The Origin Of Diagenesis Heterogeneity Of Lenticular Sandbodies In The Upper Es₄,Lijin Sag

Eocene sandstone reservoirs in Dongying Sag,Bohai Bay Basin,China,are characterised by significant hydrocarbon accumulations.samples in Lijin sag are tested by cathodoluminescence microscope,x-ray diffraction,mercury injection and grain-size analysis,comparing the grain size,sorting coefficient,compositional maturity,sedimental environment and diagenesis stage.A model is eatablished to describe the revolution of diagenesis and the petroliferous property.The result shows that there are three types of sandbody with different high porosity zone.Type ? with high porosity in the center of the sand body is in the beach bar and storm beach bar impacted by alkali fluid.Type ? with high porosity at the edge of the sand body is also in the beach bar and storm beach bar impacted by acid fluid.Type ? with high porosity dispersed distribution is in the fan delta impacted by acid fluid.The composition and structure of sediments are not the dominant factor,which could be faded by other factors,although they can affect slightly the compaction and fluid flowing.The Z value also increases from the Lower Es4 to the Es3 of the layer,indicating that the salinity increased slightly from the bottom to the top of the layer.It may be due to the closed lacustrine conditions,wherein water evaporation and infiltration into the underlying formations increase the salinity of the sediments.Most sandstones and siltstones record diagenetic temperatures higher than the mean value,whereas most claystones record temperatures lower than the mean value.Additionally,two stages of carbonate cement are found by the microscope.Therefore,the calcite in fine-grained sediments were deposited in the eodiagenetic stage without any influence of pore fluids.On the contrary,the calcite in sandstone may have been affected by the pore fluids and was deposited in the eodiagenetic stage.Early stage of carbonate cements can inhibit the compaction and preserve space for secondary pores.So,different content of carbonate cements is the beginning of the diagenesis heterogeneity.There are three stages of pore fluids in the upper Es4 in Lijin Sag.They are alkaline,aicidic and alkaline.Multi-stage fluids are the directly reason of the diagenesis heterogeneity.Aicidic fluids origin from mudstones of surrounding rock.They dissolve carbonate cements from the edge to the center of sandbodies,and create dissolved pores at the edge of sandbodies(type ?).The distance of fluids invading into sandbodies is influenced by pore connectivity.Late stage carbonate cements also cement at the edge of sandbodies first(type?).However,diagenetic fluids are hard to invade into those sandbodies which have bad pore throat structure.Both dissolution and cementation are incompletion,which do not have a noticeable high porosity belt(type ?).