In the Merjan oil field (Central Iraq), the previous 2D seismic interpretation of subsurface geometry had been revealed mound facies which was interpreted as large carbonate buildup within carbonate Najmah formation of Jurassic age. The exploration well (Me-1) had been drilled depending on these results, but carbonate buildup did not exist. The current research provides new sight to interpretation of this feature in the Jurassic succession. High accurate 3D seismic reflection data have been used. Two scenarios have been presented to interpretation. Scenario (A) includes tectono stratigraphic study which finds out that mound shape is due to fault system and tectonic activity during the sedimentation which led to the formation of a graben structure and after sedimentation led to tectonic inversion. These events cause the camouflage stratigraphic feature. Scenario (B) relies on conversion of seismic section from time into depth domain using appropriate velocity model. The resultant seismic depth section helps to present another seismic analysis of feature. The proposed structural model according to the depth section interpretation is anticlinal fault-bend fold. Quantitative geometric relationships between fault and fold shapes were established. Both scenarios explain the causes of the formation of the mound seismic facies which was previously interpreted as carbonate buildup. In conclusion, the stratigraphy interpretation based on 2D data might be very risky and 3D seismic is a must for exploration of stratigraphy traps.
Study of three dimension seismic data of Merjan survey, central Iraq has showed that the Jurassic – Cretaceous succession is affected by faulting system. Seven major normal faults were identified and mapped. Synthetic traces calculated from sonic and density logs of the well Me-1. Two exploration wells were drilled in the area Me-1 and Wkf-1 wells. The area is characterized by extension phase led to graben system forms rift sub-basin. The structural components of rift sub-basin has identified and visualized in three dimensions. Discussion about the effect of this system on the sedimentary package has been presented. Faulting framework can be divided into two groups: the first affects the Jurassic and lower Cretaceous rocks and the second affects the upper Cretaceous and lower Tertiary rocks. The first group is associated with the post rift thermal sag, passive margin progradation and gravitational collapse (lower Jurassic – mid Turonian 190–92 ma); approximately during the Sargelu – NahrUmr depositional time. The second group is limited and is associated with the rifting creating the Euphrates graben as collision wanes (Late Turonian – Maastrichtian 90–70 Ma) approximately during the Tanuma shale/Sadi – Shiranish depositional time.
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