Sand body configuration and gas-bearing properties of near source sand-gravel braided river on the northern margin of Ordos Basin

The first member of the Lower Permian Shihezi Formation in the Duguijiahan area of the Hangjinqi region on the northern margin of Ordos Basin was deposited on the southern slope of Gongkahan paleocontinent, forming a set of sand-gravel braided river deposits near the provenance area. The physical properties of sand bodies are well correlated with gas-bearing properties; however, the heterogeneity of channel sand bodies is very strong, resulting in great differences in gas-bearing properties. The sand body configuration of the sand-gravel braided river in the 1st member was analyzed. The lithofacies characteristics and main configuration units were clarified. Through core observation and logging data analysis, six typical lithofacies were classified in the 1st member, with channel bar and braided channel as the main configuration units. The channel bar generally has four lithofacies assemblages:Gm (gravel massive bedding)-Sgt (gravel-bearing sandstone trough cross bedding)-Sm (sandstone massive bedding), Gm-Sm-Sp (sandstone parallel bedding), Sgt-Sm and Sm-Sp. The braided channel generally has four lithofacies assemblages:Gm-Sp, Gm-St, Sgt-Sp and Sgt-St. This lithofacies changes are the main reason for the strong heterogeneity. The statistical data from vertical wells, horizontal wells and seismic data show that the length of the channel bar is 230-1 050 m. According to empirical formulas, the width of the channel bar is 43-386 m. The channel bar sand body and braided channel sand body are laterally symbiotic, but their physical properties and gas-bearing properties are obviously different. Statistical data show that the physical properties and gas-bearing properties of channel bar sand body are better than those of the braided channel sand body.