Role of incompetent strata and geometry of faults on the folding mechanism, a case study: the Karun oil field in the Dezful Embayment, Iran

One of the most controversial topics in the Dezful Embayment is the determination of geometry variations in different depth, folding mechanism, and the role of incompetent strata in structural evolution. Using surface and sub-surface data, this paper presents the folding mechanism of the Karun oil field by investigating the role of incompetent strata. The Miocene Gachsaran Formation, as the upper detachment level in the Karun oil field, has made a difference between structural styles of the upper and lower part of formation in the Dezful Embayment. Different sections of the Karun anticline were investigated based on geological maps, drilled wells, and seismic data. Based on geometric features on seismic sections, the folding mechanism of the Karun anticline is detachment folds. In detachment folds, it is necessary to determine the lower detachment horizon. Based on the calculated depth of detachment for the Karun anticline, considering the average thickness of formations, the Triassic Dashtak Formation acts as the lower detachment horizon. To the best of our knowledge, this is the first paper which investigates the determination of detachment horizon in the Northern Dezful Embayment. Changes in thickness of the detachment horizon have caused lateral changes within the geometry of the Karun anticline. The thickness of the detachment horizon has affected fault arrangement; subsequently, faults have changed the folding mechanism. The high thickness of Dashtak Formation, the forelimb fault, and the formation of the backlimb fault are controlling factors for the change of the folding mechanism from the detachment fold to the faulted-detachment fold type II.