Recently, deep shale
reservoirs are emerging as time requires and
commence occupying a significant position in the further development
of shale gas. However, the understanding of pore characteristics in
deep shale remains poor, prohibiting accurate estimation of the hydrocarbon
content and insights into fluid mobility. This study focuses on the
Longmaxi Formation from the Luzhou (LZ) region, southern Sichuan.
Scanning electron microscopy (SEM), low-temperature N2/CO2 adsorption,
X-ray diffraction, and geochemical analysis were performed to investigate
the micro–nanopore size distribution, main controlling factors,
and unique pore features distinct from other regions. Results showed
that the pores can be classified into four categories, organic matter
(OM) pores, intergranular pores, intragranular pores, and microfractures,
according to SEM images. The total pore volume is overwhelmingly dominated
by mesopores and contributed by pores in the range of 0.5–0.6,
2–4, and 10–30 nm. The specific surface area is primarily
contributed by micropores and mesopores in the range of 0.5–0.7
and 2–4 nm. By analyzing the influencing factors extensively,
it is concluded that the buried depth, geochemical factors, and mineral
composition can impact the pore structure in the overmature deep shales.
Specifically, the total organic carbon content plays a more effective
and positive role in the development of micropores, mesopores, total
pores, and the porosity when compared with vitreous reflectance (Ro).
The micropores are inferred to be OM-related. On the contrary, clay
mineral is detrimental to the development of micropores and mesopores
and the petrophysical properties (porosity and permeability), which
may be attributed to the occurrence of chlorite and kaolinite instead
of illite. The plagioclase conforms to the same law as clay due to
their coexistence. Quartz, carbonate minerals, and pyrite can barely
contribute to the pores. Eventually, the compared results suggest
that the Longmaxi Formation of the LZ region are qualified with a
superior pore size distribution, complicated structure, and diverse
morphology, implying a potential to generate and store hydrocarbons.
Overall, this study improves the understanding of complex pore structures
in deep shale and provides significant insights into the development
and exploration of unconventional resources in the future.