Diagenesis of Chang 6 reservoir in Zhengzhuang area,Ordos Basin and its effect on reservoir quality

The deposition of Chang 6 reservoir is controlled by delta front subaqueous distributary channel.The reservoir mainly consists of fine feldspathic sandstone.The reservoir rock is characterized by fine grain size,low compositional maturity and high texture maturity,so it is typical low-porosity and low-permeability reservoir.Its diageneses mainly include compaction,pressure solution,cementation,metasomatism,dissolution and fracturing.The study on the diagenetic evolution of Chang 6 sandstone shows that its diagenesis stage mainly is in A to B stage of middle diagenesis.Mechanical compaction and cementation result in the almost complete loss of primary pore,this is why the reservoir is of superlow permeability in the early diagenetic phase;but in the late diagenetic phase,the dissolution,especially the dissolution of laumontite,improves the porosity and permeability of the reservoir,which makes some sweat reservoirs develop in the Chang 6 reservoir and they become favorable reservoir facies belts in Ordos Basin.