The Difference Control Of Fluids To The Lacustrine Carbonate And Clastic Mixed Deposition Reservoir,Bohai Sea Area

The mixed deposition rock of lacustrine carbonate and clastic are widely developed in the first member of the Shahejie Formation in the Bohai Sea.A number of large and medium-sized oil and gas fields have been discovered in the mixed sedimentary rocks.The total proved reserves are more than 300 million cubic meters.The mixed sedimentary rock under deep burial conditions has good physical properties,high production capacity,and multi-well testing capacity exceeded 1,000 square meters,which is an important exploration area in the deep part of the Bohai Sea.Research reveals that there is a large difference in the physical properties of the mixed sedimentary rock reservoirs.In some structures,the mixed sedimentary reservoir is thick,has good physical properties and high productivity.In other structures,the reservoir is dense,has low bioclast content and no or low productivity.The high-quality reservoirs of mixed sedimentary rocks are generally dolomitized,and the dolomite content of the mixed sedimentary rocks with high intraclast content could reach 90%.Bioclastic and oolitic grains commonly dissolved in the mixed sedimentary rock,and the dissolution of external clastic debris,feldspar,gravel is also commonly observed.The interactions between different fluids and the mixed sedimentary rocks are the decisive factors of the differential evolution of the mixed sedimentary reservoirs.In this paper,a comprehensive study about the mixed sedimentary rocks of the first member of the Shahejie Formation in the Bohai Sea area is carried out,mainly including following five aspects:(1)the early sedimentary water environment;(2)the correlation between the lithologic conditions of the basement and the development of the mixed sedimentary rock;(3)the transformation of the reservoir by the dolomitized fluid of different stages;(4)the controls of meteoric water diagenetic environment on different minerals;(5)the transformation of the reservoirs by the mantle-derived CO2 thermal fluid during the late stage.The results indicate that the water environment during the sedimentation of the first member of Shahejie Formation is obviously characteristics as salty,and the comparison of index is similar to the salinity of modern salty lakes.The analysis of mudstone shows that the water of first member of Shahejie Formation is rich in magnesium during the sedimentary period.The electronic probe analysis shows that the early micrite dolomite with high magnesium content further confirms that the magnesium is enriched during the sedimentation of the first member of Shahejie Formation.High magnesium content is the characteristics of the lakes in early sedimentary stage,which is the important basis for the early dolomitization and later dolomitization.The thickness and dolomitization degree of the mixed sedimentary rock is different under the different basement lithology.The basement lithology of mixed sedimentary reservoir is composed of the Mesozoic volcanic rock,Cenozoic volcanic rock and Mesozoic volcanic glutenite,which is characterized by large thickness,high bioclastic content and high dolomitization degree.The dissolution experiment of physical simulation shows that the mineral hydrolysis of volcanic rocks provides more metal ions than quartzite.These metal ions lead to high mineral content in local lake water.Local water with high mineral content promotes the flourishing of algae,cyanobacteria and other microorganisms,thus promotes the prosperity of the biological community that feed on algae.Algal blooms further promote the development of micritic envelopes and microbial dolomites.The above early diagenesis function brings about that the more volcanic rocks overlain by mixed sedimentary rocks are developed,and the thickness of high-quality reservoir is also significantly higher than that of the mixed sedimentary rock underlain on other rock type.The following reservoir is the carbonate rock underlying on the mixed sedimentary rock,the thickness of most monolayers and accumulative layers of Paleogene clastic underlain on mixed sedimentary rock is relatively small.The reservoir has great cementation function,which is composed of the mixed sedimentary rock developed on the clastics of buried hill.This kind of reservoir with low dolomitization degree has poor physical properties.The dolomites of the mixed sedimentary rock reservoir are able to be divided into eight categories,which are micrite dolomite,granular coating dolomite,micrite envelope dolomite,multi-stage pore lining dolomite,pore filling non-straight crystal plane dolomite,microcrystalline saddle dolomite and the ankerite.The different types of dolomites are measured by different experimental methods,such as inclusion temperature,Electron probe,Micro-area Carbon and Oxygen isotopes,U-Pb dating,Carbon and Oxygen cluster isotopes ? 47.The results indicate that:(1)micrite dolomite and granular coating dolomite formed earlier;(2)the formation of pore liner dolomite has several stages,which are from shallow burial stage to medium deep burial stage;(3)pore filling non-straight crystal plane dolomite formed later;(4)the saddle dolomite filling in the innermost side of pores formed later;(5)the formation of ankerite is the evidence of hydrothermal activity,which is locally developed.The micritic envelope is a complex of mineral and organic matter formed by algal and other microbial groups acting on rock debris and bioclastics.Therefore,the micritic envelope is stable in nature and difficult to dissolve,and it forms a grid which is used to support after dissolution.During the development of pore lining dolomites,the micritic envelope becomes the attachment point of pore lining dolomite.The multi-stage growth of pore lining dolomite supports a large number of intergranular pores.The above-mentioned function is the main mechanism,which is positive correlated between dolomitization and reservoir physical properties.Both of inclusion temperature and U-Pb dating confirmed that the micrite dolomite formed earlier,and it had the characteristics of penecontemporaneous dolomite.Discovering a large amount of microbial dolomites also illustrates that a large number of penecontemporaneous dolomite formed during the early stage of the first member of the Shahejie Formation.The early penecontemporaneous microbial dolomite provided a crystalline foundation for later dolomite formation and promoted later dolomite formation.Because of shallow and wide lake of the first member of the Shahejie Formation,both seasonal changing of the water level and tectonic fine-tuning cause the exposure of the mixed beach and bars around the edge of the lake,forming a typical meteoric water diagenetic environment.Crescent-shaped cementation,cantilever-shaped cementation,a large number of kaolinite with matrix-supported,early granular cements with high Mn O content and selective dissolution pores are recognized as the typical indicators of meteoric water diagenetic environment.Comparative study shows that the meteoric water diagenesis has a wide range of influences on the mixed sedimentary rock of the first member of the Shahejie Formation.The early dissolution of gravels and debris are formed because of the lake elutriation under the meteoric water diagenetic environment,which are considered as the basis of large-scale dissolution in the later period.A large number of aragonitic bioclastic and oolitic in the upper part of the vadose zone and hyporheic zone formed under the meteoric water diagenetic environment are selectively dissolved to form amounts of bioclastic oolitic mold pores which are recognized as the important reservoir space of the mixed sedimentary rocks.Bohai Sea area is characterized by late accumulation,and the fluids formed in later period are active which have a strong effect on the reservoir.The late mantle-derived CO2 fluids in some tectonic zones are active,thus these fluids have prominent influence on the reservoir.Lots of mineralogical evidence resulted from thermal fluids in the reservoir are recorded,including saddle-shaped dolomite,high content kaolinite,high-temperature inclusions,and pyrite with high Co/Ni ratio.Mantle-derived CO2 fluids emplacement brings a large amount of non-selective dissolution with high dissolution intensity,which results in that a small number of micritic envelopes are eroded and the dissolution of gravel and rock debris are further expanded to form a large number of mold pores.Some pore filling non-straight crystal plane dolomites have high-temperature inclusions,and are characterized by excessive dolomitization.A large amount of dissolution reservoir space is formed by the erosion and modification that caused by organic acids.The comprehensive comparative study in this paper reveals that the early diagenetic conditions are the key to controlling the dolomitization and dissolution of the mixed sedimentary rock in the later stage.In the early stage of diagenesis,the mixed sedimentary reservoir with high content of bioclastics and oolitic particles are leached by meteoric water to form a large number of mold pores,which are conducive to the continuous dolomitization,dissolution and transformation of diagenetic fluids in the later stage.The reservoir with high content of terrestrial clastics in the early stage is greatly affected by compaction and its mineral components are difficult to dissolve.It is difficult to develop strong dolomitization and the dissolution under deep burial conditions.Meteoric water,dolomitized fluids and mantle-derived CO2 present the characteristics as superposition reconstruction,and the superposition results in the polarization of reservoir,which is named as “Matthew effect” of reservoir evolution.