Sedimentary Characteristics Of Lower Paleozoic In The Bohai Bay Basin

This thesis takes the Bohai Bay Basin and its periphery as the research area,the following Paleozoic Cambrian-Ordovician strata are taken as the targets.Through field outcrop survey,core observation and description,and logging data,combined with thin section identification,casting thin section identification,X-ray diffraction analysis,physical property testing and trace element analysis,sedimentology,sequence stratigraphy and reservoir geology are studied.Guided by the theory,the characteristics of Cambrian-Ordovician strata,sedimentary characteristics and favorable reservoir-cap assemblages in the Bohai Bay area are systematically studied.The Lower Paleozoic in the Bohai Bay basin was formed in the continental margin tectonic background of large-scale transgression,characterized by the formation and development of carbonate platform with a wide distribution area.At the end of Ordovician,uplift was eroded and Silurian and Devonian systems were missing.Fujunshan Formation-Xuzhuang Formation is mainly composed of clastic-carbonate mixed sedimentary formations.The rock types are mainly micrite limestone,dolomite,mudstone and silty mudstone under semi-arid climatic conditions during relative transgression stage.The Zhangxia Formation-Fengshan Formation is dominated by carbonate rocks,and the rock types are continuous transgression oolitic limestone,micrite limestone,storm-gravel limestone,bioclastic limestone and mudstone.The Ordovician mainly develops carbonate deposits in the exposed environment during relative regression stage.The rock types are mainly gravel limestone,marl,granular limestone,dolomite and dolomitic limestone.There are five types of sedimentary facies in Lower Paleozoic of Bohai Bay Basin,including tidal flat,open platform,restricted platform,far shore shoal and shore shoal shoal,among which sedimentary microfacies can be classified according to sediment type.There are two sets of relatively stable reservoir-cap assemblages in Lower Paleozoic in the Bohai Bay Basin: Yeli-Liangjiashan Formation as reservoir,overlying lower Majiagou Formation as caprock,which is a mixed reservoir-cap assemblage of dissolved pore and fracture;Fengfeng Formation dolomite and weathered crust are extremely favorable reservoir segments,which are covered by bauxite mudstone and iron ore of Upper Paleozoic Benxi Formation,and are weathered crust reservoir-cap assemblages.These two sets of reservoir-cap assemblages have laid a good condition for oil and gas enrichment.