Abstract

Fractures form in brittle strata bounded by ductile layers. Where rocks are stressed to failure, rock types such as quartzite, dolomite, sandstone, and limestone develop dissimilar fractures in similar stress fields. Various water-saturated rock types differ in permanent strain before failure. Overburden pressure and strata extension may be sufficient to fracture a less-plastic bed bounded by more-plastic beds. From these general findings, the paper describes “triple bed” rock sequences as likely sites for fracture development. The paper shows outcrop and subsurface examples of Cretaceous sequences, including a Menefee (Mesaverde Group) example. A drilling time log of the Niobrara of the Denver-Julesburg Basin is interpreted for likely sites of natural fractures based on penetration rates. Causes of natural fractures are numerous, and causes can be persistent and universal. Thus fractures are common in consolidated strata in sedimentary basins of the world. 3-D seismic imagery and amplitude coherence processing reveal shapes of sedimentary trends, subsurface fault planes, and deformation very much like styles observed in the laboratory and at the outcrop. A challenge for modelers includes scalar comparisons. Fracture styles at many scales, measurable at the outcrop and in the laboratory, can be foundation data for reservoir fluid flow models, petroleum recovery, water production, and efforts for fluid waste disposal.