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Natural Fracture Characterization and Prediction of the “Mississippian Limestone” Play, North-Central Oklahoma, U.S.A


Natural Fractures are ubiquitous in several unconventional carbonate reservoirs in both the U.S. and around the world and these natural fractures, even when sealed, can facilitate the propagation of induced fractures during hydraulic fracturing. This study is focused on correlation of fracture types and fracture density to specific petrophysically-significant facies and to an established sequence stratigraphic framework in the unconventional carbonate reservoirs of the “Mississippian Limestone” of the U.S. Mid-Continent region. Four fracture types are observed in several cores from north-central Oklahoma: ptygmatic (folded) fractures, vertical extension fractures, shear fractures, and zones of mixed types of fractures. Most of the fractures have been completely filled with predominantly calcite cement derived from basinal fluids based upon both d18O and d13C values as well as fluid inclusion homogenization temperatures and strontium values. Fractured zones are vertically heterogeneous at various scales, indicating the variability of rock mechanical properties. At the millimeter scale, fractures are commonly discontinuous and exhibit variable width. At the centimeter scale, ptygmatic fractures exhibit variable termination modes in relation to bedding planes, suggesting mineralogical control of rock mechanical properties. At the meter scale, the highest fracture abundance corresponds to facies with the highest calcite content. Such mineralogical control of fracture distribution, which is corroborated by the positive correlation between calcite content and fracture density in a “fourth-order” sequence, further contributes to the general correlation of fracture abundance with regressive phases of “third-order” sequences at the whole core scale, indicating the value of sequence stratigraphic approach in characterizing fracture distribution.