Effectiveness of nitrogen compound indices of oil-bearing sandstone extracts
Reports on the utilization of nitrogen compounds to trace oil and gas migration in oil-scarce regions are limited.This scarcity can be attributed to the lack of clarity regarding the effectiveness of nitrogen compound indices in oil-bearing sandstone extracts.To address this knowledge gap,two sets of experiments were designed,encom-passing five extraction methods,to evaluate the validity of nitrogen compound indices.The results showed that as the extraction intensity increased,the ratios of 1-/4-methylcarbazole and 1,8-/2,7-dimethylcarbazole in the extracts gradually decreased but remained consistently elevated compared to the control oils.The benzocarbazole[a]/[c]ratio remained relatively stable and closely aligned with the control oils.Upon analysis,it was concluded that the observed variations could be attributed to the differing adsorption capacities of carbazole isomers.These differences lead to the fractionation of carbazole isomers between free and adsorbed hydrocarbons under surface conditions,as well as between the extracts and rocks during the extraction process.In comparison to 1-/4-methylcarbazole and 1,8-/2,7-dimethylcarbazole,the benzocarbazole[a]/[c]ratio is less susceptible to these two fractionations,with its variations primarily governed by migration fractionation.Additionally,the indicated direction of oil and gas migration aligns with geological realities.Therefore,the benzocarbazole[a]/[c]ratio can be considered an effective nitrogen compound index in oil-bearing sandstone extracts,suitable for oil migration tracing studies in oil-scarce regions.
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