Abstract
The pore structure of marine-continental transitional shales from the Longtan Formation in Guizhou, China, was investigated using fractal dimensions calculated by the FHH (Frenkel-Halsey-Hill) model based on low-temperature N2 adsorption data. Results show that the overall D 1 (fractal dimension under low relative pressure, P / P 0 ≤ 0.5 ) and D 2 (fractal dimension under high relative pressure, P / P 0 > 0.5 ) values of Longtan shales were relatively large, with average values of 2.7426 and 2.7838, respectively, indicating a strong adsorption and storage capacity and complex pore structure. The correlation analysis of fractal dimensions with specific surface area, average pore size, and maximum gas absorption volume indicates that D 1 can comprehensively characterize the adsorption and storage capacity of shales, while D 2 can effectively characterize the pore structure complexity. Further correlation among pore fractal dimension, shale organic geochemical parameters, and mineral composition parameters shows that there is a significant positive correlation between fractal dimensions and organic matter abundance as well as a complex correlation between fractal dimension and organic matter maturity. Fractal dimensions increase with an increase in clay mineral content and pyrite content but decrease with an increase in quartz content. Considering the actual geological evaluation and shale gas exploitation characteristics, a lower limit for D 1 and upper limit for D 2 should be set as evaluation criteria for favorable reservoirs. Combined with the shale gas-bearing property test results of Longtan shales in Guizhou, the favorable reservoir evaluation criteria are set as D 1 ≥ 2.60 and D 2 ≤ 2.85 . When D 1 is less than 2.60, the storage capacity of the shales is insufficient. When D 2 is greater than 2.85, the shale pore structure is too complicated, resulting in poor permeability and difficult exploitation.
Highlights
Shale is a type of heterogeneous porous rock with a complex pore structure, including various pore shapes and sizes [1, 2]
The Permian Longtan Formation in Guizhou is a coal-bearing formation containing marine-continental transitional facies, which is generally characterized by high organic matter content and good gas indicators, indicating a great natural gas resource potential [6,7,8,9]
Scientists have found that the Longtan Formation contains a set of shale reservoirs with low porosity (0.89%-2.29%) and permeability (ð0:16 − 29:5Þ × 10−4 mD), strong heterogeneity, small pore size, and complex pore structure [10]
Summary
Shale is a type of heterogeneous porous rock with a complex pore structure, including various pore shapes and sizes [1, 2]. Scientists have found that the Longtan Formation contains a set of shale reservoirs with low porosity (0.89%-2.29%) and permeability (ð0:16 − 29:5Þ × 10−4 mD), strong heterogeneity, small pore size, and complex pore structure [10]. Quantitative methods such as the mercury injection capillary pressure (MICP) experiment can be used to study pore structure characteristics [2, 12, 13] Tight reservoirs, such as shale reservoirs, have high pore displacement pressure, requiring a higher experiment pressure to measure the pore size. This paper systematically studied the pore structures and fractal characteristics of marine-continental transitional facies shales in Guizhou as well as the relationship with other geological parameters (reservoir parameters, organic geochemical parameters, and mineral composition parameters) based on low-temperature N2 adsorption experiments, field emission scanning electron microscopy (FESEM) observations, X-ray diffraction (XRD) analysis of rock mineral composition, organic geochemical experiments, and gas content tests. The fractal dimension division standard is determined for the dominant shale reservoirs of the Longtan Formation in Guizhou Province, which provides the basis for further exploration, research, and evaluation
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