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

Abstract

One heavy crude oil from Liaohe Basin, NE China and its oxidized counterparts at 200 and 400 °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 N1, N1O1, N1O2, O1, O2, O3 and O4 classes, dominated by N1, O1 and O2. The N1 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 O1 class is generally low in raw sample and the 200 °C oxidized oil but increases sharply in the 400 °C oxidized oil. The O2 class is peaked in the 200 °C oxidized oil but its relative concentration declines drastically in the 400 °C oxidized oil. The O1 and O2 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 °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 O1 and O2 species with specific DBE values and carbon number ranges are sensitive to in-situ thermal upgrading and ratios of DBE1/DBE4-O1 and DBE1/DBE3-O2 endure the potential as geochemical proxies to monitor the degree of oil upgrading.