--> Natural Fractures and Their Relations With Facies, Sequence Stratigraphy, and Rebound Hardness, the “Mississippian Limestone” Play, North-Central Oklahoma, U.S.A.

AAPG ACE 2018

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Natural Fractures and Their Relations With Facies, Sequence Stratigraphy, and Rebound Hardness, the “Mississippian Limestone” Play, North-Central Oklahoma, U.S.A.

Abstract

Natural fractures are common in several unconventional reservoirs around the world. Although commonly sealed through mineralization, these fractures may be related to the propagation of induced fractures and may be important for production designs. This study compares fracture distribution to facies, sequence stratigraphy, and rebound hardness (RHN) in the unconventional “Mississippian Limestone” play in north-central Oklahoma.

Ptygmatic, vertical extension, shear, and mixed types of fractures are identifiable, with the folded ptygmatic fractures being the most abundant. Most of the fractures are near vertical and have been sealed with calcite cement. The highest fracture abundance generally corresponds to facies with the highest average calcite content in the regressive phases of “third-order” sequences, suggesting a mineralogical control on fracture distribution and the value of a sequence stratigraphic approach in predicting the relative fracture abundance.

Similar RHN among certain facies and among fractured and adjacent non-fractured intervals suggest the alteration of rock mechanics following fracture formation. As further suggested by a correlation between detailed mineralogy and RHN, the rock mechanics determined by mineralogy is likely retained through fracture formation into the present-day. Thus, the present-day rock mechanics is likely a combination of relatively early “unaltered” and later “altered” signals. Therefore, in addition to mineralogy, evolution of rock mechanics is also important in determining the present-day mechanical stratigraphy, and therefore, should be considered in production design. Although poorly correlating with fracture distribution, RHN correlates with reservoir quality and critical rock mechanical properties, illustrating its potential value in reservoir characterization and production design.