陕北斜坡中部泥岩压实特征分析及长7段泥岩古压力恢复

Characteristics of Mudstone Compaction and Paleo-pressure Restoration of Chang 7 Shale in the Middle of Shanbei Slope

陕北斜坡中部长7段泥岩现今欠压实幅度不是很大,因此长7段烃源岩生成的油气运移动力受到怀疑,但现今的压力特征并不能反映成藏期的压力。首先以鄂尔多斯盆地陕北斜坡中部单井泥岩压实曲线为基础,对该区泥岩压实特征进行了综合研究,结果表明研究区内泥岩压实均由正常压实段和异常压实段构成,虽然欠压实幅度不大,但欠压实现象普遍存在;正常压实趋势具有较好的一致性,地表声波时差值集中在320μs/m左右;现今压实剖面的异常压实段起始深度由东向西逐渐增大,最大古埋深期异常压实带起始深度则分布在2 250~2 400 m。然后恢复了长7段泥岩古孔隙度,并预测了主力烃源岩层（长7段）成藏期泥岩古地层压力。研究区长7段泥岩在成藏期普遍存在异常高压,南部超压明显,压力系数达1.56,反映了成藏期烃源岩具有较强的排烃动力;古压力值为17~28.5 MPa,古压力系数为1.12~1.56,两者在平面上的分布都呈现出西南部高东北部低的特征,很好地解释了研究区内现今油气的分布特征。该研究成果对下一步油气勘探具有重要的指导意义。

The undercompaction increment of Chang 7 shale is not great in the middle of Shanbei Slope,so the power of the hydrocarbon migration in Chang 7 is suspected. However,the characteristics of pressure today,does not reflect the pressure when hydrocarbons were accumulating. Firstly,this paper,based on the mudstone compaction curve,made a study on the characteristics of mudstone compaction in the middle of Shanbei Slope,Ordos Basin. The results show that the mudstone compaction sections are all composed of normal compaction and abnormal compaction. Although the undercompaction increment is not great,the phenomenon of undercompaction is widespread in this area. There is a better consistency among the normal compaction tendencies,and the acoustic travel time on the surface for wells is about 320 μs / m. The initial depths of undercompaction for present compaction section gradually increase from east to west. However,after restoring the largest burial depth,the initial depths of undercompaction distribute between 2,250 m and 2,400 m. Secondly,this paper restored the paleo-porosity and predicted the paleo-pressure of the source rock bed（ shale in Chang 7） when hydrocarbons were accumulating. The phenomenon of overpressure is widespread in Chang 7 shale during the period of accumulation,especially in the southern part of the area,and the maximum pressure coefficient is 1. 56,which reflects a strong hydrocarbon expulsion power in the source rock during that time. The pressure mainlyranges from 17 MPa to 28. 5 MPa,and the pressure coefficient mainly ranges from 1. 12 to 1. 56. They have the same distribution,with the characteristics of northeastward-decreasing,and it could well explain the distribution of reservoirs in the middle of Shanbei Slope. This research has the important guiding significance for further oilgas exploration.