In the first study of its kind, we analyze heavy ends of crude oils to characterize oil reservoirs by both in situ downhole fluid analysis (DFA) in oil wells along with heavy-end fingerprinting utilizing ultrahigh resolution mass spectrometry. One of the biggest problems in the exploitation of oil reservoirs in high-cost settings is the difficulty of characterizing flow connectivity or compartmentalization of the various permeable zones. Standard industry methods to determine compartmentalization either fail or are prohibitively expensive. A relatively new method of performing DFA on crude oils has identified compartmentalization on a regular basis and is rapidly becoming an industry standard. Here, compartmentalization is established in an oil-containing vertically stacked sand-shale sequence by performing visible-near-infrared spectroscopy on crude oils in situ in the oil well. These crude oils are then fingerprinted by electrospray ionization-Fourier-transform ion-cyclotron-resonance mass spectroscopy. This novel analysis protocol can lead to a more detailed understanding of the reservoir and provides a new utility for advanced methods of analytical chemistry; this charter fits within the new field of petroleomics.