Developmental characteristics and distribution law of fractures in a tight sandstone reservoir in a low‐amplitude tectonic zone, eastern Ordos Basin, China

Abstract

In recent years, a large number of vertical and horizontal sliding fractures were found in the Upper Palaeozoic sandstones in the eastern margin of the Ordos Basin, China, However, the effect of fractures on the distribution of tight sandstone gas is still unclear. Therefore, in this paper, a large number of cores, thin sections, petrophysical property test results, conventional and FMI (Formation MicroScanner Image) image logs, and gas‐testing results were used to study the developmental characteristics of fractures and the gas control effects in a tight sandstone reservoir, in the eastern margin of the Ordos Basin. It was found that the average linear density of the core fractures is generally less than 2 m−1, and the microfractures are characterized as ruptures of quartz particles. The underlying Taiyuan and Benxi formations have the highest fracture development degrees, and the fracture index (FI) values are 0.68 and 0.72, respectively. However, the fracture development degrees in the overlying Shanxi to Shiqianfeng formations are significantly lower, and the FI values are generally less than 0.63. There is a good positive correlation between the fracture development degree and the contents of quartz mineral or brittle minerals (quartz + feldspar). On the whole, the fractures in the uplift and slope parts of the low‐amplitude structure areas are more developed, while in the depression zones, the fractures are generally less developed. The fractures in the Zijinshan Uplift area are extremely well developed; thus, the preservation conditions of the reservoir are poor. In this case, the fractures are unfavourable for the accumulation of natural gas. For the gentle tectonic zones, the over‐developed fractures in the Benxi and Taiyuan formations are unfavourable for the preservation of natural gas. However, the moderately developed fractures in the overlying Shanxi to Shiqianfeng formations are beneficial to the enrichment of natural gas. Finally, we compared the relationship between fracture index (FI) and open‐flow productivity of the gas wells, and it was found that FI values of commercial gas wells (Types I, II, and III gas wells) are generally greater than 0.55, while the FI values of Type IV low‐yield gas wells are generally less than 0.55.