Abstract: Predrilling reservoir quality prediction includes porosity prediction and permeability prediction. The porosity of clastic rock reservoirs is determined by both sedimentary facies and diagenesis. But for the same sedimentary facies or the similar lithology, the porosity of reservoirs is mainly controlled by diagenesis. According to this, a model of porosity prediction for clastic rock reservoirs using diagenetic index I_D was established, with the predicting error of less than 2%. The Deng-3 Member (K₁d³) in Wangjiatun region, the Songliao Basin was taken as an example to illustrate the application of this model. The reservoirs were composed of lithic arkose and feldspathic litharenite, and were deposited in shallow river facies. At present, the reservoirs in northeastern Wangjiatun region are in the late diagene-tic stage B, with the maxi-mum porosity of 14%-16%; and the reservoirs in western Wangjiatun region are in the late diagenetic stage C, with poorer reservoir petrophysical properties and the maximum porosity of 10%-12%.