Abstract The generation of one or more three-dimensional (3-D), freehand drawings, based on integrated analysis of a two-dimensional (2-D) geologic database (e.g., borehole data, seismic profiles, surface geology, etc.), is proposed here as a rewarding exercise in the development of a final interpretation of subsurface geologic structures. A freehand 3-D drawing based on integration of 2-D interpretive structural contour maps (of at least two horizons) and structural cross sections can clarify and verify the 3-D details of complex subsurface geologic structures, check on the internal consistency of the interpretation, uncover untenable, interpretive, geologic configurations, and highlight possible obscure trap geometries. In some cases freehand 3-D drawings can aid in the visualization of impenetrable 3-D images produced by computer software programs. Isometric projection or linear perspective drawings are generally the most useful kinds of 3-D renditions, but strict adherence to these disciplines is not a requirement in the generation of an initial 3-D sketch. Generating a 3-D image using computer software is dominantly the functional domain of the left hemisphere of the brain (left brain), whereas the generation of freehand 3-D drawings is dominantly the functional domain of the right brain and requires penetrative visualization in the conversion of 2-D data to 3-D imagery. The right brain excels in intuitive, creative, imaginative structural interpretation. Examples of freehand 3-D drawings of complex subsurface and surface geologic structures, both self-generated and from literature, are presented along with some auxiliary 3-D analog modeling methods.

Subsurface reflection seismic and borehole data, combined with surface geologic mapping, provide a comprehensive data base for structural analysis of the central Rocky Mountain foreland province. These data, supplemented by analogue clay-model studies, constrain geometric and kinematic interpretations of the basement-involved thrust-generated folds (thrust folds) that formed structural traps for petroleum accumulations within the foreland basins. Case studies of selected intrabasin oil-field anticlines, illustrated with seismic profiles, structural cross sections, and structural contour maps, define immature, intermediate, and mature thrust folds. Because net slip is greatest at the sediment-basement contact, fold-generating thrusts must have nucleated within basement. Thrust-plane reflections on seismic profiles show that these thrusts are nearly planar within basement, and developed at an angle of between 20° and 35° to the basement surface. The thrusts then propagated and steepened upward, usually accompanied by backlimb rotation. Overlying Phanerozoic sediments were lifted and stretched over rising hanging-wall basement blocks and were ultimately offset by the propagating thrusts along the forelimbs of growing anticlines. In advanced stages of development, tapered hanging-wall basement wedges sustained significant finite strain, while in the footwalls, the basement remained relatively undeformed. Synthetic and antithetic detachment thrusts within the sedimentary column, footwall thrust wedges at the basement level, terminal tear faults, and shallow crestal extensional faults are common secondary structures. The basic elements of the thrust-fold model are incorporated in a true-scale, northeast-southwest structural transect drawn across Wyoming.