Changing in fold geometry from faulted detachment fold to fault-bend fold, a case study: The Zeloi Anticline in the Dezful Embayment, southwest of Iran

Abstract

Determining the geometry variations in the different depths reduces the uncertainties of petroleum exploration in the Dezful Embayment. In this research, the effects of the major and minor detachment horizons on the folding style of the Zeloi oil field have been investigated using surface and subsurface data. Because of the impact of the detachment horizons in the different depths, the Zeloi Anticline shows a poly-harmonic folding from surface to depth. The activity of the Gachsaran evaporites decupled the structural styles of the surface and subsurface folds. The interaction of these evaporites and syn-tectonic sedimentations influences the kinematic evolution of the Zeloi Anticline. The signatures of the sub-ordinate detachment horizons (Pabdeh-Gurpi formations and Kalhur Member) can be seen as second order folds and swell at the Asmari level. The thickness changes of the intermediate detachment horizon are the main factor of geometry variations of the Asmari and Sarvak reservoirs. The Zeloi Anticline is symmetric and asymmetric faulted detachment fold in the central part and SE plunge, respectively. But the reduction of the thickness of the Garau to Dashtak formations to the NW plunge led to the formation of shear fault-bend fold. The ENE-WSW SHmax orientation, in most of the wells, is determined using the breakout and drilling-induced fractures although, in some of the wells, the NW-SE SHmax orientation is interpreted as a result of stress perturbation that made by fore and back thrust faults. The open fractures are classified into the pre-folding, syn-folding and present-day related fractures.