Laboratory
measurements including porosity, permeability, high-pressure
mercury intrusion (HPMI), nuclear magnetic resonance (NMR) measurements,
and microscopic analysis of thin sections and scanning electron microscopy
(SEM) were performed to provide insights into the microscopic pore
structure of the Xujiahe Formation tight sandstones in Sichuan Basin.
The relationships between microscopic pore structure parameters, such
as pore geometry, pore size distribution, pore network, and macroscopic
consequences, such as permeability, reservoir quality index, Swanson
parameter, fractal dimension as well as NMR parameters, have been
investigated. The results show that the pore systems are dominated
by secondary dissolution porosity with minor amounts of primary porosity
and microfracture. NMR T
2 pore size distributions
are either uni- or multimodal. Long T
2 components are not frequently present due to the lack of the macropores,
whereas as shorter T
2 components dominate
the T
2 spectrum. T
2gm (the geometric mean of the T
2 distribution) shows good correlations with movable porosity and
irreducible water saturation. The pore throat distributions from HPMI
analysis show uni- or multimodal, and are narrower than the NMR T
2 pore size distribution. The r
apex, which is the apex of the hyperbola of Pittman (1992),
is well correlated with the entry pressure, r
35 and r
50. The pore throats larger
than r
apex, which only account for a small
fraction of the pore volume, dominate the permeability of the reservoir
rocks. Skin effect, high working pressure as well as oversimplification
of cylinder pore shapes result in the high fractal dimension of the
larger pores (>r
apex). The smaller
pores,
which can be quantitatively characterized by the fractal dimension,
control the microscopic heterogeneity of reservoir rocks. The reservoir
quality index (RQI) shows good relationships with both the NMR parameters,
such as T
2gm, and the HPMI parameters,
such as r
apex. Integration of routine
core analysis with HPMI test and integration of routine core analysis
with NMR measurements show that the RQI, which links the pore-throat
sizes resulted from HPMI tests with the pore-size distribution from
NMR measurements, is a good indicator to reservoir heterogeneity in
terms of macroscopic reservoir property and microscopic pore structure.
