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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Permian Basin (1)
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United States
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Illinois Basin (1)
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Texas
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Burnet County Texas (1)
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Fort Worth Basin (1)
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San Saba County Texas (1)
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commodities
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oil and gas fields (1)
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petroleum
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natural gas
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shale gas (1)
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elements, isotopes
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carbon
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C-13/C-12 (1)
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isotope ratios (1)
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isotopes
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stable isotopes
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C-13/C-12 (1)
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sulfur (1)
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geologic age
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Paleozoic
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Carboniferous
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Mississippian
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Barnett Shale (2)
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Pennsylvanian
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Smithwick Shale (3)
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New Albany Shale (1)
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Primary terms
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carbon
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C-13/C-12 (1)
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geochemistry (1)
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isotopes
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stable isotopes
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C-13/C-12 (1)
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oil and gas fields (1)
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Paleozoic
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Carboniferous
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Mississippian
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Barnett Shale (2)
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Pennsylvanian
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Smithwick Shale (3)
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New Albany Shale (1)
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petroleum
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natural gas
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shale gas (1)
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petrology (1)
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sedimentary petrology (1)
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sedimentary rocks
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clastic rocks
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mudstone (1)
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sandstone (1)
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shale (1)
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gas shale (1)
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sedimentary structures
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planar bedding structures
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bedding (1)
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sulfur (1)
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United States
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Illinois Basin (1)
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Texas
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Burnet County Texas (1)
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Fort Worth Basin (1)
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San Saba County Texas (1)
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sedimentary rocks
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sedimentary rocks
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clastic rocks
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mudstone (1)
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sandstone (1)
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shale (1)
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gas shale (1)
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sedimentary structures
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sedimentary structures
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planar bedding structures
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bedding (1)
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Smithwick Shale
Abstract Shale gas reservoirs are commonly produced using hydraulic fracture treatments. Microseismic monitoring of hydraulically induced fracture growth shows that hydraulic fractures sometimes propagate away from the present-day maximum horizontal stress direction. One likely cause is that natural opening-mode fractures, which are present in most mudrocks, act as weak planes that reactivate during hydraulic fracturing. Knowledge of the geometry and intensity of the natural fracture system and the likelihood of reactivation is therefore necessary for effective hydraulic fracture treatment design. Changing effective stress and concomitant diagenetic evolution of the host-rock controls fracture initiation and key fracture attributes such as intensity, spatial distribution, openness and strength. Thus, a linked structural-diagenesis approach is needed to predict the fracture types likely to be present, their key attributes and an assessment of whether they will impact hydraulic fracture treatments significantly. Steep (>75°), narrow (<0.05 mm), calcite-sealed fractures are described in the Barnett Shale, north-central Texas, the Woodford Formation, west Texas and the New Albany Shale in the Illinois Basin. These fractures are weak because calcite cement grows mostly over non-carbonate grains and there is no crystal bond between cement and wall rock. In bending tests, samples containing natural fractures have half the tensile strength of those without and always break along the fracture plane. By contrast, samples with quartz-sealed fractures do not break along the fracture plane. The subcritical crack index of Barnett Shale is >100, indicating that the fractures are clustered. These fractures, especially where present in clusters, are likely to divert hydraulic fracture strands. Early, sealed, compacted fractures, fractures associated with deformation around concretions and sealed, bedding-parallel fractures also occur in many mudrocks but are unlikely to impact hydraulic fracture treatments significantly because they are not widely developed. There is no evidence of natural open microfractures in the samples studied.