Fracture Patterns on Kuh-e Asmari Anticline, Southwest Iran

Abstract

Fracturing at air-photo scale on the Kuh-e Asmari anticline presents a simple pattern comprising two tension-fracture sets which clearly are related to Zagros folding. The absence of any marked shear-fracture sets is puzzling. Like all anticlines, this structure has its own inherent complexities. The identification of greater fracture lengths over azimuth classes corresponding to longitudinal fractures in the eastern part of the structure is suggestive of a relation to a larger, basement-controlled lineament. The recognition of such pre-Zagros features on reservoir structures may define zones of enhanced production potential. Photo-scale fractures show maximum densities over the plunging noses and central parts of the structure in that order. The contrast between the diversified small-scale, structure-unrelated patterns derived in an earlier study of the same anticline and the simple photo-scale pattern is most marked. Differing modes and times of genesis for the two scales of fracturing are proposed. The superficial nature of gravity faulting and dip-slope slumping, so prevalent in southwest Iran, suggests that fault and fracture patterns on surface structures may not be characteristic of their buried oil field counterparts. Such superficial features on surface Asmari structures results from unloading consequent on the stripping of the overlying Fars Group.