Characteristics And Origin On The Lacustrine Dolostone Of The Shahejie Formation In Tanggu Area,Bohai Bay Basin

The sha-3 member of Shahejie Formation in the Tanggu area of Huanghua Depression,Bohai Bay Basin is semi-enclosed lake bay facies sedimentary environment,at the bottom of which deposited one set of dolostone reservoir.The distribution of the dolostone is controlled by a NE trending basement fault and the main part of the dolostone developed on both sides of the Buried Hill in this area,this set of dolostone is widely spread with a cumulativethickness up to 100 meters.Based on the analysis of the drilling core,core slice,SEM and electron probe,this essay draws the conclusion of the main rock types and the mineral composition of the dolostones.The dolostone is interbeded with the mudstone,what formed this special characteristic is the alternate sediment of the endogenesis and the exogenous substance.The exogenous substance including mudstone and sandstone enter the lake through the rivers.These exogenous deposit formed shallow mud facies and sand facies.When the exogenous substances are not enough while the climate and the structure are appropriate,it deposited dolostones.According to the ancient landform,petrology and the paleontology symbols,the researched area mainly developed lake facies.It can be devided in to argillaceous dolomite,micritic dolomite and dolomitic mudstone.The main types of the mineral are ankerite dolomite,analcidite,clay minerals,terrigenous clastic deposit,pyrite and barite.It is thought that the lithogenesis include compaction,cementation,dissolution,agglutination,replacement,recrystallization and pyritization.The diagenetic stage can be effectively ascribed to the A substage of the early diagenetic stage according to the assemblage of authigenic clay minera,the authigenic mineral assemblage and the physical properties.The main reservoir spaces are microscopic pore,macro pores and fracture,most of them are pores and fractures.The study shows that this subzone has low porosity,low permeability,The reservoir type belongs to low porosity and very low permeability reservoir.X ray diffraction analysis indicates that this kind of micritic dolomite formed in penecontemporaneous is the product of rapid crystallization.Carbon and oxygen isotope and the rare earth elements analysis show the micritic dolomite formed in closed and high salinity evaporative environment,bearing coincident source of the fluid and same formation mechanism.A synthetical study of petrology,mineralogy and geochemistry suggests the micritic dolomite experience two stages:namely the reflux seepage function happened in the penecontemporaneous period and the phenomenon of isomorphism phase replacement happened in the burial environment of the diagenesis period.Apart from the micritic dolomite,we have also found fine-middle grained dolomite adherent to the surface of the fracture and the dissolution pore.Due to the lack of geochemistry data,we can only speculate it to be hydrothermal origin based on the structural feature and the mineral association characteristic.