In unconventional shale, petroleum compounds are generated during kerogen maturation and chemical fractionation occurs between fluids that are expelled out of kerogen and retained within kerogen because of different solubilities of hydrocarbons in kerogen. The compositional variation of petroleum compounds from the organic matter source (kerogen) to nanoscale pores in the organic matter and to large fractures makes the estimation of fluid storage and the multicomponent compositional distributions in shale a challenging problem. A thermodynamic model that considers the different interactions between hydrocarbons and kerogen is needed to quantify the partitioning of fluids among different phases. In this work, a new thermodynamic model is proposed that predicts both the adsorption of hydrocarbons in nanoscale pores and the dissolution of fluids in organic matter in equilibrium with the bulk fluid phase. First, by taking into account the chemical and structural similarities of kerogen with asphaltene, we leverage the perturbed chain-statistical associating fluid theory (PC-SAFT) model of asphaltene to create a cross-linked kerogen matrix model. The swelling ratios of five kerogens of varying maturities and types in different solvents are well quantified by phase equilibrium calculation using the new model. Second, the new kerogen matrix model is incorporated in interfacial statistical associating fluid theory (iSAFT) to form a nanoporous kerogen composite model. The inhomogeneous fluid distribution at the fluid/kerogen interface is then able to be solved, and the equilibrium partitioning of mixtures of fluids from the bulk phase to the nanoporous kerogen phase can be determined. The results show the enrichment of aromatic and cyclic molecules in kerogen, the preferential adsorption/absorption of high molecular weight molecules and carbon dioxide, and the significant amount of hydrocarbon storage both in pore space and kerogen matrix. Examples that compare with fluid at the same condition in a graphite pore indicate that graphite as the postmatured kerogen provides the largest fractionation effect for fluid mixtures.