Research On Characteristics And Formation Mechanism Of Ordovician Reef-beach Reservoir In The South Of Ordos Basin

The reef-beach bodies of Ordovician are developed in the south of Ordos Basin.Over the years,Reef beach facies reservoirs gained industrial airflow,however,the lower exploration level of Lower Palaeozoic,uncertain reef-beach size and the development characteristics of reservoir,restrict the direction of the next exploration.By outcrop section measurement,typical drilling core observation,rock slices identification,physical properties test and geochemical experiment analysis,the paper defines the growing rules of reef-beach bodies,selects the favorable rock types,analyzes the phases of key diagenesis,combined with karst topography to explores the formation mechanism and models of reef-beach reservoir,points out the distribution of favorable areas.According to the studies,the following conclusions have been obtained:(1)The Platform Margin Reef-beach is mainly formed in the late Ordovician.The reef-beach of Pingliang Formation,is better preserved,distribut in tongchuan,fuping area of the eastern region,bacteria algae and sponges is commonly developed,Microbialite textures could be recognized.The reef-beach of Beiguoshan Formation,is mostly bioherm and biostrome,distribut in the west of Liquan,develop bioclastic limestone and calcirudite,the biological species consist of brachiopods,echinoderms,tentaculitids,etc.(2)Favorable rock is reef limestone of Pingliang Formation and dolomite of Ma6 Member.The dolomite of Ma6 Member have the better reservoir quality.(3)Epidiagenetic karst and burial dolomitization is the key diagenesis.Effective karst area is the relatively higher position of karst topography.The Ma5 Member provides with magnesium for burial dolomitization,leading to the formation of Ma6 dolomite.The most stronger damage to the reservoir is cementation,The dissolution pores is commonly filled by the dark radiaxial fibrous calcite rims and milky white calcite.(4)The high-quality reef-beach reservoir is controlled by earlier high position of reef,karst topography and the diagenesis.The relatively higher position of karst topography and the area of dolomite of Ma6 Member is the favorable zone of reef-beach reservoir.