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Practical and Efficient Three Dimensional Structural Restoration Using “Geological Knowledge-Oriented” Earth Models

Abstract

The extension of “Balanced Cross-Section” structural validation techniques to three dimensions has been an area of active research and development for two decades. Approaches that have been applied include vertical-shear flattening of fault blocks, fault slip accommodated by vertical- or inclined-shear, fault-parallel flow, and mechanical (typically elastic) unfolding. However, none of these approaches have gained routine use for interpretation validation or quantitative reconstruction of structural evolution. The reasons for this are a combination of limited flexibility and the great effort required to properly treat complex geometry and topology inherent in geologic structures for which the tools would be most useful. We propose that the advent of so-called “Geological Knowledge-Oriented” earth models makes this traditionally difficult problem more tractable. An important aspect of these models is the ability to globally specify stratigraphic correlation and relative paleo-geographic position within the volume of interest. These tools provide the ability for practitioners to develop quantitative, robust, and topologically consistent models for complex geologic structures which provide the foundation for a practical 3D restoration. We present a workflow that uses an adaptation of the UVT transformation implemented in SKUA® to complete a three-dimensional structural restoration. The complete stratigraphy is restored using a transformation constrained only by the datum horizon. By scaling the “T” coordinate (which is vertical in the restored space) to depth in a manner that preserves volume or layer thickness, we obtain a geometric restoration approximately consistent with principles of global strain minimization. This balanced restoration provides a geometrically plausible representation of the geologic structure(s) at the time when the datum horizon was deposited, in contrast to the global volume flattening performed by conventional application of SKUA®, in which all horizons are simultaneously unfaulted and unfolded. Restorations are independent of mechanical rock properties, and may be informed by growth strata to the extent they are represented in the earth model. Once an appropriate SKUA® model has been constructed, time required for restoration is minimal. We demonstrate the validity of our solution by restoring a laboratory sandbox model with substantial structural complexity, and discuss how the tool can be used with a natural example.