Gas accumulation mechanism in Denglouku Formation of Changling fault depression, southern Songliao Basin, China

摘要

The Changling gas field is occurs in tight sandstone reservoirs of the Lower Cretaceous Denglouku Formation in the Changling fault depression,southern Songliao Basin,China,which constitutes a new gas-producing area in the depression. Using information on the source-reservoir-cap rock assemblage of the Denglouku Formation,fault activity,and single well burial history of well CS1,together with data on reservoir fluid inclusion and laser Raman spectroscopy,we described the formation of the Changling gas field and determine that this fault depression did not possess suitable conditions for hydrocarbon generation. Coal-derived methane generated from underlying hydrocarbon source rock accumulated in the Lower Cretaceous Yingcheng Formation. At the end of the Late Cretaceous Qingshankou Stage,underwater volcanic eruptions occurred in the northern part of the Changling gas field near Qian'an,resulting in the reactivation of deep faults. Mantle-sourced inorganic CO2 migrated along faults to hydrocarbon gas reservoirs in volcanic rocks of the Yingcheng Formation; Meanwhile,displaced methane( hydrocarbon gas) migrated upward to sand reservoirs of the Denglouku Formation.The methane accumulated and formed secondary gas reservoirs,Therefore fault activity was the main factor controlling the generation of gas reservoirs in the Denglouku Formation. The main accumulation period of the Yingcheng hydrocarbon gas reservoirs was 82 Ma. Whereas gas reservoir formation in the overlying Denglongku Formation was 79 Ma,slightly later than the time of formation of the Yingcheng gas reservoir in CS1 well area.At 79 Ma,the burial depth of the Denglouku Formation was 1 800--2 000 m,the diagenesis is relatively weak and the physical properties of the reservoir are relatively favorable for accumulation. This period is not only at gas generation peak time of three sets of source rock but also at the reactivation of deep faults during the formation of fault-bound depressions,thereby providing favorable conditions for the migration and accumulation of methane.