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Diagenesis and diagenetic facies of tight sandstone reservoirs in the 8th member of the Shihezi Formation,southern Sulige Gas Field

Xu Lei1,Yu Xinghe1,Yang Yong2,Li Shunli1,Tian Qianqian1,Zhou Yue1(1.China University of Geosicences,Beijing 100083,China;2.Changqing Oilfield Company,PetroChina,Xi'an,Shaanxi 710021,China)  
The low permeability tight sand reservoirs in the 8th member of the Lower Permian Shihezi Formation are the targeted pay zones in the southern Sulige as the successive resource district of the Sulige Gas Field.In order to identify the diagenetic facies with favorable poroperm characteristics in this area,we studied the diagenesis and its evolutional patterns of reservoirs in the 8th member of the Shihezi Formation by using the data of cast thinsection,SEM,etc.The following conclusions were obtained.The 8th member of the Shihezi Formation must have experienced strong compaction,leading to the complete lost of most primary pores;siliceous,clay and carbonate cementations are common in the reservoirs.So both compaction and cementation are the major factors causing the tightness and low permeability and porosity of the reservoirs.However,dissolution resulted in the well-developed secondary pores,providing a basis for the development of gas reservoirs.The identification of authigenic mineral types,grain contact states,pore types,and cement types as well as the measurement of Ro reveal that the 8th member of the Shihezi Formation is in the transitional stage from the moderate diagenetic stage B to the late diagenetic stage.Based on exploration achievements and distribution of sedimentary facies,4 diagenetic facies are recognized in the studied interval,of which coarse-grained poorly-compacted dissolution facies with relatively high permeability is most favorable for gas exploration,followed by the moderate-to coarse-grained dissolution-cementation facies with low porosity and permeability.In contrast,the moderate-to fine-grained cementation facies with low porosity and permeability and the fine-grained argillaceous tight compaction facies with ultra-low permeability and porosity are unfavorable for gas exploration.
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