Publications
Interpretation validation and reservoir properties from kinematic analyses, Rocky Mountain Association of Geologists 2001 Technical Symposium, R. Ratliff
Kinematic modeling of geologic structures analyzes the evolution of rock geometries and their associated deformation. This approach to interpretation testing and reservoir description provides unique information on the structural and depositional history of hydrocarbon traps from both a petroleum systems and a rock property perspective. In addition to the validation of competing interpretations, various scenarios on the timing of hydrocarbon sourcing and migration can be investigated, and the strain information inherent to a kinematic model can be used to map 4D deformation variation. By correlating these strain patterns with deformation mechanisms such as fracturing and pressure solution, and including present-day geometrical attributes such as curvature, a quantitative description of tectonic porosity and permeability variation is possible.
The foundation for 2D/3D kinematic analysis derives from the widespread application and success of 2D cross section restoration and "balancing". For any given data constraints (field, well, seismic, remote sensing, etc.) virtually an infinite number of 2D and/or 3D subsurface interpretations can be made (with pencil and paper or the most advanced interpretation system), but the set of physically possible interpretations are far more limited. The tests for such physical plausibility derive from our knowledge of the controls on sediment deposition and diagenesis, rock mechanics, structural evolution, erosion, and hydrocarbon systems. Interpretations that can pass these tests may not be correct, but they are more likely to be so, and can be considered 'validated'.
Validated interpretations are balanced, i.e., display geometries analogous to observable structures and, if deformed by faulting and folding, can be restored to a geologically reasonable undeformed state. However, they must also be kinematically admissible, with a demonstrably reasonable geological evolution of the structures. Whatever the reason for undertaking such kinematic analyses, there are two fundamental concerns: the plausibility of the test criteria and the limitations of the kinematic models themselves. Whenever an interpretation "fails" a validation test, the reason may be that the interpretation is wrong, or it may be due to some combination of an inappropriate test or flawed transformation algorithm being applied.
The primary validation criteria can be generally categorized as: (1) bedlength and/or area preservation, or lack of, between deformed and restored states; (2) correspondence in number and stratigraphic thicknesses/locations between hanging wall and footwall ramps and flats; (3) fault displacement consistency; (4) viable deformation intensities within structural domains, with shear strain variations being particularly diagnostic; and (5) the overall 'reasonableness' or sensu strictu 'balance' of a restored state geometry, i.e., the display of geometries comparable to natural, experimental, or theoretical examples.
Algorithms used to construct and restore or forward model an interpretation should conform with observed structural geometries and deformation histories. In both 2D and 3D the kinematic methods which have proven most useful for multiple-horizon (and intervening regions) are all simple shear based, with either a straight slip vector (i.e., vertical/oblique slip) or a curvilinear/curviplanar slip system (i.e., flexural slip and slip line). These kinematic models are also related to the techniques used for projecting bedding from the geometry of a known horizon: constant-thickness parallel geometry (angular bend or dip domain) typically defines the slip system used in flexural slip (bedlength balance) and slip line (constant fault displacement) methods, whereas similar geometry is the projection method compatible with vertical/oblique slip kinematic models.
Although flexural slip is often regarded as the appropriate restoration/modeling method for most contractional structures and vertical/oblique slip for extensional areas, many would argue that these restricted applications overly simplify actual rock deformation, particularly in areas of active salt migration or where simultaneous deposition and structural development are occurring. Furthermore, while often successful in defining subsurface structures, these techniques are based on rather crude approximations to natural rock geometries and are rarely valid at scales larger than a few meters. In the context of hydrocarbon exploration and production, such criticisms become important when using restoration to predict fracture properties in hydrocarbon reservoirs or to accurately define the size of a structural trap. We have thus recently implemented and described "complex geometry", a new projection and kinematic model that provides a significantly more accurate representation of natural rock patterns in both 2D and 3D. Critical distinctions of complex geometry are that it is defined by one or more arbitrarily weighted 2D lines or 3D surfaces, and its constraining elements may, but need not, conform to constant-thickness parallel (concentric or angular bend) or similar geometries. The weightings control not only the proportional influence of the defining entities in projecting intermediate (interpolated) and extrapolated geometries, but also the relative contributions of parallel and similar style. Complex geometry provides a robust 2D/3D horizon and/or slip system projection method along with the heretofore unavailable ability to combine multiple constraining horizons and a mix of deformation models into a single kinematic entity.
Examples of these concepts - restoration, interpretation testing and validation, incremental evolution, and fracture prediction from strain and curvature, are presented with several examples, including deformed sediments surrounding the El Papalote flared salt diapir from the La Popa Basin, Mexico, structures from the southern Alberta and Idaho-Wyoming-Utah thrustbelts, and inversion structures from offshore Oceanside, California.
