Overpressure Mechanisms and Quantitative Evaluation of Relative Contribution for Yinggehai Formation in Ledong Area of Central Diapir Zone,Yinggehai Basin
Overpressure Mechanisms and Quantitative Evaluation of Relative Contribution for Yinggehai Formation in Ledong Area of Central Diapir Zone,Yinggehai Basin
Overpressure is widely developed of Yinggehai Formation in Ledong area of the central diapir zone.The overpressure mudstone is characterized by abnormally higher acoustic travel time,lower resistivity and density than that of the normal pressured mudstones.In this paper,the contribution rate of overpressure caused by different mechanisms is quantitatively evaluated based on the comprehensive analysis of the overpressure mechanisms of Yinggehai Formation in the central diapir zone of Yinggehai basin.The results indicate that the main causes of overpressure in Yinggehai Formation in Ledong area of the central diapir zone include undercompaction,hydrocarbon generation of organic matter and overpressure transfer.The contribution of clay mineral transformation to the overpressure is not obvious.The quantitative study of overpressure contribution rate reveals the contribution of fluid pressure transfer and hydrocarbon generation to mudstone overpressure in the diapir area of Yinggehai Basin.The contribution rate of overpressure caused by undercompaction in the Ledong Area of the Central Diapir Zone of Yinggehai Basin ranges from 38％to 100％.As the burial depth increases,the contribution rate of undercompaction-induced overpressure gradually decreases.The main overpressure mechanisms in the first member of Yinggehai Formation is undercompaction with the shallow buried depth and weak hydrocarbon generation capacity.While the hydrocarbon generation capacity of mudstone in the second member of Yinggehai Formation is significantly enhanced,the contribution caused by hydrocarbon generation is up to 51.53％.Microfractures and faults are developed in the diapir area,which make the deep fluid migrate to the shallow layer and produces pressure transfer.The maximum contribution rate of pressure transfer is more than 50％in Yinggehai Formation.
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