酒泉盆地营尔凹陷异常高压形成机理

酒泉盆地营尔凹陷下白垩统砂泥岩普遍发育超压。通过对地层压实特征、黏土矿物转化及有机质演化特征的分析,揭示了凹陷超压形成的机理。结果表明:①营尔凹陷纵向上发育常压带、浅层超压带、过渡带和深层超压带4个压力带,超压带内具有明显的泥岩密度降低、电阻率减小和声波时差增大的欠压实特征,表明不均衡压实作用是形成该区超压的主要原因;②凹陷内发育3个蒙脱石向伊利石转化带,第一迅速转化带对应于浅层超压带,第二迅速转化带对应于深层超压带,反映黏土矿物转化与凹陷超压有一定的联系;③深层有机质演化程度较高(镜质体反射率Ro>0.6%),与浅层超压带相比,生烃作用对深层超压带的超压贡献更大。由此认为,营尔凹陷的下白垩统地层超压是不均衡压实作用、黏土矿物脱水作用和生烃作用共同影响的结果,并且隆起带和洼陷带的成因机理有所差异。

Overpressure-Generating Mechanisms in Ying′er Sag,Jiuquan Basin

Ying′er Sag of eastern Jiuquan Basin developes overpressure in Lower Cretaceous sequence.We have analysed the overpressure-generating mechanisms according to compaction of mudrocks,transformation of clay minerals and organic matter evolution.The results show 1) With four pressure zones-normal pressure zone,shallow overpressure zone,transition zone and deep overpressure zone dividing vertically,density decreasing,resistivity decreasing and sonic transition time increasing in overpressure zones reveals disequilibrium compaction as the main cause of overpressure;2) There are three rapid conversion zones of smectite to illite transition identified in Ying′er Sag,which the first rapid conversion zone coincides with the shallow overpressure zone while the second rapid conversion zone corresponds to the deep overpressure zone which implies that transformation of clay minerals is associated with the generation of overpressure;3) Compared to shallow overpressure zone,hydrocarbon generation contributes more to deep overpressure zone where organic matter are mature(Ro>0.6%).It can be concluded that overpressure in Ying′er Sag are generated by disequilibrium compaction with transformation of clay minerals and hydrocarbon generation.Morever,the mechanisms of uplift and depression are different.