Predicting gas-to-oil ratio (GOR) and bulk petroleum properties is of paramount concern when developing production strategies for unconventional resource plays. Subtle changes in bulk fluid composition can result in large differences in phase envelope geometry and predicted fluid types, which can cause unfavorable pressure drawdown during production. Here we present new insights into the thermal evolution of petroleum compositions in conventional and unconventional reservoirs, focusing on polar compounds, i.e., nitrogen-, sulfur-, and oxygen-containing (NSO) compounds. These compounds feature functional groups that strongly influence the sorption, solubility, and partitioning behavior of petroleum constituents in unconventional shale systems. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is a powerful tool to rapidly characterize NSO-compounds in complex mixtures and was used to compare the polar compounds in (1) extracts of six Posidonia Shale source rock samples with maturity levels between 0.48 and 1.45% vitrinite reflectance (R-o), (2) open-system pyrolysates of those six source rocks, and (3) four Posidonia Shale-sourced medium-gravity conventional crude oils. The aromaticity and degree of condensation were found to increase much more pronouncedly with increasing maturity for retained than for expelled oil NSO compounds. Pyrolysate NSO compounds have compositions intermediate between those of retained and expelled NSO compounds, pointing to preferential expulsion of smaller compounds in the crudes and enhanced cyclization and aromatization within retained fluids. Aromatization was shown to occur at the expense of aliphatic carbon. A genetic link between the fluids as well as the likely timing of petroleum expulsion is revealed by comparing the carbon number distributions of compounds from the N-1 elemental class, that is, compounds that contain one nitrogen atom (e.g., carbazoles). The here documented chemical differences between the investigated fluid types are significant and need to be taken into account when formulating production strategies.