The
pore structure characteristics of shales are controlled by
their mineralogical and organic matter (OM) compositions. However,
the contributions by different components in shales at varying thermal
maturities remain poorly understood. In this study, Devonian New Albany
Shale and Marcellus Shale samples spanning a thermal maturity from
marginally mature (vitrinite reflectance R
o 0.55%) to post-mature (R
o 2.41%) were
selected to study the control of the composition on the pore structure
properties of shales at different stages of thermal maturation. Scanning
electron microscopy (SEM) imaging was used to examine the pore types
in shales, and low-pressure N2 and CO2 adsorption
analyses were used to quantitatively characterize the mesopore and
micropore characteristics of bulk shales and major components in shales.
The results show that matrix-associated pores including interparticle
pores between silt-sized mineral grains, phyllosilicate framework
pores, and intraparticle pores within mineral grains exist in all
samples but become less common with increasing maturity, which is
likely caused by the elevated compaction, cementation, and occlusion
with bitumen. Secondary organic pores were not observed by SEM at
marginal maturity but were detected in the condensate-wet gas and
dry gas windows, with more organic pores in the dry gas window. At
marginal maturity, OM has large amounts of mesopores and micropores,
as demonstrated by low-pressure N2 and CO2 adsorption
analyses of OM isolated from shales, even though no OM-hosted pores
were observed by SEM. With increasing thermal maturity, the mesopore
and micropore specific surface areas (SSAs) of OM increase and make
greater contributions to the pore structure properties of bulk shales.
The mesopore and micropore properties of shales are controlled by
the OM content and maturity as well as by the clay mineral type and
content, and they can be estimated from the contribution of each component
at different stages of thermal maturation. Accurate evaluation of
the pore volume and SSA of shales will have important implications
for assessing gas adsorption and transport in shales.
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