Abstract：Strong amplitude anomalies occur along the horizon of Triassic Chang 8 Member on 2000km2 3D seismic data in the Southwestern Ordos Basin.The amplitude anomaly morphology and orientation contradict with the northeast extension of the channel sandstone Chang 8 Member concluded from earlier geologic studies in this region.Such a disagreement heavily affects the reservoir identification of Chang 8 according to seismic attributes and inversion method.Based on analysis of 60 boreholes and 700km2 3D seismic data in Daijiaping,the following observations are obtained:A.Chang 8 northwest amplitude orientation trend agrees exactly with Chang 7 amplitude trend on the two horizon slices.The amplitude anomalies are irrelative to loess plateau surface topography,and different from amplitude extension trend of other horizons such as Cretaceous,Jurassic,Chang 6 member and Chang 9 Member; B.The amplitude anomalies are finitly affected by coal thickness and shielding of Jurassic Yan'an Formation,and relative to Triassic Chang 7 shale thickness and velocity.The amplitude values on Chang 7 horizon slice conforms with amplitude values of synthetic traces from wells located on the corresponding positions on the slice map.The enhancement of amplitude is caused by reduced average wave impedance and increase in thickness of Chang 7 shale.The impedance reductions are brought about further by higher residual organic matter content remained in the shale where faults and fractures do not occur.Therefore,northwest amplitude anomalies on Chang 8 horizon slice are caused mainly by change in thickness of Chang 73 shale incised and thinned by Chang 72 gravity flow from the NW direction.and somewhat by a comprehensive effect of both lack of faults and fractures and inadequate hydrocarbon expulsion from Chang 7 shale; C.The false amplitudes impact heavily reservoir identification on 3-D seismic in the area and need to be removed in the next seismic explorations.
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