The sand reservoirs in the Zaoyuan Oilfield are low in compositional maturityand textural maturity, as revealed by the investigations on the diagenesis andpore structures on the basis of the core analyses. The great variation of the con-tents af various components in the reservoirs is one of the geological factors respon-sible for the digenetic change and complex pore structure of the reservoirs. Thediagenesis in the First and Second Members of the Kongdian Formation is repre-sented by compaction, cementation, replacement, dissolution and authigenic mineralfilling. The pores in the upper part of the Fiist Member of the Kongdian Forma-tion are mainly primary ones with high porosity and permeability and large porethroats, but poorly sorted pore throats. The primary and second pores coexist inthe lower part of the First Member of Kongdian Formation. Most pores in theSecond Member of the Kongdian Formation are secondary ones with fine throatsand poor sorting. The porosity and permeability are not so good as those in theupper part of the First Member of the Kongdian Formation.