Investigation of the pore structures and fractal characteristics of marine shale reservoirs using NMR experiments and image analyses: A case study of the Lower Cambrian Niutitang Formation in northern Guizhou Province, South China

The pore structures and fractal characteristics of the organic-rich marine shales of the Lower Cambrian Niutitang Formation in northern Guizhou Province were investigated using nuclear magnetic resonance (NMR) and field emission scanning electron microscopy (FE-SEM). In this paper, the NMR fractal dimensions and pore morphology fractal dimensions were calculated based on NMR T-2 spectra and SEM image analyses. The relationships among the total organic carbon (TOC) contents, mineral compositions, porosities, permeabilities, CH4 adsorptions and NMR fractal dimensions (D-N1 and D-N2) of the shales are discussed, and the properties of the different pores are quantitatively analyzed using pore extraction based on SEM images. Moreover, the geological significances of fractal dimensions are discussed. The results reveal that D-N1 is positively correlated with TOC contents, whereas D-N2 does not have an apparent relationship with TOC contents. The quartz content presents a positive linear correlation with D-N1 and D-N2, whereas the clay content is positively correlated with D-N1 but is not correlated with D-N2. D-N1 increases with decreases in porosity and permeability, and D-N2 increases with increases in porosity but is not correlated with permeability. The organic matter (OM) pores are smaller in size and have a larger number than the interparticle pores and intraparticle pores. The interparticle pores have the smallest average roundness values, implying that their roundness is the worst. Moreover, the fractal dimension of the OM pores is the largest, whereas the fractal dimension of the intraparticle pores is the smallest. The pore morphology fractal dimension has no apparent relationship with the roundness but shows a negative correlation with pore size. Shale samples with larger surface fractal dimensions have higher methane adsorption capacities.