Effect of pore structure on oil‐bearing property in the third member of Paleogene Funing Formation in Subei Basin, East China

Abstract

Mercury injection capillary pressure (MICP) tests, nuclear magnetic resonance (NMR), (fluorescence) thin section, X-ray diffraction (XRD), and scanning electron microscope (SEM) analyses were used to describe the size and distribution of entire pore-throat structures in the sandstones of the E 1 f 3 (the third member of Paleogene Funing Formation) in the Subei Basin. The lithologies of E 1 f 3 in the Subei Basin are mainly dark-gray, very fine-grained sandstones and siltstones, interbedded with dark mudstones. The pore systems predominantly feature secondary intergranular and intragranular dissolution pores, micropores coexisting with minor amounts of intergranular pores, and microfractures. The high threshold pressure and bulk volume of irreducible fluids values and the significant variation in the NMR and MICP parameters indicate that the E 1 f 3 reservoirs are characterized by complex and heterogeneous microscopic pore structures. Microscopic pore-throat parameters are linked with macroscopic properties through the reservoir quality index (RQI). The NMR T 2 (transverse time relaxation) spectrum is unimodal or bimodal but with weak right peaks, indicating the rarity of large intergranular pores. However, large-scale pore throats, though only account for a minor part of the total pore volume, significantly contribute to the total permeability. The abundance of small-scale pore-throat systems (short T 2 components) results in high irreducible water content. Therefore, the oil saturation in E 1 f 3 sandstones is low, and the pore structure, especially the number of micropores, determines the oil-bearing property. Oil primarily occurs in the intragranular dissolution pores with minor amounts occurring in the large intergranular pores. Most of the micropores are bound by capillary water. The sandstones with chlorite clay minerals tend to be oil-wet and have high oil-bearing potential, while the abundance of detrital clay or illite contributes to a low oil-bearing grade. The combination of core and microscopic observations and the MICP and NMR analyses have allowed the determination of the pore structure characteristics and their coupling effects on oil-bearing property. |