Study On The Diagenesis Of The Tight Sandstone Of The Chang 8 Member Of The Triassic Yanchang Formation In The Xifeng Area Of ??the Ordos Basin And Its Influence On The Quality Of The Reservoir

The Triassic Yanchang Formation is an important oil-producing horizon in the Ordos Basin.The existing diagenesis research results of the tight sandstone of the Yanchang Formation can not predict the irregular high-porosity and high-permeability“dessert zone”in the reservoir.Therefore,this paper takes the case of the tight sandstone reservoir of the Yanchang Formation Chang 8 Member in Xifeng Oilfield as an example.Detailed petrological and geochemical analysis studies were carried out to determine the type of diagenesis,reconstruct the history of diagenesis,and analyze the control effect of diagenesis system on the reservoir.It is hoped that the results of this study will provide important guidance for the later prediction of reservoir desserts and exploration deployment.The study included the analysis of 268 slices of 60 wells,measurement of pore permeability in 1073 samples,carbon and oxygen isotope analysis of 59 carbonate cements,scanning electron microscopy analysis of 59 sandstone samples,cathodoluminescence analysis of 19 sandstone samples,20 pieces of double-thick sheet fluid inclusion analysis and 40 sets of formation water data analysis,and the basic data obtained are then used to study reservoir characteristics and diagenesis.The tight sandstone reservoirs of the Yanchang Formation Chang 8 Member in this area are dominated by lithic feldspathic sandstones and feldspathic lithic sandstones,the types of reservoir spaces are such as intergranular pores,intergranular dissolved pores,intragranular dissolved pores and structural joints.The average porosity is 9.83%and the average permeability is 0.96mD of tight sandstone,which belongs to the low porosity and low permeability reservoir.The correlation is good between porosity and permeability by positively correlation to show a pore reservoir.The types of diagenesis of the Chang 8 Member mainly include compaction,pressure solution,dissolution,cementation and metasomatism.The main manifestations of sandstone compaction in the Chang 8 Member of the study area are:a.Plastic particle bending deformation.b.Rigid particle rupture.c.Detrital particles directional alignment or semi-directional alignment;the form of pressure solution is the concave convex contact to suture contact between the quartz particles;most of the feldspar has been dissolved,but it can be identified from the shape or residue of the mold hole,and some of the mold holes preserve the outline of the plate-like body of the feldspar crystal.Some of the mold holes preserve the residue materials of the original feldspar along the cleavage joint;siliceous,carbonate and clay cements precipitate between debris particles,or in the form of secondary enlargement of particles,occupying the pore space of sandstone;the metasomatism is represented by iron calcite and iron dolomite metamorphic quartz particles and dolomite cement.The diagenetic sequence in the Chang 8 Member was established by mineral combination characteristics and inclusion analysis:compaction?early chlorite filmfeldspar dissolution calcite/dolomite cementation?authigenic kaolinite?quartz secondary enlargement and authigenic quartz crystal illite/smectite mixed layer and illite/montmorillonite mixed layer?iron calcite and iron dolomite metamorphic dolomite cement?pores filling by illite and second-phase chlorite.Scanning electron microscopy showed that authigenic quartz and clay minerals were always associated with the dissolution of feldspar as a by-product,and did not transport over long distances,that consistent with closed geochemical systems.The dissolution of feldspar has little effect on the change of absolute porosity of sandstone,and only the redistribution of pore space position is shown.The analysis data show that the water type is mainly CaCl2 from current pore water,and the current pore water salinity with the pore water salinity in the diagenetic period calculated by the inclusion freezing point temperature both show an increasing trend with the increase of the buried depth,combined with the distribution pattern of authigenic mineral and feldspar dissolution suggests that the geochemical system during the diagenesis is closed in tight sandstone reservoir of the Chang 8 Member.In the early stage of diagenesis,23.7%-28.6%of the pores disappeared due to compaction.Closed diagenetic geochemical system is important during the mid-late stage of diagenesis,the authigenic quartz and clay mineral cements from the dissolution of feldspar always coexist with the feldspar dissolved pores,which have little effect on the change of the absolute porosity,it shows the redistribution of the pore space.Carbonate cements are formed due to the decarboxylation of organic matter in adjacent mudstones,which makes reservoir quality worse in sandstone edge,while the residual pore space is preserved in the middle of the sandstone(the distance from the sand/mudstone interface exceeds 1m),include residual intergranular pores and feldspar dissolved pores.Therefore,the high quality reservoir tends to be the middle part of the sandstone,especially the sandstone layer over 2m thick is a potential oil-bearing reservoir because it retains the best porosity and permeability.