Configurational-biased Grand Canonical Monte Carlo and Molecular Dynamics simulations were used to investigate the adsorption and structure of n-alkanes in realistic models of kerogen (the organic phase in gas shales). Both the effects of the n-alkane length (from methane to dodecane) and of the adsorbent porosity/density and connectivity were considered. For all n-alkanes, due to the subnano pore size of kerogen, adsorption follows a Langmuir-type adsorption isotherm as the adsorbed amount increases continuously with pressure until maximum loading is reached. While all n-alkanes get adsorbed very efficiently in low density kerogen, size exclusion in high density kerogen lead to low adsorbed amounts for molecules larger than hexane. Using Molecular Dynamics simulations reflecting the setup of actual adsorption experiments, we also probed the effect of pore accessibility on n-alkane adsorption in kerogen. All pores are connected to the surface and, hence, accessible for the low density kerogen. In contrast, most pores are isolated and inaccessible in the kerogen with the high density. We also address the effect of mesoporosity and its connection to nanoporosity on alkane adsorption in kerogen.