A new method to estimate the oil and gas potentials of coals and kerogens by solid state 13C NMR spectroscopy

Abstract

Samples of 33 Chinese terrestrial kerogen concentrates from different oil fields and 32 oil shales, coals and coal macerals have been analysed by solid state 13C NMR for structural characterization. Based on pyrolysis data, the aromatic carbons are found to contribute little to oil and gas formation; the methylene (CH 2) and methine (CH) carbons are directly related to oil yield, so they are classified as oil prone carbons; the aliphatic carbons other than methylene and methine, together with carbonyl and carboxyl carbons, are classified as gas prone carbons. The aliphatic band of the 13C NMR spectra was deconvoluted by a curve fitting technique. A ternary diagram using these three carbon constituents as vertices is suggested to characterize the type and maturity of coals and kerogens. Equations for the estimation of oil and gas potential ( P o and P g) per unit weight of source rock have been derived: P o = 1.2( TOC) 2[k(f o ) 1 − (f o ) 2] P g = 1.3( TOC) 2[k(f g ) 1 − (f g ) 2] k = (f a ) 2 (f a ) 1 where TOC is the total organic content, f o, f g and f a are the oil prone, gas prone and aromatic carbon fractions respectively, k is the recovery factor.