Fractal Analysis of Donghetang Sandstones Using NMR Measurements

Routine core analysis, thin section epifluorescence and scanning electron microscopy (SEM) analyses were performed to delineate the reservoir qualities, pore systems, and pore network characteristics of the Devonian Donghetang sandstone samples. Nuclear magnetic resonance (NMR) measurements were used to gain insight into pore size distributions of these sandstones. Fractal analysis was performed on the NMR T2 spectrum measured at echo spacings of 0.6 and 1.2 ms. Then the relationships between NMR parameters and fractal dimension were investigated. The results show that the pore systems are dominated by primary intergranular pores with a combination of secondary dissolution pores, micropores, and microfractures. Samples with the highest reservoir quality are characterized by a coexistence of primary intergranular and secondary dissolution pores. The parameter T2gm (the geometric mean of the T2 distribution) is a sensitive one for characterizing the pore size distribution and microscopic pore structure since it shows good correlations with T2peak (value of T2 showing the highest frequency on T2 spectrum), permeability, and reservoir quality index (RQI). By using a short echo spacing, more smaller pores can be detected, resulting in a higher fractal dimension. The calculated fractal dimensions show strong positive relationships with the NMR T2peak and T2gm values. Fractal dimensions can be used here to represent the complexity degree and heterogeneity of pore structure, and the coexistence of dissolution pores and large intergranular pores contributes to a heterogeneous pore throat distribution and a high value of fractal dimension. The results are important for petroleum exploitation in the Donghetang sandstones and can provide insights into pore structure characterization in sandstones with similar geological settings.