Enrichment factors and play fairway mapping for tight gas in the 5^th member of the Permian Shiqianfeng Formation, Shenmu gas field, Ordos Basin

The study intends to clarify the undefined dominant factors controlling the tight sandstone gas enrichment and challenges associated with play fairway mapping through the exploration of the 5^th member of the Permian Shiqianfeng Formation (hereafter referred to as the Qian 5 Member) in the Shenmu gas field, Ordos Basin. A combination of seismic, drilling, log, and laboratory data is applied to examine the characteristics of tight-gas reservoirs, analyze the impacts of various factors including faults, reservoirs, cap rocks, barriers, source rocks, and the elevation on tight gas enrichment, and map play fairways for tight gas exploration. The results indicate that the sand bodies of the Qian 5 Member originate from the distributary channels of the braided river delta plain subfacies and that inherited strike-slip faults play a role in improving the reservoir properties and transporting natural gas. The gas reservoir in the member exhibits a porosity mainly ranging from 6.0 % to 12.0 % (average: 9.1 %) and permeability from 0.20×10^-3 to 0.80×10^-3 μm² (median: 0.49×10.0^-3 μm²). The reservoir is of a lenticular tight gas reservoir, exhibiting a quasi-continuous distribution characterized by lateral contiguity and vertical superimposition. Primary factors controlling tight gas enrichment include faults and reservoir physical properties. In contrast, the thicknesses of cap rocks, barriers, and coal seams in source rocks produce relatively weak controlling effects on tight gas enrichment, and the elevation delivers no impact in this regard. In the case where the distance between a gas well and faults exceeds 9 km, the reservoir shale content is greater than 12 %, and the thickness of mudstone barriers exceeds 210 m, the degree of natural gas enrichment increases as the values of these parameters decrease. Conversely, under conditions where the porosity is below 9 %, the permeability falls below 0.5×10^-3 μm², the thickness of mudstone cap rocks is less than 120 m, and the coal seam thickness within source rocks is less than 15 m, the degree of natural gas enrichment improves with increasing values of these parameters. It is proposed to employ the gas enrichment indices to map the play fairways of tight-gas reservoirs. Using this method, a total of 14 Class Ⅰ and 22 Class Ⅱ play fairways primarily distributed in the eastern Shenmu gas field are identified.