The acid and neutral nitrogen compounds characterized by negative ESI Orbitrap MS in a heavy oil before and after oxidation

One heavy crude oil from Liaohe Basin, NE China and its oxidized counterparts at 200 and 400 degrees C have been characterized geochemically by negative-ion electrospray ionization Orbitrap mass spectrometry to investigate the variation of acid and neutral nitrogen compounds during in-situ combustion. The heteroatom compounds of acid and neutral nitrogen detected in the samples are mainly N-1, N1O1, N1O2, O-1, O-2, O-3 and O-4 classes, dominated by N-1, O-1 and O-2. The N-1 class predominated by carbazole and benzocarbazole with double bond equivalents (DBE) 9 and 12, is relatively stable and resistant to oxidation, while the N1O1 and N1O2 classes vary without a clear trend during oxidation. The O-1 class is generally low in raw sample and the 200 degrees C oxidized oil but increases sharply in the 400 degrees C oxidized oil. The O-2 class is peaked in the 200 degrees C oxidized oil but its relative concentration declines drastically in the 400 degrees C oxidized oil. The O-1 and O-2 class compounds show a similar DBE and carbon number (CN) distribution trend with increasing temperatures. Lower DBE and CN compounds are enriched in the 400 degrees C oxidized oil after alkyl side chain cleavage, where small proportion of high DBE compounds derived from aromatization and condensation has also been observed. The relative abundance of O-1 and O-2 species with specific DBE values and carbon number ranges are sensitive to in-situ thermal upgrading and ratios of DBE1/DBE4-O-1 and DBE1/DBE3-O-2 endure the potential as geochemical proxies to monitor the degree of oil upgrading.