Lithology And Physical Property Distribution Patterns Of Sandy Braided River Near Architectural Bounding Surfaces

The overlapping interface of sand body is complex due to the frequent migration and diversion of sandy braided river.Lithology may not change abruptly on both sides of the architectural interface of different orders in thick sandstone.More kinds of lithologies near the interface are gradually changed,and this kind of interface is not completely a fluid seepage barrier,which has an important effect on reservoir heterogeneity near the architectural bounding surfaces.This paper aims to analyze the distribution patterns of lithology and physical properties near the architectural interface of different orders in sandy braided fluvial reservoir,by taking the sandy braided fluvial reservoir of Guantao Formation in the west 7th block of Gudong Oilfield as the main research area,and combining the typical field outcrops of the sandy braided river in Datong,Shanxi Province.The 5th-order interface is the boundary of a single channel(braided channel belt).Channels mainly present three types of isolation,vertical superimposition and lateral migration superimposition.The 4th-order interface is the boundary of a single midchannel bar or braided channel-fill.The mid-channel bar mainly develops three types of isolation,lateral migration superimposition and vertical superimposition.The 3rd-order interface is the boundary of the accretion inside the mid-channel bar.The interface presents two types of nearly horizontal and top convex.There are positive correlations between the width of the channel belt and the width of the mid-channel bar,the length and the width of the mid-channel bar,the width and thickness of the mid-channel bar,and the width and the thickness of the accretion.There are 11 kinds of distribution patterns of lithology near the architectural interface of different orders of sandy braided river,based on the analysis of the outcrops of the Middle Jurassic Yungang Formation in Datong Basin and the underground thick sandstone reservoir of the Guantao Formation in the west 7th block of Gudong Oilfield.There are 3 modes of lithological changes near the 5th-order interface.The scouring surface at the bottom of the isolated channel is a lithological abrupt change surface,below which the lithology changed to floodplain mudstone.The bottom of the isolated channel with a thin layer of scoured muddy gravels is also changed abruptly to floodplain mudstone.There is no obvious lithological change above and below the superimposed interface of the two channels.There are 5 modes of lithological changes near the 4thorder interface.When the overlying mudstone remains between two superimposed midchannel bars,the overlapping interface is an obvious scouring surface,and the overlying mudstone is actually composed of mudstone and muddy siltstone.When there is no overlapping mudstone between the superimposed mid-channel bars,the scouring surface is not obvious,there is actually a thin layer of scoured muddy gravels near the superimposed interface.Another pattern is that there may also be no obvious lithological changes above and below the superimposed surface of two mid-channel bars.There is no obvious lithological changes near the superimposed surface of mid-channel bar and the braided channel-fill with full sand.When the mid-channel bar and the abandoned channelfill are superimposed,the lithology gradient pattern of muddy siltstone and mudstone appears near the interface.There are 3 types of lithological changes near the 3rd-order interface where the accretions are superimposed inside the mid-channel bar.The superimposed interface of the accretion is a lithological abrupt change surface,the upper and lower lithology of the interface is changed from sandstone to thin layer mudstone.There is no obvious scouring surface between the accretions,and there is a fine-grained muddy siltstone near the overlapped interface.There can also be no obvious lithological changes.According to the physical property test of the coring wells of the Guantao Formation in the west 7th block of Gudong Oilfield,the analysis shows that there are 11 modes of physical property changes near the architectural interfaces of different orders.There are3 modes of physical property changes near the 5th-order bounding surface.The first one is that the permeability abruptly changes to 0 below the bottom surface of the isolated channel.The second one is that the permeability gradually decreases above the bottom surface of the isolated channel and changes to 0 abruptly below the surface.Thirdly,there is no obvious difference of permeability above and below the superimposed surface of two channels.There are 5 modes of physical property changes near the 4th-order bounding surface.The first one is when the overlying mudstone remains between two superimposed mid-channel bars,the permeability near the superimposed interface presents a high-low-high trend and can be reduced by 90%.The second one is when muddy gravels deposit near the superimposed interface of two mid-channel bars,the permeability presents a high-low-high trend and can be reduced by 40%.The third pattern is that there is no significant change in permeability near the superimposed interface of mid-channel bars.The fourth one is that there is no obvious change in the permeability near the superimposed interface of mid-channel bar and the braided channel-fill with full sand.The fifth one is the permeability near the superimposed interface of mid-channel bar and the abandoned channel-fill presents a high-low trend,and changes to 0 above the interface.There are 3 distribution modes of physical property near the 3rd-order interface:The first is that the permeability near the superposed interface of thin mudstones between the accretions shows a high-low-high trend,and the permeability can be reduced by 25%.The second is that the permeability near the superposed interface of thin muddy siltstones between the accretions shows a high-low-high change pattern,and the permeability can be reduced by 20%.Thirdly,there is no significant change in the permeability near the superimposed interface of two accretions.Innovate the 3D architectural modeling method for sandy braided fluvial reservoir,focus on describing the lithological change zone model near the 5th-order and 4th-order bounding surface.Divide the grid of the architectural unit and the lithological change zone near the bounding surface separately,and make fine simulation of lithology and physical properties inside the thin lithological change zone.Therefore,the changes of lithology and physical properties would not be coarsened in the later stage,which could ensure the fine architectural modeling results to be fully applied in in the industrialized numerical simulation of reservoirs.