Abstract: The paper introduces the granularity distribution testing method of reservoir cap rocks, including status quo of granularity distribution testing technology, importance of cap rock granularity testing, sample preparation,testing result,and parameter evaluation,etc.Sample preparation process of mudstone and siltstone granularity testing are the more complicated,the main work is granularity disassociation which directly affects the accuracy of data.In the meantime,the precision and accuracy of granularity testing using laser method are discussed.Through testing the granularity distribution of cap rock sample,the data of evaluating microscopic cap rock quantitatively are provided.The result of testing and statistic distribution of rock granularity shows that the lower sand-bearing mudstone has the better sealing property,so the granularity distribution testing technology is meaning to evaluate the sealing property of cap rock. Key words: rock;granularity distribution;cap evaluation

Main titles 1 Status quo and importance of rock granularity testing 2 Preparation of rock granularity sample 2.1 Selecting sample is key to granularity testing 2.2 Affecting granularity distribution data little to deoil way 2.3 Sample disassociation and cement remove are necessary procedures 2.4 Particle disassociation is key to reality of granularity data 2.5 Understanding the relationship of disassociation and association to prepare the stable suspension sample 3 Experiment analysis of rock granularity 3.1 Disassociating representative suspension must contraction and separation 3.2 Testing the accuracy and repeatability using Standard sample to verify the reliability of device testing data 3.3 Setting up reasonable device testing parameters to obtain the more accurate granularity testing result 4 Granularity distribution characteristics and its geological application 4.1 The rocks which granularity consist of fine sands-silt sands and muddy grade have better sealing property 4.2 Granularity'D5-D95≤32µm shows that the fine rock granularity is higher 4.3 Cap rocks have characteristics that the sealing ability is from medium to good,selecting degree is worse,deflecting degree is closer symmetry-positive deflection,peak degree is medium 4.4 Suspending component occupies mostly,shows probability cumulative curve of two and three section modesm 4.5 C-M plot shows that sediment is suspending substance in static water and rock particle belongs to Ⅷ of granularity type 5 Conclusions