This study applied high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS), to examine the effect of varying thermal maturity level on NSO compounds in a well constrained suite of related crude oils. These oils are derived from a common marine carbonate source rock, and span most of the liquid petroleum generating portion of the oil window (0.61 – 1.15% vitrinite reflectance equivalent (%Re)). The oil suite was analyzed directly, i.e., without fractionation, using electrospray ionization (ESI) in positive-ion mode, for basic polar compound species (comprising nitrogen, sulfur, and oxygen (NSO) heteroatom containing organic compounds). Significant compositional and distributional changes in NSO compound classes were detected with increasing oil maturity level. In particular, the relative abundance of nitrogen (N1) and nitrogen-sulfur (N1S1) containing compounds detected herein, were found to decrease systematically with higher oil maturation levels. Observed compositional changes and trends were correlated against existing bulk property and conventional biomarker maturity parameters, obtained for the same samples. From these correlations we were able to derive several novel NSO compound parameters with potential as new geochemical indicators for maturity level estimation of crude oils generated from marine carbonate sources.