Changes in the cross-link density of a type 1 kerogen during maturation

A five sample maturation series of type I kerogen from the Uinta Basin has been studied. Kerogen samples were isolated by classical HCVHF demineralization. The bitumen was extracted with toluene and its molecular weight distribution was determined using {sup 252}Cf plasma desorption mass spectrometry. The kerogen samples were all swollen in a set of 10 solvents. In all cases, solvent-kerogen interactions followed regular solution theory. The solubility parameter of the less mature kerogens is {approximately}9.5 cal cm{sup 3/2} and may increase slightly with increasing maturity. The number average molecular weight between branch points (reciprocal of the cross-fink density) calculated using the Flory-Rehner model for the 5% mature kerogen is about 2000 daltons. This decreases to about 1400 daltons at 80% maturity and plunges to about 300 daltons at 1000/6 maturity. During maturation, groups fragment off of the kerogen while the kerogen undergoes a polymerization reaction. This polymerization is slow for the first 80% of the maturation process then accelerates rapidly. This behavior is quite different from coals in which the maturation process appears to be a depolymerization.