Pore structure heterogeneity of Chang 7 shale in the Ordos Basin: insights from NMR and multifractal theory

Quantitative characterization of the heterogeneity of shale nanopores and its influencing factors have a significant impact on the occurrence quantity, pore size and mobility of shale oil. Twenty-five Chang 7 shale samples were analysed for their geochemical properties and mineral composition, along with field emission scanning electron microscopy and nuclear magnetic resonance. Combined with multifractal theory, the heterogeneity characteristics and influencing factors of the shale pore structure were studied, and a reservoir classification was carried out. The results show that the pore types include microfracture, intercrystalline pores, intergranular pores, intragranular dissolved pores and organic matter shrinkage pores. The pore size distribution of shale samples was divided into four groups according to the geometric mean value of T 2 spectrum ( T 2,gm ) and T 2 value corresponding to 50% of the cumulative curve of T 2 relaxation time ( T 2, 50 ), in which the pore-scale composition gradually transitions to micropores (&lt;100 nm), while the heterogeneity gradually diminishes. The total organic carbon (TOC), clay matrix and pyrite content were positively correlated with multifractal parameters, while the maximum pyrolysis yield temperature ( T max ) and content of quartz, feldspar and carbonate content show opposite trends. The T 2 spectrum parameter T 2,gm and multifractal parameters <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>α</mml:mi> </mml:math> and D 1 / D 2 can effectively classify shale oil reservoir quality. This study provides insights into and reference for the characterization of pore heterogeneity and the classification of shale oil reservoirs.