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Fractured Reservoirs of the Shaikan Anticline, Northern Iraq


Oil-bearing reservoirs of the Shaikan Anticline in northern Iraq, with billions of barrels of oil in place, consist of highly fractured Mesozoic carbonates. Extensive field and core work documents an interconnected network of fractures dominated by closely spaced, bed-normal extension fractures, supplemented by strike-slip conjugate shear fractures and local low-angle thrust planes. Younger fracturing consists of flexural-slip bed-parallel shearing and shear reactivations of the earlier bed-normal fractures. Characteristics of these natural fractures vary by lithology, diagenesis, and structural position, with fracture heights being greater in the more homogeneous formations and bed-parallel shear being best developed on the steep forelimb of the anticline. Fractures in certain formations have been subjected to significant dissolution, with open, millimeter- to centimeter-scale slots developed along what were originally calcite-mineralized fractures. Most fractures in core, even the dissolution slots, are strata-bound, but vertical interconnectivity is enhanced by faults. Fracture patterns and bed-normal stylolites provide information on the structural development of the Shaikan Anticline, recording a pre-fold maximum compressive stress that was parallel to bedding and aligned in the NE-SW direction, evolving into the present-day, NNE-SSW oriented stress system.