The Discussion Of Mechanism Of Tight Sandstone's Formation

In order to understand the formation mechanism of tight sandstone,the paper take Shanxi formation of the eastern part of Ordos basin as example,through thin sections,cathodoluminescence,scanning electron microscopy and other means of observing and analyzing the characteristics of rocks and minerals.By means of quartz precipitation power analysis of the diagenetic environment siliceous mineral deposition rate,inclusion temperature measurement calibrated silica precipitation from diagenetic stage.by basin modeling software restore ancient diagenetic environment,and introduced the process of tight sandstone from the various stages of diagenetic.In the research area,the coal source rock is ? kerogen,with a strong acid production capacity,the production of acid is about 50~220mg/g kerogen.? kerogen make the micro diagenesis to be acidic environment,the pore fluid control the diagenetic authigenic minerals and mineral dissolution and precipitation.In the acidic environment of the micro-pore well developed quartz overgrowths side,and the main is ? and ? overgrowth edge,? overgrowth edge had rarely seen,the thickness of the overgrowth edge is in 10~70?m,the majority concentrated in 20~50?m,the individual is less than 10?m or is more than 100?m,due to kaolinite,overgrowths side is jagged edges.Quartz overgrowth side widely developed the fluid inclusions,? overgrowth of quartz sides of the homogenization temperature is usually in the 80~120 ?,? of quartz overgrowths edge homogenization temperature is usually in the 130~150?,? and above of quartz overgrowths edge homogenization temperature is usually in the 140~175?.Combined with inclusions data of the quartz overgrowth edge,quartz precipitation and basin dynamics model diagenetic burial history data analysis,in the 280~260Ma,Shan 2 deposition,the original porosity is about 34%,in the 225 Ma of the early diagenetic stage A,at this point palaeotemperature is 65? and the depth is 1150 m,mechanical compaction reduce the porosity to 13.18%,the total porosity reduced to 20.82%;in the 225~220Ma,as early diagenetic stage B period,at this point palaeotemperature is 65~85? and the depth is 1150~1650m,mechanical compaction reduce the porosity 6.87%,? grade of overgrowth quartz edge reduce the porosity 2.06%,dissolution increased porosity 0.8%,in total porosity reduced to 12.69%;in the 220~175Ma of late diagenetic stage A,at this point palaeotemperature is 85~140?and the depth is 1650~3650m,mechanical compaction reduce the porosity 4.91%,? grades of quartz overgrowth edge development reduced the porosity 6.12%,the dissolution increased porosity 5.7%,the total porosity reduced 7.36%;in the 175~158Ma,as late diagenetic stage B period,at this point palaeotemperature is 140~170? and the depth is 3650~4100m,mechanical compaction reduce the porosity 0.8%,? grades of overgrowth quartz edge reduce the porosity 1.24%,dissolution increased porosity 1.5%,in total porosity reduced to 6.82%;158~93Ma is late diagenetic C stage,at this point palaeotemperature is higher to 170? and the depth is higher to 4100 m,mechanical compaction reduced the porosity 0.61%,Siliceous cementation reduced the porosity 0.81%,the total porosity eventually reduced to 5.4%.Sandstone densification further intensified development,it is the result of the combined effects of tight sandstone diagenetic environment and diagenesis.