Log facies types of tight sandstone reservoir in the Daniudi gas field

Daniudi gas field is located in the north of Ordos Yishan slope of the basin,the main gas bearing strata are the upper Paleozoic Carboniferous Taiyuan formation,Permian Shanxi formation and Shihezi formation,which belongs to tight sandstone reservoir.Daniudi gas field mainly experienced the transgression in late Carboniferous,regression in early Permian transgressive delta in Shanxi period,sea-land transitional fluvial deposits in lower Shihezi formation and there formed various log facies types.By using natural gamma curve,the reservoir can be divided into eight rock types,the reservoir and favorable sedimentary facies under different sedimentary systems were made clear.He_3,He_2 and He_1 of Shihezi formation are mainly of braided fluvial facies deposit and the favorable facies belt is braided river main channel.Shan_2 and Shan_1 of Shanxi formation are mainly of delta plain sedimentary and the favorable facies belt is distributary channel.Tai_2 of Taiyuan formation is mainly of barrier island-lagoon deposits and the favorable microfacies is barrier bar.