Relationship between shale gas reservoir pores structures/connectivity
and organic matter/inorganic matrix in Wufeng-Longmaxi formation of
Southeast Sichuan basin, China
Abstract
Abstract: In Sichuan basin, the Longmaxi-Wufeng formation is rich of the
shale gas, and the study of control factors of pores structure and
connectivity are extremely significant for exploration of shale gas.
Organic matter and inorganic matrix attribute to shale reservoir
characters strongly. Though this work has been carried out by others
before, it still has some arguments for this topic. In this project, we
collected rock samples from oilfields, we did SEM, CO2 adsorption CH4
adsorption N2 adsorption, high pressure mercury injection, microscope
observation, NMR, XRD, TOC test some related experiments. Analytical
results show that TOC contents of the samples range from 0.96% to
6.12%, and the Wufeng-Longmaxi formation shale organic matter content
is high. The samples porosity and permeability range from 0.49%-9.55%
and 0.0006mD-0.73mD respectively, and the I/S of clay mineral amount
ranges from 10%-60%. A plenty of graptolites were observed in the
formation under microscope, and reflectivity of graptolites and solid
bitumen can reveal that organic maturity is high in the formation. The
higher the TOC percentage of sample, the higher the graptolite
reflectance is. There are hydraulic fractures under microscope and SEM
observation, and some of hydraulic fractures formed by stress, and some
of them formed by hydrocarbon generation. Inorganic pores including
bedding fracture, structural pores, intercrystalline pores,
intergranular pores and dissolved pores and organic pore including
corrosion pores, gas pores, bitumen spherulite pores can be observed. We
observed quartz, calcite, feldspar under SEM to identify their
cementation, filling, different periods’ growth, and aeolotropism. Based
on these analysis data, authors inferred the shale diagenetic evolution
is at the diagenetic B period, the mineral bitumen mass and solid
bitumen macerals organic matters are contributed to the shale gas
mainly, and by calculating the plane porosity by MATLAB, the fractal
dimension is about 2.57. Pores diameters of these samples range from
1-55nm, and the maximum is 4nm. Most pores are around medium pores. In
summary, shale gas migrates obeying non-Darcy flow along pores/fracture
several nanometers, and gas migration affected by organic matter mainly
in Wufeng-Longmaxi formation in Southeast Sichuan basin, China.