Abstract:

The shale within Chang 7 Member of Yanchang Formation in Triassic is an important source rock of Mesozoic reservoirs in Ordos Basin. The paleopressure in the period of accumulation plays a vital role for the hydrocarbon migration, and the changes of paleoporosity represent the characteristics of the paleopressure evolution. So according to the mechanism of hydrocarbon fluid expulsion and overpressure release from the period of accumulation to the maximum burial depth，this article presents a new method that can extrapolate the paleoporosity of shale within Chang 7 Member in the period of accumulation on the basis of the relationship between porosity increment and the maximum depth. Through analyzing the characteristics of present shale compaction profiles, there is an exponent relationship between the porosity increment and the maximum depth of undercompacted shale within Chang 7 Member. Then by using this relationship, the increment of shale porosity in the period of accumulation can be extrapolated and by combining the porosity of normal compaction trend, the total porosity of undercompacted shale within Chang 7 Member in the period of accumulation can be restored effectively. The paleoporosity of shale within Chang 7 Member in the period of accumulation is generally high and the porosity ranges from 23% to 35%, indicating that the undercompaction of shale in the accumulation period is strong, and the abnormal high pressure is the main source rock expulsion power.

Key words: porosity, undercompacted shale, abnormal high pressure, the shale within Chang 7 Member, Yanchang Formation, Jiyuan area