The use of curvature for the analyses of folding and fracturing with application to the Emigrant Gap anticline, Wyoming

Fractures, which are common structural heterogeneities in geological folds and domes, impact the charge, seal, and trapping potential of hydrocarbon reservoirs. Because of their effects on reservoir quality, the numerical prediction of fractures has recently been the focus of geologists, geophysicists, and petroleum engineers. The need to predict fractures has preceded an adequate understanding of their formation mechanisms, so existing techniques for prediction, such as curvature analysis and conceptual fracture-fold models, are based on simplifying assumptions. This research is concerned with furthering our understanding of the processes of fracturing and folding, and also with improving techniques for the prediction of fractures in folded strata.; The curvature of folded bedding has been used both to describe these geological surfaces and to predict the distribution of deformation in folded or domed strata. A technique is presented here for the exact calculation of the normal curvature of surfaces, which is based on the First and Second Fundamental Forms of differential geometry. For the description of geological surfaces, the first and second fundamental forms offer a unique quantitative tool. A challenge associated with geological curvature calculations is that continuous geological horizons are usually sampled at discrete points. The geometry of the sampled horizon includes surface undulations at all scales, which are then also included in the curvature analysis, thereby leading to noisy and questionable calculated curvatures. Thus, the source data needs to be filtered prior to curvature analysis. A numerical technique based on Fourier analysis is successfully tested for extracting meaningful curvatures of various scales.; Most conceptual fracture-fold models assume symmetric relations between synfolding fractures and fold geometry. This view is contradicted by field observations from the Emigrant Gap, an anticline located near Casper, Wyoming. There, observations suggest that the orientation of synfolding fractures was influenced dominantly by preexisting joints rather than by the fold geometry. Moreover, these field observations indicate a change in fracture style from mostly opening mode on the limbs to shearing mode near the hinge. Because models of symmetric fracture-fold relationships fail to account for these observations, an alternative is presented, which includes the influence of prefolding fractures.