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Knowledge of subsurface structures and stress is important to the petroleum and mining industries. These factors have a direct impact on reservoir performance and mine roof stability. Local structures and stresses typically cannot be defined by normal exploration drill spacing. Underground mine mapping, however, provides a unique opportunity to measure these local features and to determine how they affect petroleum and coal production.
A geologic mapping program was conducted at Old Ben Coal Company's Mine 27, which operates in the Illinois No. 6 Coal in Franklin County, Illinois. The mine is in an area of "pod-type transitional roof," in which isolated Energy Shale pods are overlain by Anna Shale and Brereton Limestone. Each facies has a distinctive structural and deformational assemblage. Mapping shows that an excessive east-west horizontal stress also exists in the area. The stress and the structural and lithologic discontinuities are the primary cause of roof failure in the mine.
The detailed structure and stress data acquired in the Mine 27 investigation can also be used to model oil and gas reservoirs. The measurements of joint density, fracture orientation, etc, can be integrated with petro-physical and mechanical data to evaluate fracture permeability. Because underground stresses control hydraulic fracture propagation, knowledge of the subsurface stress field can be used to evaluate stimulation techniques. The orientation of hydraulically induced fractures can be approximated if the magnitude and direction of the stresses are known.
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