Sedimentation environment and characteristic of highyield gas horizon of the third member of Shihezi Formation in Daniudi gas field

Geographically located in the boundary of Shaanxi and Inner Mongolia,Daniudi gas field is in the north of eastern Yishan slope of Ordos basin.Its upper Palaeozoic group is a- bout the erosional surface of caledonian movement,and the sedimentary setting was evolved from the marine to nonma- rine facies,the fluvial to the lacustrine facies,and the humid to arid lithofacies palaeogeography.The third member of lower Shihezi Formation was deposited in this sedimentary setting,with mainly the meandering stream deposit in arid palaeoclimate.Marginal banks are the dominated sedimenta- ry type.The rock types of the sandstone in the medium and low part of marginal bank are mainly debris quartz sandstone and glutenite,with coarse clastic particles,fair separation and rounding,high maturity of mineral and more than 85% of quartz contents.The rigid granulometric quartz of high content and coarse granularity enhances the capacity against mechanical compaction,and is favorable to the conservation of primary pores and the generation of secondary of seconda- ry pores.The pore types are multifold in Daniudi gas field, of which the aperture of solution hole is relatively large;the aperture of intergranular dissolved pore is 30～300μm and the maximum is up to 600μm;and the intragranular dis- solved pore is 20～200μm.The high-yield gas horizon is characterized by low spontaneous gamma value,medium to high resistivity,and medium to high sonic differential time.