Keywords: 
• Ordos Basin /
• diagenesis /
• clastic rocks /
• reservoir /
• laumontite

Abstract: An intensive study has been done in the J ingan oil-field by observing five cross sections and eleven wells,analyzing 108 samp leswith thin section, cathodelumination, SEM, X-ray diffraction and related test materials. It is believed that Chang 6 oil-bearing beds underwent a series of diagenesis, such as compaction, cementation, dissolution and so on. The diagenetic phenomenona are very rich and the diagenetic characters are very obvious and complex.According to the analysis of petrology, it is found that Chang 6 oil-bearing interval is composed of middle size sand-stone and fine sub-sand. After analyzing of the whole situation, granularity is much smaller. Little of them are rock fragments feldspar sandstone. Diagenesis of Chang 6 oil-bearing interval can be divided to two stages, early and late stage. Furthermore, they can also be subdivided into two sub-phases, A and B sub-phase. The diagenetic processes that control reservoir properties include the mechanic compaction, cementation and dissolution of grains. At the early diagenesis, mechanical and chemical compaction is very strong, which reduced the primary porosity. Cementation is an important factor to reservoir, including carbonate, clay and laumontite cementation and so on. Laumontite is special mineral in OrdosBasin and is very abundant. Cementation filled the space among grains and blocked the path between grains are another factors resulting in sandstone compaction and very low permeability. But, at the same time,cements also protect grains from further compaction. At the late diagenesis phase, the dissolution of framework grains,especially the dissolution of feldspar and laumontite, is the key to form secondary porosity. They changed the porosity characteristics of sandstone; engendering new benefit storing beds consisted with secondary porosity. Feldspar is dissolved very commonly and severly. They often form dissoluble porosity among grains. Laumontite has deficient dissoluble forms and its dissolution mostly appear along rein, forming irregular size room among grains. This indicates the direction of exp loration and development for oilfield. In a word, the diagenetic study of reservoir property show that mechanical compaction and cementation is two important diagenetic controlling factors, which make oil-bearing physical property bad and porosity down in reservoir.Dissolution, particularly dissolution of laumontite and feldspar is one ofmost important diagenetic factors controlling the physical p roperty of oil2bearing interval and forming better reservoir.