Keywords: 
• diagenesis

Abstract: On the basis of a careful analysis of thin section observation, scanning electron microscope and Xdiffraction data, the diagenetic stage of Yanchang Formation in Fuxian exploration area is the A stage of late diagenesis and the main diagenic metamorphism include mechanicalchemical compaction, consolidated fill and dissolution. The mechanicalchemical impact and consolidated fill are the main factors causing the loss of porosity for the sandstones. Carbonate, hydrosyalite and quartzfeldspar overgrowths are the main cements that reduced the reservoir quality of the sandstones．Thin chlorite film around detrital grains prevented from part of quartz and feldspar overgrowth and carbonate cement precipitation, which cause a part of primary pore preserved and hence caused a better reservoir quality. The main factor for the great quantity secondary pore is dissolution partly of detrital feldspars and laumonite cements developed in early stage by the acidic liquid related to decomposition of organic mater during hydrocarbon enrichment in the late diagenesis and meteoricowaters during epidiagenesis.
Laumonite cement developed mainly in Chang 3 Oilbeds which were owing to the high content of feldspar clastic and the alteration to feldspar at the diagenetic process. Kaolinite content is high in the Chang 2 and Change 3 Oilbeds which probably were owing to feldspar corrosion by fresh water in atmosphere and the kaolinite deposit at IndoChinese epoch. 
The main pore types are residual intergranular pore and feldspardissolved pore and the porethroat structure can be divided into five types which are eyeletthin throat, eyeletmicro thin throat, eyelet micro throat, microthin pore microthin throat and microthin pore micro throat in which the last type is unavailabile reservoir.