Abstract:

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 mainly ranges 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 oil-gas exploration.

Key words: normal compaction, abnormal compaction, paleo-porosity, paleo-pressure, hydrocarbon migration