Fractal characteristics and its controlling factors of pore-throat with different scales in tight sandstones of the Yanchang Formation in the Ordos Basin

Abstract

Pore-throat structure is one of the important factors affecting the characteristics of tight sandstone reservoirs. Pore-throat structure of tight sandstone is complex and with strong heterogeneity, thus, conjunction study with fractal theory bas become a promising approach. In this study, casting thin section, scanning electron microscopic observation and rate-controlled porosimetry were performed on tight sandstone samples of the Yanchang Formation in the Ordos Basin to study the distribution of pores and throats. With fractal principle and method, the fractal dimensions and influencing factors of tight sandstone pores and throats were studied. Results showed that there are clear inflection points on the fractal curves of lgSHg versus lgPc, and according to the mercury pressure corresponding to the inflection points of fractal curves, pores and throats of tight sandstones can be classified to be large and small ones respectively. Fractal dimensions (D) were calculated by the means of slope of the straight part of each curve, and the fractal dimensions of pores and throats greatly varied between large pore-throats and small pore-throats. For the small pore-throats, compaction and cementation effects are strong, mainly developing intercrystalline pores, residual intergranular pores and necked throats. The heterogeneity of small pore-throat is weak, the pore-throat space is less deformed, and the fractal dimensions are small (2 7). It was indicated that diagnosis, including compaction, cementation and dissolution, greatly affected the size and shape of pore-throat in tight sandstone, and determined the fractal characteristics of pore-throat with different scales.

Keywords

• pore-throat structure
• fractal dimension
• rate-controlled porosimetry
• tight sandstone
• yanchang formation
• ordos basin