Characterization of kerogen and bitumen in Type II-S organic-rich chalk as a function of maturity using 2D NMR relaxation

As kerogen matures over geological times, bitumen is generated inside kerogen. With further maturation, bitumen can coat the inorganic meso-macro pores, which complicates the interpretation of important petrophysical properties such as wettability, permeability, acoustics, and electrical resistivity. To address these challenges, a “2D splice NMR” method consisting of ¹H NMR T₁-T₂ relaxation with solid-echo (T_2G^∗) and spin-echo train (T_2e) is used to quantify the kerogen and bitumen content in cores from a Type II-S organic-rich chalk formation as a function of thermal maturity (i.e., depth). In the first of its kind approach, 2D splice NMR is used to quantify the bitumen readily extractable by solvents, which is interpreted as bitumen coating the kerogen, and the remaining bitumen after solvent extraction, which is attributed to bitumen residing in kerogen nano-pores. 2D splice NMR and Rock-Eval analysis are used to determine the elemental H/C ratio of the organic matter, the swelling factor of kerogen from dissolved heptane, and the nano-pore size of dissolved heptane in kerogen versus depth. A similar T₁ value is found for Mahogany shale, the organic-rich chalk, and Athabasca bitumen, which have different origins and span a wide range of maturity, indicating universal ¹H-¹H dipole–dipole relaxation instead of paramagnetism.