Effects of secondary pores within K-feldspar and laumontite on the reservoir quality of tight sandstone from the Upper Triassic succession, Ordos Basin, China

The Chang-6 member of the Upper Triassic Yanchang Formation in the Ordos Basin is a productive tight sandstone reservoir. In addition to residual primary intergranular pores, secondary pores within K-feldspar and laumontite are key components of the pore system in these reservoirs. However, the specific roles of these secondary pores in reservoir quality remain unquantified. This study integrated back-scattered scanning electron microscopy with energy dispersive spectrum (BSEM-EDS) analysis, image processing, and fractal analysis to characterize the secondary pore structures within K-feldspar and laumontite. To address challenges in quantifying their contributions to reservoir quality, a novel method combining the results of BSEM-EDS, mercury intrusion capillary pressure (MICP), and nuclear magnetic resonance (NMR) was developed. The results indicate that the secondary pores within K-feldspar and laumontite primarily serve as reservoir throats, controlling reservoir permeability. K-feldspar dissolution exhibits a negative correlation with its Na⁺ content, especially when primary intergranular pores are incompletely filled by interstitial materials. Laumontite dissolution is inhibited by high Ca²⁺ concentration in the formation water and a closed diagenetic environment. Mineral brittleness and cleavages significantly influence the pore structure complexity of K-feldspar and laumontite. Compared with laumontite secondary pores, K-feldspar secondary pores contribute more substantially to reservoir quality, providing critical insights for “sweet spot” identification in tight sandstone reservoirs.