A series of experiments
including porosity and permeability measurements,
thin section and scanning electron microscopy (SEM) observations,
incremental pressure mercury injection (IPMI), and nuclear magnetic
resonance (NMR) were conducted to systematically characterize the
pore structure of tight sandstone from the Lower Shihezi Formation
of Permian (P2x) in the northeastern Ordos Basin, China.
The influences of pore types, pore size distribution, and fractal
characteristics on reservoir quality of tight sandstones are also
investigated. Results show that the studied tight sandstones generally
possess poor quality and complex pore structure. The porosity and
permeability range from 4.08% to 17.56% (average 9.22%) and from 0.05
to 16.66 mD (average 2.49 mD), respectively. Five pore types were
observed in thin section and SEM images: primary intergranular pores,
intergranular dissolution pores, intragranular dissolution pores,
micropores within clay aggregates, and microfractures. The pore throats
are mainly hairy/fibrous, inhibiting the connectivity between pores.
Three types of pore structures were identified in the mercury-injection
curves and pore size distribution curves from the IPMI experiment
and in the T
2 relaxation time spectrum
obtained by NMR. Both experiments yielded consistent classifications,
and their combination was necessary to analyze the pore structure
effectively. In general, permeability and porosity are positively
related and depend on pore types. Large numbers of small pores confer
high storage capacity, whereas small numbers of larger pores improve
the flow capability. In the high porosity–permeability zone,
larger pores also determine the storage capacity. The P2x tight sandstone is fractal, and macropores are more heterogeneous
while micropores are more homogeneous. The fractal dimensions of macropores
are good indicators of the reservoir quality of the P2x
tight sandstone as larger fractal dimension values of macropores reflect
poor reservoir quality.
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