Characteristics and development models of high-quality dolomite reservoirs in Middle Permian Qixia Formation,central Sichuan Basin
Dolomite reservoirs in the Middle Permian Qixia Formation of central Sichuan Basin are thin and strongly heterogeneous,with their development models unknown.It is needed to accurately understand the reservoir characteristics and development mod-els for purpose of beneficial exploitation.By means of core observation,thin section identification,cathodoluminescence and geochemical analysis for wells of the Formation,the reservoir characteristics and development models were studied.The results are as follows:(i)The Qixia Formation dolomite reservoirs,belonging to beach facies,are dominated by fine-to medium-crystal residual psammitic dolomites.(ii)The dolomite reservoirs are thin layers,with an average thickness of only 5.8 m per well,featured by low porosity and low permeabili-ty.They can be divided into three types:the fractured-vuggy,the vuggy,and the porous,in which the fractured-vuggy and vuggy reservoirs are of high quality.(iii)Grainy beach is the material basis for the development of dolomite reservoirs.The reservoirs are formed by beach bodies subjected to penecontemporaneous karstification and penecontemporaneous dolomization.In the burial stage,the fault system fur-ther transformed the reservoirs to form high-quality dolomite reservoirs with developed fractures and vugs.In other words,the develop-ment of dolomite reservoirs shows a"three-control"mechanism,i.e.,control of beach body on reservoir formation,control of dolomization on reservoir scale,and control of fault on reservoir quality.It is concluded that the high-quality dolomite reservoirs of Qixia Formation have two development models:high-energy beach facies + penecontemporaneous karstification and penecontemporaneous dolomiza-tion;and high-energy beach facies + penecontemporaneous karstification and penecontemporaneous dolomization + fault transformation.
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