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ABSTRACT

Natural fractures are common in several unconventional reservoirs in the U.S. and around the world and, even when sealed with cements, can facilitate the propagation of induced fractures during hydraulic fracturing. This study is focused on correlating fracture types and intensity to distinct petrophysically significant facies and to an established sequence stratigraphic framework in the unconventional carbonate reservoirs of the “Mississippian limestone” of the U.S. midcontinent region.

Four fracture types are observed: ptygmatic, vertical extension, shear, and mixed types of fractures. Most of the fractures have been completely sealed with predominantly calcite cement. Fractured zones are vertically heterogeneous at various scales, indicating the variability in rock mechanical properties. At the millimeter scale, fractures are commonly discontinuous and exhibit variable kinematic aperture. At the centimeter scale, ptygmatic fractures exhibit variable termination modes in relation to bedding planes, suggesting a mineralogical control on rock mechanical properties. At the meter scale, the highest fracture abundance corresponds to facies with the highest calcite content. The mineralogical control of fracture distribution is also represented by the higher fracture intensity within the regressive phases of “third-order” sequences, indicating the value of sequence stratigraphic approach in characterizing and predicting fracture distribution in these unconventional reservoirs.

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