Abstract

Seismic reflection data show that the Kelasu structure, a frontal structure of the south Tian Shan foothills thrust belt, is a triangle zone with the upper detachment following an overpressured lower Tertiary anhydrite zone and the lower detachment zone probably in the lower Mesozoic seismic reflectors. The south-verging thrusts emerging at the surface are the small roof structures rooted in the upper detachment. Although the upper detachment was a relatively lubricated surface, the passively folded formations show strong resistance to the blind wedging underneath, particularly in the anticlines, during the triangle zone development. As a result of the massive shear forces created by the opposing movement of the two tectonostratigraphic units, the top part of the thrust sheets immediately under the crest of the frontal anticline was ripped off in the form of backthrusting and brecciation. The large backthrust fault blocks and breccias were retained in the anticlines, whereas the underlying duplex continued wedging toward the foreland. In the early formed duplex structures in the hinterland, the tips of the thrust sheets were completely shredded, leaving sharp angular contacts between the thrust sheets and the upper detachment surface. Different types of fractures and present-day geostress (stress of the earth measured along the wellbore) interpreted from the borehole image logs suggest a complex deformation history and reversal of geostress. The Kelar gas reservoirs are located within the backthrust fault blocks under the frontal anticline. The current model reduces the previous concerns about potential damage to the anhydrite seal by the surface thrusts because the thrusts occurred entirely above the seal. More importantly, because the reservoir rocks are repeated many times in the duplex thrust sheets below the continuous anhydrite seal, any anticlinal structure of the upper detachment surface is a potential prospect.

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