Coalbed
is the carrier for coalbed methane (CBM) enrichment and
migration. The pore structure characteristics of coal and their main
geological controlling factors are critical to the exploration and
development of CBM. In this paper, 20 coal samples were collected
from eastern Yunnan and western Guizhou, China. Based on vitrinite
reflectance, proximate analysis, maceral analysis, and low-temperature
N
2
adsorption/desorption (LT-N
2
GA) experiments,
the hysteresis coefficient of low-temperature N
2
desorption
was proposed, the types of pore structure were identified, and the
effects of coal facies and rank on the pore structure were revealed.
The results show that the
R
o,max
values
of the 20 coal samples are between 0.74 and 3.38%, which belong to
medium- and high-rank coal. In the coal macerals, the vitrinite is
mainly collodetrinite. The inertinite is dominated by semifusinite,
and some coal samples contain exinite. The coal samples investigated
can be divided into two types. Type A samples mainly contain open
pores, while type B samples are rich in bottle-shaped pores. Compared
with type A coal samples, type B samples have the characteristics
of smaller total pore volume (TPV), smaller average pore diameter
(APD), larger specific surface area (SSA), and larger hysteresis coefficient.
The coal samples are located in three regions of different coal facies,
including low-level swamp (reed) facies, wetland herbaceous swamp
facies, and wet forest swamp facies. The tissue preservation index
(TPI) values of most coal samples are less than unity, which indicates
that herbaceous plants have absolute dominance in the coal-forming
plants in eastern Yunnan and western Guizhou. The maximum vitrinite
reflectance (
R
o,max
), gelification index
(GI), TPI, vitrinite content (
V
), inertinite content
(
I
), Barrett-Joyner-Halenda pore volume (
V
BJH
), Brunauer–Emmett–Teller SSA
(
S
BET
), and low-temperature N
2
desorption total hysteresis coefficient (
H
t
) were clustered using the R-type cluster analysis method.
It is found that TPI is the main controlling factor of the pore structure
of type A coal samples, while the pore structure of type B coal samples
are jointly controlled by TPI and coal rank. Type B coal samples are
mainly located in Zhuzang and Laochang high-rank coal research areas,
while the distribution of type A coal samples is mainly in other medium–high-rank
coal research areas. These results will contribute to the exploration
and development of CBM and also guide the study of pore structures
of other unconventional gas reservoirs.
