沉积盆地主要超压成因机制识别模式及贡献

Recognition Model and Contribution Evaluation of Main Overpressure Formation Mechanisms in Sedimentary Basins

沉积盆地中超压形成往往受到多种因素的控制,使得对每种超压机制的识别及其评价较为困难。将超压的形成机制总结为4类:不均衡压实、流体膨胀、超压传递和侧向构造应力。建立了4类主要超压形成机制的综合识别模式:不均衡压实和侧向构造应力增压地层中孔隙度表现为明显的高异常,而流体膨胀、超压传递增压地层中孔隙度无明显异常;4种类型增压机制在声波速度与垂向有效应力、密度与声波速度的变化关系曲线上表现出明显差异,不均衡压实增压地层中声波速度与垂向有效应力的变化应遵循正常压实作用的指数变化关系,而流体膨胀、超压传递、侧向构造应力增压地层明显偏离正常压实趋势线;不均衡压实、侧向构造应力增压地层中声波速度与密度的变化遵循正常压实趋势线,而流体膨胀、超压传递增压地层则偏离正常压实趋势线。依据垂向有效应力的减小量等于流体膨胀、超压传递、侧向构造应力中一种或多种共同作用产生的流体增压量的假定,结合实际地质条件分析,确定并评价相应增压机制地层中产生的流体增压量及其对地层超压的贡献率。尽管该假设下的评价结果会低估该机制的增压作用,但仍可为沉积盆地中复杂地区的超压识别和评价提供较好的方法。

Overpressures in sedimentary basins are usually controlled by many factors,which make recognition and evaluation of each overpressure mechanism be difficult.The paper divided the causes of overpressure into four general categories：disequilibrium compaction,fluid expansion,overpressure transfer,and lateral tectonic stress.The comprehensive recognition model of these overpressure mechanism types was established.Disequilibrium compaction and lateral tectonic stress overpressures are typically associated with abnormally high porosities and overpressures generated by fluid expansion and overpressure transfer mechanisms are not associated with a porosity anomaly.Sonic velocity-vertical effective stress and sonic velocitydensity plots are used to distinguish between overpressures generated by the four types.On a sonic velocity-vertical effective stress plot,sediments that have undergone disequilibrium compaction remain on the exponential function curve of normal compaction loading.However,overpressures generated by fluid expansion or overpressure transfer or lateral tectonic stress follows a sonic velocity-vertical effective stress path away from the loading curve.On a sonic velocity-density plot,sediments that have undergone disequilibrium compaction or lateral tectonic stress remain on the loading curve.However,overpressures generated by fluid expansion or overpressure transfer follow a sonic velocity-density path away from the loading curve.On the basis of the hypothesis that the decreasing magnitudes of vertical effective stress are equal to the overpressures generated by fluid expansion or overpressure transfer or lateral tectonic stress,the overpressure mechanisms are assured and the magnitudes of overpressure and the relative contribution generated by these overpressures were evaluated combined with the regional geological conditions.Although the evaluation results will underestimate the overpressure generated by these mechanisms,these can provide a good method for recognition and evaluation of overpressure in complex regions of sedimentary basins.