Abstract
Nitrogen–Sulfur–Oxygen compounds of crude-oil samples were analyzed to study oil migration in Block TW. The 15 crude-oil samples of the Upper Cretaceous Napo Formation M1ss Member had similar geochemical characteristics; a similar normal alkane distribution mode (forward peak-single peak model); and similar terpane, hopane, and regular sterane distribution characteristics, which are similar to those of the Upper Cretaceous marl in the eastern flank. The nitrogen compounds of the crude oils from the M1ss Member in Block TW show a prominent geochromatographic effect-an obvious decrease in the total absolute abundance of pyrrole (a kind of biomarker), absolute abundance of exposed pyrrolic hydrogen. The ratios of “shielded pyrrolic hydrogen/exposed pyrrolic hydrogen” and “partially shielded pyrrolic hydrogen/exposed pyrrolic hydrogen” increase with oil migration while the ratios of “shielded pyrrolic hydrogen/partially shielded pyrrolic hydrogen” exhibited almost no change. The ratios of 1,8-/2,6-dimethylcarbazole, 1,4-/2,6-dimethylcarbazole, and 1,5-/2,6- dimethylcarbazole were good indicators of oil migration, which greatly decreased in short-distance migration in the Johanna reservoir and Alice West reservoir and could be used to semiquantitatively analyze the oil migration direction within the same reservoir. The main oil migration in Block TW was from southwest to northeast, and the geochromatographic effects between different reservoirs were not obvious, which might be the results of different oil charging points, different migration pathways, or the simultaneous supply of local and far sources. Heteroatomic compounds of oxyfluorene and thiofluorene series have strong biodegradation resistance and can assist in identifying the direction of oil migration. The variation trend of total abundance of oxyfluorene and thiofluorene in Block TW is similar to that of carbazole series compounds, but the migration fractionation effect is not as obvious as that of pyrrole series compounds. A set of parameters should be used in studying oil migration, which can be determined according to most of their variation trends. The ratios of 1,8-/2,6-dimethylcarbazole, 1,4-/2,6-dimethylcarbazole, and 1,5-/2,6-dimethylcarbazole in the M1ss Reservoir of Block TW have similar and obvious geochromatographic effects, which can be used as regional oil and gas migration indicators. The pyrrole compounds (particularly the carbazole series) have obvious geochromatographic effects along with oil migration, which could be used to analyze its direction, particularly in highly explored areas with several wells.
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