东营凹陷陡坡带油井前兆异常及其动力学成因讨论

PRECURSORY ANOMALY OF EARTHQUAKE OBSERVED IN OIL-WELL IN STEEP-SLOPE ZONE OF DONGYING DEPRESSION AND ITS DYNAMIC GENESIS

分析了东营凹陷陡坡带断裂体系 ,研究了东营凹陷陡坡带油井前兆异常与地震活动的关系 ,探讨了油井前兆异常的动力学成因。孕震过程是应力场变化和调整的过程 ,应力场变化导致孔隙压力变化 ,使得油井产生前兆异常。特定的地质环境是油井前兆异常的基础 ,应力转化为孔隙压力是油井前兆异常的前提。油井前兆异常是正异常还是负异常 ,取决于油井所处的地质环境在孕震过程中地应力是增加还是减小 。

The fault systems in the steep-slope zone of Dongying depression consist of NEE-and NWW-trending faults.The faults are deep and narrow,having dip angle of above45°and a throw of more than3000m.The modern stress field in the steep-slope zone is currently very active.As the petroleum resources in this zone are very abundant,a great deal of oil wells have been drilled.All this specific geological conditions provide the foundation to seek for the relation of precursory anomaly in oil well to seismic activity and to discuss the dynamic genesis of this anomaly. This paper presents the analysis of the relationship between the precursory anomaly and earthquake observed in several tens of oil wells in steep-slope zone of Dongying depression in the past3decades.The results of analysis show that the precursory anomaly observed in oil well is very complex.Some wells may exhibit precursory anomaly,and the other may not.The type of anomaly is also very complicated.It might be a positive or negative anomaly,and a short-term or long-term anomaly,while the amplitude of anomaly is variable in different wells. Furthermore,the dynamical genesis of oil well precursory anomaly is discussed.It is generally ac-cepted that the generation of earthquake is the process of transformation and adjustment of regional stress field.The concentration of stress under a specific geological condition is the main factor that causes the precursory anomaly in oil well.As a lot of faults and lateral discontinuity of strata occur in the steep-slope zone of Dongying depression,the zone becomes stress concentration zone during the development process of earthquake,so precursory anomaly of earthquake is easy to occur in oil well of this zone.The precondition of the occurrence of precursory anomaly depends on the facts that the stress change is translated into the change of pore pressure.If the oil well is located in compression area during the development process of earthquake,the positive anomaly will occur in the well,otherwise the negative anomaly will occur.The amplitude of anomaly depends on the variation amplitude of pore pressure.Precursory anomaly of earth-quake in oil well is affected synchronously by various factors,such as the change of stress,tectonic set-ting,and the property of stratum etc,so the analysis of the dynamic genesis of precursory anomaly is criti-cal to the understanding of the relationship between earthquake and oil well precursory anomaly.