Sedimentary Characteristics And Reservoir Diagenetic Reconstruction Model Of Thin Sandbodies Of Southern Gentle Slope Belt In Dongying Depression

Thin sandbodies are separated by stable mudstones with the thicknesses are usually lessthan8m and lateral distributions are wide in interbedds of sandstones and mudstones insedimentary basin. The ratio of thickness of thin sandbodies and thickness of strata is usuallybetween0.1and0.6. The lithology is usually siltstone, fine sandstone and medium sandstone.The lithology of thin sandbody separated by stable mudstones can be single type or verticalsuperimposed of multiple types. Thin sandbodies of gentle slope belt in rift lacustrine basinare important type of sandbody. Usually, thin sandbodies are characterized by frequentchanges of thickness in vertical, wide distribution in plane, various types of sedimentaryorigin, complex diagenesis and difficult prediction of reservoir development. We take the thinsandbodies of Ek1-Es4of sounthern gentle slope belt of Dongying depression, the typicalrepresentative of rift lacustrine basin, as the study object. The sedimentary environment andthe corresponding genetic types, the distribution and sedimentary evolution models, thereservoir characteristics and diagenesis, the burial diagentic environment, the diageneticreconstruction models and their reservoir distribution of thin sandbodies were studied by theanalysis of drilling data, logging data, laboratory analysis data, thin sections, casting thinsections, scanning electron microscopy, mercury intrusion, physical properties,carbon-oxygen isotope and fluid inclusions.During the sedimentary period of Ek1-Es4x, the climate was arid and characterized byfrequently altertive of arid and relatively moist climate, the lake was frequently oscillatory.During the sedimentary period of Ek1-Es4xx, the sedimentary environment was low waterlevel frequently oscillatory floodover lake-salt lake, the gentle slope belt of Dongyingdepression was floodover lake, which controlled the development of floodover lacustrine delta and floodover lacustrine beach-bar. During the sedimentary period of Es4xs, thesedimentary environment was high water level frequently oscillatory floodover lake-salt lake,the gentle slope belt of Dongying depression was fluvial-floodover lake, which controlled thedevelopment of shallow water delta and floodover lacustrine beach-bar. During thesedimentary period of Es4s, the sedimentary environment was saltwater shore-shallowlake-semideep lake, the gentle slope belt of Dongying depression was shore-shallow lake,which development of large scale of shore-shallow lacustrine beach-bar. The genetic types,develop positions, internal stuctures, accumulation laws and develop abundances of thinsandbodies are controlled by the paleoclimate, paleogeomorphology, paleohydrodynamic,paleo-base level and paleoprovenance. Based on the study of genetic types and distribution ofthin sandbodies, the sedimentary evolution models of different sedimentary environmentswere studied.Analysis of diagenetic products, carbon-oxygen isotope and fluid inclusions indicatedthat the diagenetic fluids of floodover lacustrine thin sandbodies mainly experienced theprocess of early alkaline-early acid-late alkaline-late acid, the formation fluids occurred asupwelling currents in open diagenetic environment; because of the tectonic uplift, thediagenetic fluids of floodover lacustrine thin sandbodies of Gaoqing area experienced theprocess of early alkaline-medium atmospheric water eluviation-late acid, the formation fluidsmainly occurred as downwelling currents in open diagenetic environments; the diageneticfluids of shore-shallow lacustrine thin sandbodies experienced the process of early weakalkaline-early acid-late alkaline-late acid, the closeness of diagenetic environment of west partexperienced the early open-medium open-late close evolution process, and the closeness ofdiagenetic environment of east part experienced the early close-medium open-late closeevolution process. Influenced by the differences of formation temperature, diagenetic fluidsand closeness of diagenetic environment, the differences of compaction, develop degree ofcemented case, types and content of dissolution pores and distribution of diagenatic productsbetween reservoirs of floodover lacustrine thin sandbodies and reservoirs of shore-shallowlacustrine thin sandbodies are obviously. The diagenetic sequence of floodover lacustrine reservoirs was mainly early carbonate and gypsum cementation, early quartz dissolutionâ†'feldspar and carbonate cement dissolution, authigenic quartz cementationâ†'late carbonate andanhydrite cementation, late quartz dissolutionâ†'late a spot of carbonate cement and feldspardissolution, a spot of authigenic quartz cementation. The diagenetic sequence of floodoverlacustrine reservoirs of Gaoqing area was early carbonate cementation, early quartzdissolutionâ†'feldspar and carbonate cement dissolution (eluviation zone), authigenic quartzand carbonate cementation (precipitation zone)â†'late a spot of carbonate cement and feldspardissolution, a spot of authigenic quartz cementation. The diagenetic sequence ofshore-shallow lacustrine reservoirs was early weak carbonate cementationâ†'feldspar andcarbonate cement dissolution, authigenic quartz cementation (early carbonate cementation atedge of sandbodies)â†'late carbonate and authigenic feldspar cementation, late quartzdissolutionâ†'late carbonate cement and feldspar dissolution, a spot of authigenic quartzcementation.Based on the study of burial diagenetic environments and their diagenetic responses, thediagenetic reconstruction models of reservoirs of floodover lacustrine thin sandbodies andshore-shallow lacustrine thin sandbodies were researched. The floodover lacustrine thinsandbodies mainly developed diagenetic reconstruction model of alternate evolution ofmultiple alkaline and acid diagenetic environments-action mechanism of upwelling currents,the high quality reservoirs mainly developed at middle part of thick sandstones at themiddle-lower part of fault blocks, the reservoir spaces were mainly primary intergranularpores and feldspar and quartz dissolution pores. The floodover lacustrine thin sandbodies ofGaoqing area developed diagenetic reconstruction model of evolution of alkaline and aciddiagenetic environments-action mechanism of downwelling currents, the high qualityreservoirs mainly developed within a certain depth below the unconformity, and the quality ofreservoirs became worse gradually witt the increase of distance to the unconformity, thereservoir spaces were mainly primary intergranular pores, feldspar dissolution pores and closegrain joints. The shore-shallow lacustrine thin sandbodies of west part developed diageneticreconstruction model of alternate evolution of multiple alkaline and acid diagenetic environments-early open-medium open-late close diagenetic environments, the high qualityreservoirs mainly developed at middle part of thick sandstones at the middle-lower part offault blocks, the reservoir spaces were mainly primary intergranular pores and feldspardissolution pores. The shore-shallow lacustrine thin sandbodies of east part developeddiagenetic reconstruction model of alternate evolution of multiple alkaline and acid diageneticenvironments-early close-medium open-late close diagenetic environments, the high qualityreservoirs mainly developed at middle part of thick sandstones at the middle part of faultblocks, the reservoir spaces were mainly primary intergranular pores and feldspar dissolutionpores. Alternate evolution of multiple alkaline and acid diagenetic environments controlledthe distribution patterns of diagenesis and reservoir spaces. The diagenetic closeness and itsresponsed flow patterns of formation fluids controlled the differentiation of reservoirproperties. Controlled by the characteristics and the strength of diagenetic reconstruction indifferent positions of thin layer sandstone, their may develop a considerable number ofdiagenetic trapps in thin layer sandstone reservoirs.