ABSTRACT
The occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial, diagenesis, uplift, and erosional unloading. Variations in the state of stress within rock arise, for example, from changes in temperature, pore pressure, weight of overburden, or tectonic loading. Hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress, and this information used to compute a stress history. From this stress history, predictions may be made as to when in the burial cycle to expect fracture (joint) formation, what type of fractures (extension or shear) may occur, and which geologic factors are most favorable to development of fractures.
A stress history is computed for strata of the naturally fractured Altamont oil field in Utah’s Uinta basin. Calculations suggest that fractures formed in extension, that the well-cemented rocks are those most likely to be fractured, that fractures began to develop only after strata were buried to great depth, and that the fracture system continued to develop as strata were uplifted and denuded of overburden. Geologic evidence on fracture genesis and development is in accord with the stress history prediction.
Stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays.
