A tentative discussion on oil migration orientation: a case study of 1st structural belt of Nanpu sag

Abstract：Based on the composition and concentration variations of nitrogen compounds in the studied oil samples as well as the maturity indexes of saturated and aromatic biomarkers, the authors investigated the oil migration direction of 1st structural belt of Nanpu sag. The results show that the distribution and composition characteristics can obviously indicate oil fractionation effects. The concentration of nitrogen compounds decreases whereas the ratios of shielded nitrogen compounds to exposed ones increase with increasing migration. The oil will migrate to high position along the sand body on the outer side of 1st fault. This is in consistent with oil migration directions deduced from the gradient changes of hydrocarbon biomarkers. They are both consistent with the regional modes of occurrence and exploration facts of oil; nevertheless, because of fractional distillation, these two indexes are somewhat different. Due to weak fractionation, biomarker varies slowly between near-by wells; in contrast, due to strong fractionation and relatively insignificant influence of maturity, data of nitrogen compounds change significantly, which is favorable for the judgment of hydrocarbon migration.