Reservoir Study Of Paleogene Shahejie Formation In Southern Laizhouwan Depression,BohaiWan Basin

The southern area of Laizhouwan is located in the southern part of Laizhouwan Depression in the southern sea area of Bohai Sea.Laizhouwan Depression is sandwiched between the East and West branches of Tanlu strike-slip fault in the southern part of Bohai Sea.It is the key exploration area of Bohai Oilfield in recent years.Shahejie Formation is the main oil-producing section of Laizhouwan Depression.However,in the southern area of Laizhouwan,the exploration degree is low,the reservoir geological conditions are complex,the number of faults is large,and the number of exploration wells is small.Because of Engineering reasons,the drilling wells are mostly located in the high part of the structure,which leads to the unclear understanding of the distribution law of high-quality reservoirs.Therefore,to study the distribution of high-quality reservoirs in the sequence stratigraphic framework of Paleogene Shahejie Formation and to find out the distribution law of high-quality reservoirs of Paleogene Shahejie Formation in the study area can lay a solid geological foundation for improving exploration success rate and reducing exploration risk.In this paper,the clastic rock reservoirs of the Paleogene Shahejie Formation in the southern part of Laizhouwan Depression are taken as the research object.Through the comprehensive analysis of core description,thin section identification,scanning electron microscopy,X-ray diffraction analysis and mercury injection analysis,the rock types and characteristics of the clastic rock reservoirs of the Paleogene Shahejie Formation in the Southern part of Laizhouwan Depression are identified,the diagenetic facies types are determined,and the reservoir space is summarized.Structures and characteristics,clear reservoir physical properties and vertical and horizontal distribution law,discussed the controlling factors of clastic reservoir in the study area,put forward the evaluation criteria of high-quality reservoir suitable for clastic rocks in the study area,determined the distribution range of high-quality reservoir,and pointed out the favorable areas for further exploration.The results are as follows.The rock types of reservoirs in the study area are mainly lithic feldspathic sandstone and feldspathic lithic sandstone.There are impurities and cements in the fillings.The types of cements are abundant,including clay mineral cements,carbonate cements and siliceous cements.Clay mineral cementation is the most common and siliceous cementation is relatively rare.Generally speaking,the sandstone is of low composition maturity and low structure maturity.Diagenesis includes compaction,dissolution,cementation,metasomatism andfragmentation.According to the classification criteria of diagenetic stages,the diagenesis stage of clastic rocks in the study area is from early diagenesis B to mi-ddle diagenesis A.From the point of view of actual exploration,combined with the actual situation of the study area,the diagenetic facies can be divided into four types:medium compaction-clay mineral cementation-medium dissolution-frac-ture face,medium compaction-clay mineral cementation-medium dissolution face,medium compaction-carbonate cementation-medium dissolution-fracture face and medium compaction-siliceous cementation-medium dissolution facies.Reservoir space includes primary pore,secondary pore and fracture.The rese-rvoir space is mainly secondary pore and fracture.There are three types of pore: type I pore,with large pore radius,small displacement pressure and good pore connectivity,is the best pore structure type of reservoir performance in the study area.Class II pore,with larger pore radius but less than Class I pore,medium displacement pressure and general pore connectivity,is a type of pore structure with medium reservoir performance in the study area.Class III pore,with small pore radius,high displacement pressure and poor pore connectivity,is the worst type of pore structure.Along the inner slope zone,the outer slope zone and the sub-convex slope reservoir,the physical properties gradually deteriorate.With the increase of burial depth,reservoir porosity and permeability gradually decrease.Reservoirs are influenced by sedimentation,diagenesis and tectonism.Sandstone in subaqueous branching channel and estuary bar is well sorted,and is a reservoir with good reservoir performance,which is conducive to the development of high-quality reservoirs.In diagenesis,dissolution plays a constructive role in reservoirs,while cementation and compaction play a destructive role in reservoirs.Fractures formed by tectonism are not only reservoir space,but also can connect isolated pore,which greatly improves reservoir performance.Clastic reservoirs can be divided into three types.The first type of reservoir is the best one in the study area.Class II reservoirs are moderately evaluated and Class III reservoirs are poorly evaluated.Class I reservoir and Class II reservoir are high quality reservoirs.Class I reservoirs are mainly distributed in the southwest of the study area and belong to Long-source high-quality reservoirs in the outer slope zone.Class II reservoirs are mainly distributed in the fan delta deposits in the East and South and in the braided river delta regions in the west.The optimal reservoir is located in the southwest of the study area,which is the key area for further exploration and development.