Research On The Characteristics Of Under-salt Reservoir Of Ordovician In Mid-Eastern Ordos Basin

Ordos basin is located in the west of north China platform.The basin area is about 320000km2,which stretches northward to Yinshan Mountain and Daqing Mountain,southward to Qinling Mountain,westward to Helan Mountain and Liupanshan Mountain,and eastward to Luliang Mountain and Taihang Mountain.Ordos basin has a complex tectonic system and various types of sedimentary strata,and the major movement of the basin is lifting tectonic movement,it's tectonic features are distributed north-south and west high east low.Ordos basin experienced lots of important structural events,then it can be divided into 6 primary tectonic units such as Yimeng upthrust,Weibei up thrust,Jinxi plyzone,Yishan slope,Tianhuan depression and Western Boundary obduction zone.The subsalt layer of Ordovician of Lower Paleozoic in Odros Basin is well-developed,which has great thickness and larger distribution area.The percolation ability of the subsalt layer is great,well-developed reservoir property and source rock-reservoir collocation make it possible to form commereial gas reservoirs.After years of changqing oilfield exploration and research,put forward a new knowledge is the salt rock under the coal source forlateral hydrocarbon generation and accumulation?dolomite lithology trap trap accumulation and regional lithology transformation band enriched with multiple series of strata,and has made significant exploration breakthrough under the guidance of this theory.Although the region has obtained industrial gasflow,and has a good exploration prospects,but we lack of overall and deep understanding on the characteristics of dolomite reservoir?the distribution law of dolomite reservoir and the main controlling factors of dolomite reservoirs.So we need to carry out targeted research to guide the exploration and development on the subsalt layer of Ordovician of Lower Paleozoic in Odros Basin.Based on the detailed study of more than 40 exploration wells and development wells,and using large amounts of log data combing maker beds identification,detailed classification and correlation of every layer in Ma6 to Ma10 Member have been accomplished;Through the observation and study on the cores,rock slices,logging data,and on the basis of single well and well tie study,we conclude that the study area belongs to tidal-flat facies and lagoon facies.Tidal-flat facies can be divided into 3 types of tidal flat subfacies such as supralittoral zone,intertidal zone and subtidal zone,8 types of tidal flat microfacies such as mud flat,mudly dolomite flat,containing mud dolomite flat,dolomite flat,gypsodolomite flat,gypsum-dolomite flat,limestone contained dolomite flat,limestone-dolomite flat,grain beach,mudly limestone flat,dolomite-limestoneflat and limestone flat.Lagoon facies can be divided into lagoon subfacies and gypsic saline lake microfacies.Through the observation and study on the cores and rock slices,we conclude that the study area developed mudstone dolomite,gypsum contained mudstone dolomite,silty dolomite,bioclastic silty dolomite,limestone silty dolomite,silty-fine dolomite,aplite-dolomite,bioclastic aplite-dolomite,gypsum contained aplite-dolomite,grain dolomite and gypsum-dolomite.The favorable reservoirs of Ma6 to Ma10 Members are silty dolomite and aplite-dolomite.Through diagenesis study of reservoirs in layers,we suppose that the extensively developed diagenesis types are penecontemporaneous dolomitization,reflux seepage dolomitization,dissolution,cavity filling,recrystallization,compaction and pressolution.Penecontemporaneous dolomitization,reflux seepage dolomitization and dissolution were beneficial to the formation of reservoirs,cavity filling,recrystallization,compaction and pressolution took moreserious destruction of the reservoirs.Through the observation and study on the casting thin sections,scanning electron microscope,electron probe and laser scanning confocal microscope,we conclude that the study area Ma6 to Ma10 Members developed 7 types of pores such as intergranular pores,intracrystalline pores,intercrystalline solution pores,gypsum mould pores,dissolution pores,microcrack and intercrystalline pores.Reservoir space is mostly composed by intracrystalline pores and other various pores.Permeability were controlled by the pore development in the sardstone,also associated with pore-throat size and pore connectivity.The reservoir physical property of Ma7,Ma9 Members are relatively well,oppositely Ma6,Ma8,Ma10 Members are relatively bad.Reservoir physical property obviously controlled by different lithological characters and different sedimentary facies.Through the comprehensive index of main reservoir rock type,sedimentary facies,diagenesis,physical parameter,pore characteristics and reservoir thickness et al,to establish reservoirs evaluation standard.We divided the study area into class?reservoirs,class?reservoirs and class ? reservoirs.We selected 4 Class?favorable reservoirs and 4 Class?favorable reservoirs in Ma7 Members,Class ? favorable reservoirs are located around Wushenqi-Jingbian-Zhidan-Yulin;We selected 1 Class?favorable reservoirs and 6 Class?favorable reservoirs in Ma9 Members,Class ? favorable reservoirs are located around Wushenqi-Jingbian-Zhidan-Yulin.