Rediscussion on Pore Genesis of Donghe Ssndstone——An example from Tazhong area

Key words: 
• Donghe sandstone /
• deeply buried /
• porosity /
• filling /
• dissolution /
• genesis /

Abstract: Donghe sandstone is typical shore-shallow-sea facies quartz sandstone with burial depth ranging from 3500 to 5100m and the largest thickness 129 meter formed during late Devonian (or early Carboniferous) in Tarim basin. Although it has great depth, it still has better reserroir quality (Porosity is about 20% and permeability 200 ×10^-3μm² ) and that is quite rare in the world. Different vie wpoints on its genesis yet exist.Some believe that the well-developped porosity was formed by dissolwtion of calcite cement in late diagenesis,but the others think it is primary proe retained after compaction and cementation. On the basis of mrmeraus data collected from casting section,thin section and SEM,the anthor draw a conclusion that pare in Dongre sandstone is chiefly primary porosity retained. after compaction and cementation; and the proofs are as follows:First,tire cornpaciion intensity on porasity is not strong and pore fillings come mainly from autirigenic clay,quartz and calcite nriner}als ilr wtrich tire clayfilm is the most common infillins form,and next clay fill,quartz overgrowth as well as carbonate fill. The carbonate cement is abundantly distributed around the sardbody, but irl the middle they present in spot,grutneleuse and laminae. Second dissolotion is raot well developped in the sandstone snd they mainly take place in class, quartz overgrowth and quartz grain us vela as feldspars while the carboncate cereut shows no any dissolution and they are not the remains produced by intersive dissaieaion. Under the background of the remained intergranular pore,the mixed pares formed by the remained primary pore and the secondary dissalution pore. Dissolution has the eminent contribution to the sandstone permeability. By quantitative analysis on tire evolution of porosity and related physical property parametevs,the author believe that the primary pore. suchas the pare with well-developped clove film and substrate loss porosity 17.82% by canpaction,and the other forms of loss by filling and compaction is 20%,27 % and 34.3%, respectively. The secondary dissalution can increase part of porosity by 1.49%. The protection condition of primary porasity has close relatian to the intgerated gealagicalfactors.