A review of the mechanics and occurrence of natural fractures in rock as applied to th-e development of the tight Western gas sands

Data on natural rock fractures and measurement or estimates of stress show promise as an aid in predicting the orientation and continuity of both natural and artificial fractures in natural gas reservoirs. Overbey and Rough, for instance, have shown that one set of surface joints in the Bradford field in Pennsylvania nearly parallels the direction of artificial hydraulic fractures in the reservoir. The "tight gas sands" in the Rocky Mountains are thin, discontinuous, and have low permeability. They are found in gently folded rocks and often have abnormally high pore pressure. In a relaxed tectonic setting, one principal stress is most often nearly vertical and the other two horizontal. Natural fractures usually approximate the same orientation to stresses as those produced experimentally. Increased pore pressure extends the depth to which extensional fractures can occur: these tend to form perpendicular to the direction of the least principal stress. Local stresses that control small features such as joints may be different frcm, although compatible with, regional stress. In folded rocks, sliding and bending forces appear to be most important in forming the smaller structures, such as joints. Fractures can be formed by flexing during downwarp and uplift; more intense folding or faulting is not necessary. Fractures are controlled by the physical environment, the stresses, and the nature of the rock. Changes in stress due to burial and uplift are also important. Jointing frequency is greater in weak and thinly bedded rocks and is directly related to intensity of folding. Two sets of both shear and extensional fractures may develop: one oriented parallel, the other normal, to axes of folding or downwarp. These relations may be used to extrapolate fracture patterns laterally or into the subsurface.