Reservoir Characteristic And Origin Of Secondary Porosity In The 3^rdZhuhai Formation In Wenchang-A Sag,Pear River Mouth

Several gas bearing structures have been found in Wenchang-A sag,Pear River Mouth Basin since the 1980 s,revealing a good prospect of resources.The poorer physical properties and lower production,however restrict the further exploration and development of gas in this area.Therefore,how to find relatively high quality reservoirs in the low porosity and low permeability reservoirs is important.The main purposes of this thesis are to:?1?study the origin of primary pore preservation under ordinary pressure based on the analysis of differences of reservoir characteristics and diagenesis between the area of faults,No.6 fault belt and the South fault zone in the 3rd Zhuhai formation.;?2?point out the mechanism of secondary pore and the changes of physical properties by simulation experiment;?3?get the classification standard of high quality reservoir according to the microscopic pore structure parameters;?4?predict the favorable area with relatively high quality reservoirs which has a significant meaning to the next step of exploring through the comprehensive analysis of controlling factors of low porosity low permeability reservoirs.The petrography of 3rd Zhuhai formation which varies frequently,could be divided into 7 types and 15 subgenera.Five types of lithofacies combination can be recognized in this region.The best reservoir is located in the area of faults,the second is in No.6 fault belt and the South fault zone has the worst reservoir.The reservoir quality of Tidal channel?underwater distributary channel?mouth bar which are composed of coarse grain are better than that of sand flat?mixed flat.The reservoir in study area can be divided into 4 types??-??based on the physical property,common mercury penetration analysis,rate-controlled mercury penetration analysis and nuclear magnetic resonance?NMC?.The reason of low permeability of the 3 rd Zhuhai formation is related to the deep burial which leads to intense compaction.Meanwhile,the late stage of carbonate cementation and silicon cementation are also led to the low permeability.The sandstone underwent intense diagenesis alteration and generally got to mesodiagenesis B stage.Dissolution is common in study area and plays a good effect to improving reservoir property.Two types of dissolution,including meteoric water,hydrothermal alteration,CO₂ corrosion,are certified in this study.The pore types in study area are composed of primary pore and secondary pore,but the secondary pore takes a little advantage due to intense dissolution.Formation mechanism of secondary pore is explained by simulation experiment about cylindrical plug in condition of CH3COOH/C2H2O4/CO₂/meteoric water under different temperature and acidity.In organic acid?CH3COOH/C2H2O4?condition,H+replace metal cation,the solution turn to alkline,along with temperature rising the organic acid decompose,leading calcium carbonate supersaturated and precipitated.Buffering capacity of CO₂ is much better,the reaction process is sustainable,as a result the cements are not easy to come out.The solubility of meteoric dissolution gets higher along with the temperature rising,conforming to diffusion reaction mechanism.In the study area,secondary pore of the 3rd Zhuhai formation are mainly distributed in XX9-2 area,due to the multi-stage dissolutions,especially by the dissolution of mantle-source CO₂.Second pores are not common in Region XX10-3 and.XX11 around No.6 fault zone and the east South fault zone.By pore structure parametesr and pore throat distribution high-quality reservoir can be divided into two types,one is relatively higher porosity and permeability reservoir,another is lower permeability reservoir,of which low permeability reservoir is the major one.The 3rd in Zhuhai formation develops relatively higher porosity and permeability type ?,low permeability type ?,and low permeability type?.Fine sandstone develops low permeability reservoir;relatively higher porosity and permeability type ? appears in tidal channel sandstone in well XX9-1-1,low permeability type ? appears in underwater distributary channel sandstone around well XX10-2-1;Sand flat and mixed flat around region XX9-2 develops low permeability type ?.