Pore structure and fractal analysis of Lower Carboniferous carbonate reservoirs in the Marsel area, Chu-Sarysu basin

Evaluation of carbonate heterogeneity, closely related to pore geometry and connectivity, is important in oil and gas field production and reservoir prediction. To profoundly understand the heterogeneity of Lower Carboniferous carbonate reservoirs in the Marsel area, including Visean (C1v) and Serpukhovian (C1sr), the pore structure and fractal characteristics are investigated using routine petrophysical measurements, X-ray diffraction (XRD), cast thin section analysis, scanning electron microscopy (SEM), and mercury injection capillary pressure (MICP) tests. The origin of potentially prolific reservoir, the relationships between fractal dimension and reservoir physical properties, mineral composition and pore structure are also discussed. XRD results reveal that the mineral compositions are highly heterogeneous in C1v and C1sr. Routine petrophysical experiments indicate that C1v and C1sr are generally tight. However, some high porosity-permeability zones exist in C1sr, which is described as a potentially prolific reservoir. The cast thin section and SEM analyses identify three main pore types in the potentially prolific reservoir: interparticle dissolution pores, intercrystalline dissolution pores, and moldic pores. The MICP data reflect that the pore-throat radii in the prolific reservoir range from 0.29 to 4.6 μm, whereas those in tight section are typically <0.14 μm. The fractal dimensions are computed based on MICP data, and the fractal characteristics of the potentially prolific reservoir in C1sr are divided into two segments. In the tight section, relatively larger fractal dimensions are observed, indicating highly heterogeneous pore structure. The potentially prolific reservoir in C1sr is formed mainly because of penecontemporaneous dolomitization and leaching due to meteoric freshwater ingress. Different mineral compositions have differing effects on the fractal characteristics of the pore structure. Relatively good correlations of fractal dimension with porosity, permeability, sorting coefficient, and skewness indicate that fractal dimensions are helpful in reservoir quality evaluation and quantitative characterization of pore networks.