Geochemical Characteristics of Chang 3 Oils in the Ordos Basin, Central China: Implication for Its Migration Process

The Chang 3 Member in the Ordos Basin is still an important pay zone for increasing reserves and boosting production. However, the key factors controlling the differential enrichment of the Chang 3 oils remain unclear. In this study, whole-oil gas chromatography, saturated hydrocarbon chromatography-mass spectrometry and nitrogen compound analysis were conducted on the Chang 3 oils in the basin. Both the Chang 3 oils and Chang 7 black shales exhibit a C20 < C21 < C23TT pattern and are consistent in ααR C27/C29 sterane ratios and gammacerane indices. The Chang 3 oils have been mature, but their maturities (Ts/Tm and C30diahopane indices) are significantly lower than those of the Chang 7 source rocks with equivalent Ro values ranging from 0.5% to 0.7%, indicating a charging time from the Late Jurassic to early Early Cretaceous. Chang 3 oils migrated vertically along NE-trending faults in the Zhenbei area, and along NW-trending faults in other areas. Except for the Shaanbei area, Chang 3 reservoirs are mainly located above thick source rocks, and both maturity parameters and benzocarbazole[a]/[c] ratios show no significant lateral variation along the Chang 3 sand bodies, suggesting that Chang 3 oils migrated predominantly vertically, with limited lateral migration.