Paleostrain and fracture potential of deep-seated wrench-fault systems in the post-Hith of Block 11 (Qatar)
Authors: Suess Michael Peter , Plesch Andreas , Shaw John H. , Zahran Mamdouh 
Published: May 2016

The Post-Jurassic strata of Block 11, offshore Qatar, are dissected by en-echelon normal faults above deep-seated wrench faults below the Hith. The Hith, a thick Upper Jurassic anhydrite dominated unit, acts as mechanical decoupling layer between the fault systems in the Pre- and Post-Hith sections. The paleo-strain field constrains the evolution of the fault system constrains the paleo-strain field, which influenced the reservoir-scale fracture generation. Fracture systems are an important element of many carbonate reservoirs in Qatar. They are not only conduits of fluids, but also pathways of dolomitization. Structural restoration is a major tool for understanding the time-dependent evolution of faults and strain, which can express themselves in the patterns of reservoir scale fractures.

We perform a series of 3D structural restorations of the Cretaceous reservoirs in Qatar using a mechanical modeling technique. Subsequent stripping of layers allows for incremental restoration. The method enables us to analyze amounts of displacement and strain within the main levels of the structure.

Currently our model consists of nine stratigraphic horizons from Mishrif to Khuff. Each stratigraphic unit can be restored and the incremental strain can be computed. Our analysis helps us to define the deformational history of the target structures, including the spatial and temporal evolution of faults and folds that affect the reservoir. In addition, we compare the 3D strain properties calculated from the restorations with seismic attribute data and other geophysical attributes to establish correlations with natural fracture patterns in the reservoirs.

Introduction and background

The Post-Jurassic of Offshore Qatar in the area of Block 11 has been only affected by small amounts of deformation. The area is characterized by relative low amplitude folding concentrated along a system of en echelon normal faults above the Hith, an Upper Jurassic anhydrite layer, and a lower system of strike-slip faults below the Hith level. Fault offsets are normally around 20 m and reach a maximum of about 25 m. The Post-Jurassic succession is dominated by carbonates, but intermittend shaly units induce a layering with strong mechanical anisotropy. While deformation may be comparably small, fracture occurrence in the Post-Jurassic offshore Qatar has proven to be a crucial element of the carbonate reservoirs. Fractures provide both additional access to hydrocarbon volumes and the potential of early water break-through. It is therefore the aim of this study to develop a new workflow, which allows the prediction of fracture occurrence in areas with relative low amounts of deformation.

At this point various approaches exist to generate full geomechanical restorations in 3D (Moretti et al. 2006; Maerten & Maerten 2006; Muron et al. 2005, Müller et al., 2005; Guzofski et al., 2009). We use a tool developed as a plugin in gOcad, a geological modeling application where the model construction and computations are performed (Muron et al. 2005). The method applies finite-element algorithms which enable a full geomechanical restoration. The model consists of a tetrahedral representation of the volume which honors the faults and the major stratigraphic horizons. Each stratigraphic unit can have its own geomechanical properties which are defined by Lame's constants. The method minimizes the total strain energy imposed on a model by restoration of a datum horizon. The restoration is constrained by pinpoints, pinlines or fixed boundaries and carried out to a restoration datum or surface. Because of the low offsets in the area of interest, a high resolution representation of the fault and horizons becomes necessary (more then 380.000 nodes and 1.9 million tetrahedral), and the software must compute deformations of the resulting large computational mesh.