Through core and thin section observation, cathodoluminescence,
calcite U–Pb dating, single well basin simulation, fluid inclusion
microthermometry, laser Raman spectroscopy, and other methods, combined
with gas content calculation, the evolution of pressure and gas content
was reconstructed during the uplift of shallow shale gas reservoirs.
The results show the following. (1) The homogenization temperature
in fractured calcite veins primarily ranges from 80 to 160 °C.
The Raman spectral shift ranges from 2912.21 to 2913.16 cm–1. The capturing pressure of methane inclusions in shale fracture
veins ranges from 37.05 to 57.6 MPa, with a pressure coefficient of
1.04–1.33. (2) The pressure evolution during the gas reservoir
adjustment process was carried out in three stages. In the initial
uplift stage (114–70 Ma), the gas reservoir was in an overpressure
state. In the slow uplift stage (70–25 Ma), the gas reservoir
gradually transitioned from weak overpressure to normal pressure.
Finally, in the last uplift stage (25–0 Ma), the gas reservoir
reached a state of normal pressure to weak low pressure. (3) During
the burial stage, the amount of free gas gradually increases with
the depth of burial of the formation, reaching its maximum at the
maximum burial depth. Subsequently, in the uplift and denudation stages
of the formation, the quantity of free gas decreases, while the amount
of adsorbed gas shows the opposite trend. During the uplift process,
a significant portion of the free gas converts into adsorbed gas,
with only a small portion being lost. The loss of shale gas is about
37.4% of the total gas content in the maximum burial depth stage.
The research findings will contribute to enhancing the understanding
of marine shale gas enrichment in China and can be directly applied
to the study of shallow shale gas reservoirs in the middle-upper Yangtze
block.
