Stratigraphic Controls on 3D Structural Traps Developed in Carbonate/Shale Multilayer Sequences

Folded and fractured carbonates form extensive hydrocarbon traps. Fold geometries in multilayered sequences are governed by fold mechanisms and the mechanical properties of the layers. In sedimentary environments where laterally variable thicknesses of interbedded carbonates, clastics, and evaporates may be present, predicting the sub-surface geometry of structural traps can be challenging.

In the Zagros fold and thrust belt of SE Turkey, Neogene shortening has affected an Ordovician-Miocene sedimentary sequence of carbonates, shales, sandstones and minor evaporites. Miocene carbonates capping the sequence allow 3D fold geometries to be reliably mapped at surface. The underlying ~1km of shales and mudstones allow folds to develop that depart substantially from standard text-book geometries. Beneath these incompetent units a further 1.7km of carbonates and clastics overlie Ordovician shales up to 1.5km thick. Where the amount of shortening is relatively low, asymmetric, angular, kink-like folds form long wavelength structures, and thrust faults rarely reach the surface. With increasing strain, the wavelength/amplitude ratio decreases and thrusts cut through the fold limbs.

Folds are interpreted to detach above a thick sequence of Ordovician shales. They originate by buckling of the competent units within the two weaker horizons, with initial perturbations probably provided by sedimentary heterogeneities. As the folds amplify, thrusts form in the more competent units above the shale. These then propagate upwards with fold amplification dominated by fault-tip tri-shear. The mechanical heterogeneity between the near surface shales and overlying carbonate leads to the kink-like geometries seen at surface. In this model, thrust formation is governed by the location of the initial buckle folding. Since the thrusts mostly originate from the top of the Ordovician shales, they form a linked system detaching into the Ordovician so that displacement can transfer from one structure to the next along strike of the orogen. It is the interaction of folds with varying amplitude which is essential in the identification of viable hydrocarbon traps.
We use numerical models which incorporate both fracturing and folding to test the influence of multilayer stratigraphy on the geometry of folds that develop in the competent carbonates. Inputs for the models are constrained using field observations combined with borehole data and satellite and seismic interpretation.

Keywords: Fractured carbonates; Structural traps; Multilayer reservoirs; Mechanical stratigraphy