Abstract:

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 SiO₂ is 72.76%-74.96%,that of Al₂O₃ is 16.17%-16.17%,K₂O 1.93%-1.93%,CaO 0.08%-0.08% and Fe₂O₃ 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.

Key words: bleached Pisha sandstone, petrology, geochemistry, natural gas, Ordos Basin