Sequence Stratigraphy,Sedimentary Facies And Reservoir Quality Of Es4_s,Southern Slope Of Dongying Depression,BOHAI Bay Basin,East China

Thin-bedded beach-bar sandstone reservoir is widely deposited in the upper fourth member of the Eocene Shahejie Formation(Es4s)of Niuzhuang Sag,along the southern gentle slope of the Dongying Depression.The very first work of this thesis is to establish a linkage between sequence stratigraphy,sedimentary facies and diagenetic effects on reservoir quality.Seismic,wireline logs,core observations and analyses are used to interpret depositional settings and sequence stratigraphic framework.Petrographic study based on microscopic observation of optical,cathodoluminescence(CL),confocal laser scanning(LSCM)and scanning electron microscope(SEM)and X-ray diffraction(XRD)analyses are carried out to discuss the fabric,texture,allogenic and authigenic mineralogy of highly heterogenic sandstone reservoir.Es4 s is interpreted as third-order sequence,composed of a lowstand systems tract(LST),a transgressive systems tract(TST)and a highstand systems tract(HST).On the fourth order,twenty-nine parasequences and seven parasequence sets have been identified.Sand bodies were mainly deposited in the shoreface shallow lake beach-bar(clastic beach-bar),semi-deep lake(carbonate beach-bar)and the fluvial channels.The depositional and diagenetic heterogeneities were mainly due to the following factors:(1)fine grain size,poor sorting,and continuous thin inter-bedded mud layers with siltstone/fine-sandstone having argillaceous layers in regular intervals,(2)immature sediment composition,and(3)even with the dissolution of grains and several fractures,destruction of porosity by cementation and compaction.Secondary pores from feldspar dissolution are better developed in sandstones with increased cementation.Grain coating smectite clays preserved the primary porosity at places while dominating pore filling authigenic illite and illite/smectite clays reduced permeability with little impact on porosity.Pore size distribution and associated heterogeneities of thinly bedded beach-bar sandstone reservoir from upper fourth member of Eocene Shahejie Formation(Es4s)are characterized with help of mercury injection capillary pressure(MICP)and nuclear magnetic resonance(NMR)transversal relaxation time(T2)data.Permeability and porosity are two represented important characteristics of rocks that control the movement and storage of fluids.Aim of this part of thesis is to establish relationships between pore throat sizes and reservoir quality.The results derived from thin-section petrography,scanning electron microscopy(SEM),MICP,NMR T2 relaxation time,and 3D ?-CT are compared to characterize pore space dimensions and types comprehensively.Average pore throat size from MICP ranges between 0.47 ?m to 2.83 ?m while maximum pore throat size ranges between 2.48 ?m to 7.36 ?m.The combination of pore size distribution obtained from MICP,NMR seems appropriate to cover the range of pore size from beach-bar sand and overcome the individual method limits.Digital 3D micro-CT tomographic images are used to characterize and visualize pore space and pore network model to compare with the experimental data.MICP and NMR experiments showed generally bimodal(Meso and Micro)pore size distribution.Usually mesopore corresponding to intergranular pore are dominant,while heavily cemented sandstones show large amounts of intercrystalline micropores.To identify and elucidate pore types,connectivity and distribution in a texturally low maturity,thin bedded,medium to low permeability silt to sand sized clastic reservoir,precise and fine mapping of minerals,pores and microporous phases with 3D X-ray micro-computed tomography(3D ?-CT),2D back-scattered scanning electron microscope(BSEM)imagery and 2D quantified,automated SEM-EDS mineral analysis(QEMSCAN)can play a significant role in reservoir characterization.In this part of the thesis,an integrated core to pore scale approach is adopted to compute macro scale petrophysical rock parameters such as porosity,permeability,formation factor,and pore throat size and distribution directly from 3D digitized rock images.A sub-plug of the sandstone was imaged in 3D ?-CT in dry and saturated states.A quantified,3D pore space map,including microporosity,was created by registering into perfect geometric alignment the two tomograms.Quantified,3D mineralogical mapping along with association of pore and grain structure was performed by registering the high-resolution BSEM image and quantified 2D mineral map with the 3D ?-CT images.The association was characterized by the matching of microporosity to the mineralogical information.Interconnectivity of primary and secondary porosity and nature of secondary porosity was also determined and quantified.Computational results of porosity and permeability from digital rock show good agreement with laboratory core measurements.Results show that the computed properties from this technique are reliable and can be used for reservoir quality assessment and field development programs.Complex and heterogeneous beach-bar sandstone reservoir requires comprehensive study program to evaluate the reservoir.Imaging and visualization of core material with 3D ?-CT at the pore scale and subsequent analyses gives significant insights into properties of low permeability reservoir core material and further developments will make this technique more common and easy to use.Due to the high degree of heterogeneity in the Es4 s beach-bar interval,it is recognized as middle to low permeable reservoir.The aforementioned study reflects significant insight into the understanding of the properties of the beach-bar sands and are valuable in the comprehensive reservoir characterization and overall reservoir bed quality.