Structural geometry and evolution of the Dehdasht Basin in the Central Zagros fold-and-thrust belt: Implications for the timing of oil generation and trap formation

Abstract

Structural analysis and temporal evolution of the Dehdasht Structural Basin in the Central Zagros through the constructed cross-sections and 2D sequential restoration have defined the timing of trap formation in the basin. At the level of the competent layer (Early Cretaceous to Oligo-Miocene), the basin is deeper toward the center where it is covered by the thicker Miocene evaporites and syn-tectonic clastic deposits. The evolutionary history of the basin based on the sequential restoration proposes that the anticlinal traps are formed since the Pabdeh deposition at Paleocene-Eocene on the boundaries and during the middle Miocene Mishan deposition within the basin. The proposed structural evolution and the suggested oil window using the burial depth and the present thermal gradient for the Dehdasht Basin give insight into its hydrocarbon potential. Accordingly, the Cretaceous Kazhdumi source rock has started oil generation since Paleocene-Eocene (deposition of the Pabdeh Formation) in the deeper central part until the present in the subsurface relatively large-amplitude anticlines close to the boundaries. We also suggest that the Paleocene-Eocene Pabdeh Formation contained oil mainly in the subsurface low-amplitude anticlines in the central part of the basin during deposition of the Gachsaran and Mishan formations at early-middle Miocene. The older oil generation in the basin is attributed to its thicker deposits in the Paleocene-Eocene and early Miocene resulting in deeper burial depth for the source rocks. The obtained results propose that the subsurface relatively large-amplitude anticlines near the basin margins with the possible hydrocarbon (mainly generated from the Cretaceous Kazhdumi shale) are the most likely targets for future oil exploration. Additionally, we obtained that the boundary fore-thrusts and the caprock erosion due to the lateral anticlinal growth control the oil and gas seepages on the Dehdasht Basin boundaries.