Flexural Shear Associated with Transpressional Folding of Abu Dhabi Onshore Fields

Abstract

The Abu Dhabi onshore Anticlinal Fields are trending NE-SW and N-S, with their eastern flanks being of steeper dip (~3°-5°) than their western flanks (~1°-2°). In previous research, kinematics, and mechanics of onshore Abu Dhabi folded anticlines had not been addressed. The main objective of this paper is to understand the kinematics and mechanics of the folding systems.A new approach to understanding the mechanical evolution of these anticlines is presented here through the use of conceptual modelling and theoretical approaches. A combination of geological and geophysical tools is used for determining the direction of the shear stresses along the flexural slip planes. Although textural evidence, such as karstification and diagenesis, suggests local material redistribution on different scales, compositional trends support negligible volume flux at the scale of the anticlines. The preservation of bed length and bed thickness in most of the onshore Abu Dhabi Anticlines requires the development of bed-parallel shear. Abu Dhabi Anticlinal fields are mechanically evolved by flexural shear and flexural slip folding, which is characterized by differential simple shear along the bedding planes, where deformation is concurrent with the diagenesis. The layered horizons of these folds are mostly of carbonates, where the competency contrast is low and the friction between the layers is high leading to flexural shear. In the locations, where incompetent shale layers alternate with the carbonates, flexural slip is considered -although this is very limited within Abu Dhabi anticlinal structures. In these folds, the bedding planes of less competent layers act as boundaries of shear zones. Therefore, most of the internal deformation occurs within the less competent layers by partial recrystallization and development of fractures. However, the competent layers undergo pressure solution, development of sigmoidal extensional fissures, and stylolites in most cases. Thus, the overall folding system is heterogeneous and the competent layers simply slide over one another. The dominant direction of the over-riding shear is directed toward the fold axial surface and the stresses are released along the bedding plane discontinuities. The bedding plane discontinuities are considered to represent the finite neutral surface of the fold. Folds having parallel geometrical forms have opposite directions of structural shear in transtensional and transpressional regime.In the Abu Dhabi flexural slip shear folds, deformation in the hinge zone is characterized by pure shear, and deformation on fold limbs is characterized by components of both simple and pure shear. Strain models of pure bending, layer-parallel shear, and shear parallel to the hinge plane are shown to be the main mechanism of these folds. Preservation of these will play an important role in revealing a new play concept for the Abu Dhabi fields.