—Petroleum geochemical characteristics are a major parameter of hydrocarbon field development. The present study is an attempt to decipher the unusual Bangestan (Cretaceous age) oil distribution throughout the Ahvaz oil field, SW Iran, based on the SARA test and GC–MS analysis of selected oil samples. The results indicated that all analyzed oils belong to the paraffinic group. Hydrocarbon indicators, such as tricyclic triterpane C22/C21 (high), C24/C23 and C26/C25 (low), and hopane C31R/C30 (high) ratios and C25/C26 ratio vs. C25/C26 tet ratio, show that these oils are sourced from carbonate–marl rocks. The high saturation/aromatic hydrocarbon ratio in these oil samples may be related to long migration or high maturity. The biomarker variation exhibits a marine environment for the source rocks deposition. The observed oil maturity trend is showing a good correlation with the prevailing geothermal gradient and possible basement faults and fractured system. The increasing oil maturity in the eastern part of the field may be related to a heat flow anomaly. It seems that the area around well C (the area of wells C, D, or E) can be considered a petrochemical separator of fluids for two sides of the field. Therefore, it can be concluded that the observed petrochemical pattern is a complicated response of several factors: the presence of a palaeohigh, basement-controlled faults, petroleum source rocks, fractured system, and geothermal gradient variation in this deep reservoir.