Bleached Pisha sandstones are widely distributed in Ordos Basin and mainly occur in Yan'an Formation of the Mesozoic

In Dongsheng area，the bleached Pisha sandstone overlying on the coal seam is an important evidence of the existence and migration of hydrocarbons.In this paper

the samples collected during field geological investigation were analyzed by using polarization microscope

scanning electron microscopy(SEM)

X-ray diffraction and X-ray fluorescence spectrum and inductively coupled plasma mass spectrum.The results show that the main mineral composition of the bleached Pisha sandstone are quartz and alkali feldspar

followed by illite

kaolinite and a small amount of illite/smectite mixed layer.The content of SiO

2

is 72.76%-74.96%

that of Al

2

O

3

is 16.17%-16.17%

K

2

O 1.93%-1.93%

CaO 0.08%-0.08% and Fe

2

O

3

0.01%-0.69%.Hf

Zr

Ss

Ba and U are relatively enriched.The ∑REE and LREE/HREE was are in the ranges of 67-296 μg/g and 7.36-13.67

respectively.REE distribution pattern features in light REE enrichment

but heavy REE deficient

with significantly negative anomaly of Eu and weak deficient of Ce(the content of Ceanom being in the range of -0.07--0.17

averaging at -0.12)

suggesting an anoxic reducing environment.The studies show that the bleached Pisha sandstone is the result of sulfate and natural gas reduction

and thus can provide a geologic basis for the formation and dissipation of oil and gas in the study area.