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Implications on Porosity and Fluid Flow From Micro-Scale Parasitic Folding

Abstract

Structures developed in sedimentary rocks due to deformation such as folds are often a combination of macro- and micro- scale deformation features. While the macro-scale geometry, amplitude and shape of a fold defines that structure’s capability to form a structural trap, the meso and micro-scale deformation features such as parasitic folds, cleavage and fabric orientation have significant impact on the formations’ porosity and permeability distribution within the macro structure. Parasitic folds are of particular interest since they are common in multilayer buckle folding. The resulting meso- and micro-scale asymmetric ‘S-‘ or ‘Z-‘ and symmetric ‘M-‘ shaped fold structures represent a complex strain distribution which can significantly affect the porosity, permeability and associated fluid flow pathways within the macroscopic structural trap. This study utilizes a 2-D plane strain finite element modeling approach to simulate multi-scale, multilayer, viscoelastic buckle folds under in-situ stress and drained pore pressure conditions. The sensitivity of the resulting porosity distribution is analyzed with respect to variations of several model input parameters such as the elastic modulus contrast, number of layers, viscosity contrast, strain rate and layer thickness ratio. The results show that the parasitic fold shapes developed depend upon the interaction of buckling of both the large- and small-scale folds and are influenced to greater and lesser degrees by the various parameters. The resulting porosity distribution shows a large degree of deformation induced heterogeneity as the volumetric strain changes drastically in different locations of the folds during the multi-scale, multi-layer buckling process. Three regions, including the hinge and limb of the less competent layer in the M-shaped folds and the limb of the less competent layer in the Z-shaped folds, feature significant porosity changes. In addition, the numerical simulations, through the applied volumetric strain-porosity-permeability coupling, show that resulting fluid flow regimes in multi-scale, multilayer buckling systems include localized pervasive and focused regimes. This indicates that localized meso & micro-scale folding has significant impact on potential reservoir heterogeneity which may not be considered when only analyzing macro-scale deformation.