--> Fracture Characterization and Its Relation to Geomechanical Stratigraphy in a Heterogeneous Reservoir Analog: Implications for Fractured Sandstone Reservoirs From Outcrop Analysis, Cretaceous Mannville Group, Southwest Alberta

AAPG ACE 2018

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Fracture Characterization and Its Relation to Geomechanical Stratigraphy in a Heterogeneous Reservoir Analog: Implications for Fractured Sandstone Reservoirs From Outcrop Analysis, Cretaceous Mannville Group, Southwest Alberta

Abstract

Comprehensive analysis of natural fracture network geometry is an important step in the realistic modelling and characterization of reservoir targets as fractures can provide preferential flow pathways, impact wellbore stability, and influence hydraulically induced fractures. Outcrop exposures of subsurface reservoir analogs provide detailed information regarding fracture development, interaction, and geometry in relation to host-rock properties that are not easily discernable from subsurface data alone. Exposed bedding planes of the Cretaceous Mannville Group along Livingstone River in southwestern Alberta were the focus of this study as they provided detailed multi-dimensional fracture network data of a heterogeneous reservoir analog. At the study site, the Mannville Group is characterized by thickly bedded sandstones interbedded with fine-grained siltstones allowing for the comparison of natural fracture geometry across several lithologies and lithofacies. Classical methods for fracture sampling, including scanline and window sampling, were applied in outcrop and compared to fracture analysis conducted on high-resolution orthophoto data using newly developed software tools for digital fracture characterization. The mechanical character of the section was defined using rock hardness rebound values and compared to the distribution of fractures within the section. A resulting fracture stratigraphy model was established for analogous reservoir targets as changes in fracture geometry including fracture height and intensity correspond closely to changes in geomechanical stratigraphy. Two nearly orthogonal, systematic stratabound fracture sets are well-developed in the sandstone lithofacies. The intervening softer siltstone facies are relatively barren of well-developed fracture planes, and act as barriers to fracture propagation between sandstone packages. Preferential horizontal connectivity and permeability is likely to be favored in such reservoirs, and geomechanical stratigraphy can be expected to strongly influence hydraulic fracture propagation. This study highlights the complexity of natural fracture networks in heterogeneous reservoirs. Furthermore, the results provide guidelines for accurate fracture model generation and fractured reservoir characterization.