Effect of transverse faults on fracture characteristics and borehole instability in the Asmari reservoir of Zagros folded belt zone, Iran

Abstract

In this paper, the effect of transverse faults on the orientation, density and fractures apertures and amount of mud loss is investigated in Asmari Formation as a major oil reservoir in Zagros folded belt zone, Iran. This carried out on three selected anticlines as the surface Kuh-e-Kamarab, and subsurface Marun and Aghajari oil felds. Subsurface data on fracture characteristics were obtained from Image logs, mud losses, and reported drilling data. These are compared with outcrop fracture characteristics gathered from feldwork and satellite image interpretation. The results show that fracture orientations are almost identical in both outcrops and subsurface data, whereas the fractures density and aperture are different along and across the folded layers and are generally governed by their location with respect to the transverse faults. The closest sector to the faults displays the highest fracture density and greater aperture. Quantitative analysis of fractures from the Kuh-e-Kamarab Anticline indicates average fracture density almost two times greater close to the Izeh-Hendijan transverse (IZH) fault than the other parts of the fold. In the Marun and Aghajari oil felds, fracture density shows three to fve times greater close to the transverse faults. Analysis of the Breakout (BO) and Drilling Induce Tensile Fractures (DITF) indicate that the directions of maximum horizontal stress (SHmax) in the wells located in the bent part of the Marun feld consistent with the stress state of the Mn transverse fault. Moreover, the SHmax orientation of wells in the southern periclinal closure of the Aghajari Field is also affected by the IZH transverse fault. The result of Isomud loss maps in the oil felds reveals that the severe mud loss occurred in areas close to the transverse faults.