Origin of a Negative Gravity Anomaly over an Anticlinal Structure: A Case Study from the Rumaila Oil Field, Southern Part of Iraq

The area of the southern part of Iraq, Basra Governorate is well-known as an area of giant oil fields (such as West Qurna 2, Rumaila, Tuba, Zubair, and Nahr Umr) associated with negative gravitational anomalies. The negative gravity anomaly over the Nahr Umr structure was interpreted from seismic sections as a piercement salt dome like Jebel Sanam (a hill located 80 km to the south). But, no evidence for salt existence is found beneath West Qurna 2, Rumaila, and Tuba structures in the seismic sections. The main problem facing the interpretation stage is the ambiguity in the source of this negative anomaly. In the South Rumaila oil field, the negative anomaly still appears despite the disappearance of salt; this case produces confusion in understanding the geological situation. The present study discussed the case and gave an interpretation for the presence of the negative gravity anomaly over these structures. A residual gravity map was prepared, synthetic case examples are given, and sedimentary successions from well data are studied and compared with the seismic section to define the thickness of formations, their density contrast, and structural geometry conditions. Considering the geometry, the results showed that an arch-shaped anticline consists of less dense rocks (Gotnia Formation) compared to denser adjacent formations (limestone). In this case, the density contrast can lead to a negative density contrast and a negative gravity anomaly. The density contrast values extracted from well data: 1- Nahr Umr (2.32 g/cm3) and Mauddud (2.5 g/cm3) formations. 2- Gotnia (2.36 g/cm3) and Sulay (2.7 g/cm3) formations imported as constraints in 2D forward gravity modelling. All the results of residual gravity, seismic section, and 2D modelling suggest that the reason for the negative anomaly above the South Rumaila oil field is due to the low-density layer of the Gotina Formation.