Synergistic Diagenetic Evolution Of Deep-Water Gravity Flow Sandstones-Mudstones And Genesis Of High Quality Reservoirs In The Third Member Of The Shahejie Formation, Dongying Depression

The synergistic diagenetic evolution processes of sandstone and interbedded mudstone while burial are the research hotspot of clastic reservoir diagenesis,although they are very complex.This work is vital for understanding the porosity and permeability variation,and oil distribution in reservoirs,which are dominated by diagenesis.Deep-water gravity flow sandstones and interbedded mudstones in the third member of the Shahejie Formation of the Dongying Depression,Bohai Bay Basin,Eastern China were taken as the research objects.The research methods were used in this work included core and thin section observation,micro X-ray fluorescence spectrometry composition analysis,X-ray diffraction composition analysis,SEM and EDX analysis,porosity and permeability testing,image analysis,cathodoluminescence,electric microprobe analysis,fluid inclusions,carbon and oxygen isotopes,X-ray fluorescence spectrometry,laser ablation inductively coupled plasma mass spectrometry analysis as well as diagenetic numerical modeling.On the guiding of research concept of nature science,“Description-Interpretation-Prediction”,the key scientific questions of the dissertation will be solved.The purposes of this research work are to analyse the genesis and characteristics of sandstone and mudstone diagenesis,to discuss the synergistic diagenetic evolution processes of sandstone and interbedded mudstone,thus to make the genesis of high-quality reservoirs clear.The deep-water gravity flow deposits are dominated by middle-to-thin bedded massive sandy debrites sandstone and mudstone lithofacies associations.The lithological character of the sandstone is mainly lithic arkose while the mudstone is the subclass mudstone which is dominated by clay minerals.The compaction of sandstone is moderate to strong,while cementation dominated by carbonate cementation.Tight carbonate cementation is common in the sandstone and mudstone interface,and occasionally in the inner high permeability part of thick-bedded sandstone.The silica cementation exhibits as microcrystalline quartz and two periods of quartz overgrowth have been distinguished.Kaolinite is the most common clay minerals.The dissolution of feldspar is well developed without a large amount of pore space increased.The precipitation of carbonate minerals in mudstone is abundant with calcite.The shapes of calcite are varied for different formation mechanism.Pyrites are distributed along the horizontal organic materials.Authentic quartz exhibits as microcrystalline quartz and quartz overgrowth.Albite is characterized by short columnar parallel orientation.Clay minerals appear as illite and illite-smectite mixed layer minerals,and it's common that the transformation of clay minerals from smectite to illite,kaolinite to illite,and kaolinite to dickite.The replacements in mudstone include calcite replacing detrital grain,and kaolinite or albite replacing k-feldspar.Organic matters experienced microbial fermentation in eodiagenesis,and generation of organic acid,hydrocarbons,carbon dioxide in mesodiagenesis.The pH of the formation water has been controlled by the partial pressure of carbon dioxide nowadays.The positive?¹³C?+1.3‰⁺3.3‰?and Ce negative anomaly of calcite in sandstone indicate that the source of carbon comes from the dissolution of carbonate in mudstone by synsedimentary water and carbonic acid caused by organic matter fermentation.The?¹³C value of ferroan calcite has a comparatively wider range from-2‰to+3.6‰,Ce negative anomaly disappears and the content of trace element is larger than that of calcite.Besides,the ferroan calcite mainly distributed in the sandstone and mudstone interface.The above features indicate that the ferroan calcite resulted from the calcite in the mudstone dissolved by organic acid and carbonic acid,which origin from the thermal evolution of organic matters in the closed geochemistry system.At the same time,the ions like Ca²⁺,Fe³⁺,Fe²⁺,Mg²⁺released to the flow while smectite transfer to illite are an important source of ions for ferroan calcite precipitation.The?¹³C value of ankerite ranges from+1.0‰to+3.6‰,and the content of light rare earth element is the same to ferroan calcite,which implies the same carbon source.At the same time,ankerite is accompanied with biotite and hydrocarbon.Fe²⁺released by biotite transformation and the reduction of Fe³⁺by hydrocarbon was the main source of Fe²⁺for ankerite precipitation.The positive correlation between Al and Ge in the first period of quartz overgrowth indicated the silica source form feldspar dissolution as well as smectite transfer to illite.The relatively high homogenization temperature of fluid inclusions in the second period of quartz overgrowth and the same element content between quartz grain and quartz overgrowth imply the silica source from compaction dissolution and kaolinite transfer to illite.The organic acid and carbonic acid source from organic material thermal evolution as well as K⁺consumed by smectite transfer to illite will promote the dissolution of feldspar.The diagenetic fluid in the mudstone is controlled by synsedimentary flow and organic matter microbial fermentation in eodiagenesis,while in mesodiagenesis controlled by clay minerals transformation and organic matter thermal evolution.In the sandstones,Al and Si are immobile during burial diagenesis,whereas K lost and Ca,Mg,Fe gained.The gain of Ca,Mg,Fe included Ca²⁺transported from mudstone to sandstone by advection in early burial and Ca²⁺,Mg²⁺,Fe³⁺transported from mudstone to sandstone by diffusion in late burial.The loss of K was dominated by diffusion caused by the concentration gradient.In the mudstones,Al and Si are immobile,however,K gained and Ca,Mg,Fe lost,which was an opposite tendency of sandstone.When the burial depth was less than 2000 m,the pore fluid in mudstone transported to sandstone driven by compaction was the main mass transport mechanism.When it became deeper than 2000 m,overpressure fluid and diffusion dominated the mass transfer process.The precipitation of carbonate in mudstone was earlier than that in sandstone,while the transformation of smectite to illite in sandstone was earlier than that in mudstone.The dissolution of feldspar and carbonate in mudstone was earlier than that in sandstone.The structures between sandstone and mudstone,composition difference,evolution degree and the effect of conducting fault have controlled the synergistic diagenetic evolution processes of sandstones and interbedded mudstones.The precipitation of vermicular kaolinite resulting from feldspar dissolution would promote the wettability of the reservoir change from water wettability into oil wettability,thus beneficial for early oil charge.The early oil charg would inhabit the cementation of carbonate and promote the later oil charg.At the same time,the later oil charg would inhabit the late carbonate cementation.The over processes are the main genetic mechanism of the high-quality reservoir in the research area.