Micro characteristics and formation mechanism of low-quality gas reservoirs in Taiyuan Formation of Shenmu Gas Field, Ordos Basin

Abstract

Shenmu Gas Field in Ordos Basin shows the characteristics of low quality gas reservoir, and its deve-lopment effect is quite different from that of surrounding gas fields, which brings some problems to the exploration deployment and sustainable production of the gas field. In order to reveal the origin of low-quality gas reservoirs, the microscopic characteristics and formation mechanism of Taiyuan Formation reservoirs in Shenmu Gas Field were studied based on microscopic experiments at different scales. The results show that the reservoir of Taiyuan Formation in Shenmu Gas Field is characterized by "rich in quartz, poor in feldspar, and high content of rock detritus". The reservoir of Taiyuan Formation is a low porosity and ultra-low permeability tight sandstone reservoir with small pore-fine throat combination, poor pore throat connectivity, and weak reservoir permeability. The formation of low-quality reservoirs in Taiyuan Formation is affected by the matrix, the content of rock detritus in eruptive rocks and diagenesis. During the late Carboniferous to early Permian, the formation of the reservoir was significantly affected by the volcanic activity of the Inner Mongolia ancient uplift in the north of the Ordos Basin. The content of matrix and rock detritus of the eruptive rock in the sandstone was generally high. The rock detritus of the eruptive rock provided the main material basis for the development of secondary pores. While the matrix blocked the pores, it also produced a certain number of matrix dissolved pores, which had a dual impact on the reservoir. The dissolution during diagenesis is critical to the formation of Taiyuan Formation reservoir, and the increased porosity accounts for 64.3% of the current porosity. At the end of the Late Cretaceous, the late Yanshan movement led to the structural inversion of the Ordos Basin, the readjustment of gas and water, and the escape of natural gas along the fault in the eastern part of the basin. Finally, the low quality gas reservoir of Taiyuan Formation in Shenmu Gas Field is formed. The next exploration focus of Shenmu Gas Field should be based on the clear macro distribution law of diagenesis and favorable lithologic traps, and further search for the development area with high content of eruptive rock detritus and low content of matrix.

Keywords

• diagenesis
• pore throat structure
• tuffaceous matrix
• structural inversion
• taiyuan formation
• shenmu gas field
• ordos basin