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Relationship Between Shortening in the Zagros Fold, Thrust Belt, and Natural Fracture Orientation: A Case Study From Kurdistan Region of Iraq


The Zagros Fold-Thrust Belt (ZFTB), a relatively underexplored hydrocarbon province in the Kurdistan Region of Iraq (KRI), extends from southern Iran across northern Iraqi Kurdistan to southern Turkey. In this area of northern Iraq, prospective, hydrocarbon-rich surface anticlines are common. Tertiary, Cretaceous and Jurassic carbonate reservoirs that are prospective in KRI rely on porosity and permeability from natural fractures for commercial production. To date in Kurdistan, 3D seismic surveys are not widespread, so both regional interpretations and prospect evaluations rely on sparse 2D seismic and few wells. Therefore, traditional tools for Fold-Thrust Belt analysis such as the construction of regional and field scale structural cross sections, mapping the bend in the thrust belt based on axial traces of surface anticlines, and determining the depth to detachment levels are used to focus exploration interest. The relationship of fracture formation to fold belt shortening direction and fold kinematics is a key to the understanding of the productivity of these anticlines. We recognize a strong correlation between structural style, the location of an anticline in the ZFTB, and orientation of the natural fractures within most of the major anticlinal structures. In general, large, simple anticlinal structures with low shortening contain more hydrocarbon resources than tight fault-related anticlines with high shortening. In the Jurassic reservoirs that we evaluated, the productive reservoirs, as determined from drill stem tests (DSTs), contain a high density of fractures trending NNE-SSW, which is subparallel to the present day maximum principal stress. We interpret these as extension fractures, a subset of which are reactivated with a lateral slip with progressive deformation. In the Cretaceous reservoirs, we recognize a NNE-SSW productive fracture set, proven by DSTs, as well as a set of NW-SE trending permeable fractures. To date in the Tertiary reservoirs, we have only seen the NW-SE trending permeable fractures. Evidence from core indicates the NW-SE fractures are in part dip-slip shear fractures, probably accommodating outer-arc extension during folding. Near-vertical dips of these fractures in the Cretaceous reservoirs indicate that some of them are also joints. Despite their orientation perpendicular to the maximum principal stress, the NW-SE fractures appear to be permeable, as evidenced by mud losses during drilling and hydrocarbon shows that are coincident with fractured intervals.