Division of diagenesis reservoir facies and its control — Case study of Chang-3 reservoir in Yangchang formation of Fuxian exploration area in northern Shaanxi

By dividing the diagenetic reservoir facies and studying its distribution, we can effectively evaluate reservoirs. Using an analytical method for mutual relationships among diagenesis of reservoirs, structural characteristics and the origin of types of sand, we classified the diagenetic reservoir of the Yanchang formation Chang-3 oil layer into different types and analyzed the factors controlling this high quality reservoir. The results show that: 1) the catagenesis of the Chang-3 oil layer in the Fuxian region is complex and variable; 2) mechanical compaction, pressure solutions and cementation of all kinds of authogenous minerals decreases porosity by 28.13% and 3) corrosion increases porosity by 3.39%. Cementation and corrosion of laumontite is a kind of catagenesis which makes this region different from other exploratory regions. The original amount and rate of corrosion of laumontite are the main factors for the increase in laumontite porosity. The Chang-3 oil layer developed four kinds of diagenetic reservoir facies, of which the weak compaction facies, i.e., laumontite corrosion and overgrowth, of the original pore reservoir facies has the most promising diagenesis. Its distribution is controlled by a sedimentary facies zone and is widely developed in the original truncated tributary channel facies belt of the delta plain. The development of this high quality reservoir is controlled by the distribution of sedimentary sand bodies and secondarily-eroded porosity of laumontite in the Chang-3 oil layer of the Fuxian exploratory region.