Study on bleaching of sandstone of different ages in the world indicates that the formation of bleached sandstone is relevant to organic fluid flows.It can provide some clues to hydrocarbon migration

accumulation and dissipation

thus can be used as an evidence for predicting the existence or destruction of oil/gas reservoirs(or fossil oil/gas reservoirs) in nearby areas.The occurrence of oil and gas in Ordos basin would have been altered during the later reconstruction of the basin.Based on analysis of the major elements

rare-earth elements and petrologic characteristics of Mesozoic bleached sandstone in the northeastern Ordos basin

it is preliminarily believed that the bleached sandstone in the study area would have been formed in an acidic and reducing environment.The bleached sandstone is characterized by its relatively high content of kaolinite and pyrite

and the color of ferric oxide minerals on the edge is lighter than that in the center.The major element analysis of sandstone samples shows that they have relatively high Al

2

O

3

content and low Fe

3+

/Fe

2+

ratio;the REE analysis shows that the Eu contents in both core and outcrop samples are negative;all these indicate a reducing environment.At the same time

the geochemical characteristics of oil sand in the northern part of the basin indicate that the oil is exactly condensate as a result of the migration and dissipation of Permo-Carboniferous humic gas in the south to the surface.In a word

the comprehensive study suggests that the bleaching of Mesozoic sandstone in the study area indicates dissipation of natural gas.