydrocarbons in the fault depressions are characterized by their annular distribution,i.e. gas zone, oil & gas zone and oil rim are successively distributed from the center of fault depression to the swell at the boundary.Gas are enriched vertically in three horizons, i.e. Nong'an gas horizon in lower Quantou Formation, Xiaochengzi gas horizon in Denglouku Formation and Wujiazi gas horizon in Yingcheng Formation. All of them are characterized by cross-over and multistage accumulations.

Key words: Southern Songliao basin, Lower Cretaceous, natural gas, accumulation pattern