Novel geochemical proxies derived from 1D simulation of Liaohe oil sand bitumen to monitor in-situ combustion process

Abstract

1-D combustion tube in-situ combustion (ISC) simulation experiment has been performed on a natural bitumen bearing core. Five ISC zones were distinguished on the basis of the temperature filed and oil saturation analysis of the experiment. A suite of six oil sand residues at different locations after ISC simulation (initial, oil bank, condensation, coking, combustion zone and burned zone, respectively) were obtained for further analysis by using gas chromatography-mass spectrometry to assess the ISC impact and possible proxies monitoring the ISC process. Compounds in the saturated and aromatic hydrocarbon fractions change systematically with increasing temperatures. The concentrations of various compound classes increase generally from the initial zone to the combustion zone, then decrease drastically in the burned zone. However, different compounds show distinct behaviors during ISC and several geochemical proxies were temporary proposed. The amount of light molecular weighted n-alkanes increases significantly, while isoprenoids and heavy molecular weighted n-alkanes show the same trend but less dramatic changes. The ratios of n-C17/pristane (Pr) and n-C18/pyhtane (Ph) and ∑n-C20−/∑n-C20+ increase continuously from the initial zone to the combustion zone and clearly reflect ISC impact. A similar proxy can be derived from the ratio of summed n-alkanes to summed pentacyclic terpanes (∑n-alkanes/∑PTs), which increase more perceptibly due to easy decomposition of biomarkers. In the aromatic fraction, the most sensitive proxy to monitor ISC is derived from degree of alkylation of aromatic homologues. Non-substituted compounds especially naphthalene and phenanthrene increase constantly from the initial zone to the burned zone accompanied by the depletion of highly alkylated counterparts. Some newly formed compounds such as 2-methylanthracene and benz[a]anthracene are defined combustion evidence. However, commonly used maturity parameters based on relative stability of different isomers show no significant variation and cannot serve as valid ISC proxies.