Characteristics And Origin Of Deeply-buried,High-quality Es1 Sandstone Reservoirs In 2^nd And 3^rdstructural Belts In The Nanpu Sag,Bohai Bay Basin

The deeply buried Es1 sandstone of the Shahejie Formation in the Nanpu Sag,Bohai Bay Basin consists of lacustrine and braided river deltaic sandstones and interbedded mudstone with excellent reservoir characteristics.The reservoir characteristic are evaluated with different methodology and advance techniques including Scanning electron microscopy(SEM),Cathodoluminescence(CL),Carbon and Oxygen isotope analysis,Thermometry fluid inclusion,X-ray diffraction analysis(XRD),l laser scanning confocal microscope(LSCM),petrographic thin section and mercury injection capillary pressure analysis(MICP).The Es1sandstone consisting primary intergranular pores as well as dissolution and secondary pores that enhancing the reservoir characteristics.Multi-scale heterogeneity indicate the tough tasks in hydrocarbon exploration and production.An integrated methodology is used to characterize the heterogeneity by lithofacies scale,architecture elements scale and pore scale.Core description,wireline log analysis and seismic interpretation are assimilated to demarcate the structural and sequence stratigraphic framework of studied sandstone reservoir.Core data are used to establish the lithofacies and interpret the depositional history.Several analysis including thin section petrography by using optical microscope,scanning electron microscope,X-ray diffraction,confocal laser scanning microscope,and cathodoluminescence analysis are used to evaluate the heterogenic sandstone texture,fabric and allogeneic and autogenic mineralogy.The present study aims to evaluate the reservoir characteristics,diagenesis impact on reservoir and genetic mechanism of high-quality,deeply-buried reservoir sandstone.Statistical data,thin section images and laser scanning confocal microscopic data is more reliable to evaluate the reservoir characteristics.The Es1 sandstone mostly deposited in lacustrine,shallow and semi deep lake,and braided river deltaic environment.The depositional heterogeneity and various diagenetic process are controlled by grain size and their sorting in addition to continuous deposition of mud layers,siltstone and fine sandstone at different intervals.Compaction,cementation,immature sediment composition,dissolution or leaching of feldspar,carbonate cement,unstable rock fragments and fractures are the various diagenetic processes that effect the reservoir quality.Clay minerals clogs the pores and reduced the reservoir quality.The kaolinite,illite and illite/smectite are acting as pore blocking cement and reduce the reservoir porosity and permeability.Reservoir pores,pore size distribution(PSD)and their associated heterogeneities of Es1sandstone are characterized by mercury injection capillary pressure(MICP)data.Porosity and permeability are significant characteristics of reservoir rock that provide the storage capacity and migration conduit for reservoir fluids.Whereas the pore size distribution is basically developed a link between pore throat/connectivity and their effect on reservoir quality.Data and results are combinly used to evaluate the reservoir pore space,their type and their dimension.Pore throat data is mostly derived from MICP and maximum pore throat size ranges from 0.006 to?63?m,while average pore throat ranges 0.073?m to 15.492?m.MICP results indicates meso and micro size pore distribution,generally matching that intergranular pores are dominant,whereas high cemented sandstone show high content of intercrystalline micropores.The Porosity of deeply buried rock enhanced due to fracturing and dissolution.Secondary pores are evaluated with the study of scanning electron microscope,laser scanning confocal microscope and casting thin sections.All effective fractures were acting as a passage for the hydrocarbon migration.Secondary pores due to extensive feldspar dissolution suggested that the acidic diagenetic environment for the evaluation of the secondary pores.Fractures and faults extended to deeper depth and provide favorable condition for atmospheric water causing leaching and dissolution that causes Es?formation water desaltation and rock leaching.Similarly the fluids at deeper horizon migrated towards the shallow horizon along fractures and faults due to temperature difference,concentration difference and pressure difference making the subsurface water convective circle system in Nanpu Sag.This water play important role in dissolution of soluble feldspar and alteration of carbonate minerals in deep depth.Thermal degradation of organic matter causes to generate the large amount of CO₂ and organic acid.Acidic water is significant for the generation of secondary pores that apparently improve the physical properties of deeply buried reservoir rocks.So the main genesis for the formation of secondary pores within the studied reservoir is organic acid and CO₂ acid water.The porosities of braided river facies and channel bar are higher than meandering river facies and lacustrine facies(zone I).Whereas porosities of reservoir in braided channel facie are high,followed by meandering facies&channel facies,and porosities of flood plain facies are worst(Zone II).In Zone III,The fluid overpressure data indicates that the shallow depth generated fluid overpressure effectively delay the compaction,with increasing burial depth leading to point contact followed by point-line,line-line and stylolite contact of detrital grains.Hence the shallow depth generated overpressure plays significant role to developed high quality reservoir(high porosity)in deeply buried sandstone reservoir.It is also found that hydrocarbon emplacement also prohibited the carbonate cementation,this early formed hydrocarbon emplacement play important role to develop high porosity zone in deeply buried reservoir rocks.