--> Bedding-Parallel Fractures in Shales

AAPG Annual Convention and Exhibition

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Bedding-Parallel Fractures in Shales

Abstract

Natural, sealed bedding-parallel fractures in shale are common and may impact hydraulic fractures by inhibiting height growth while facilitating horizontal propagation. In this presentation we present data on thickness, lateral extent and morphologies of these fractures in several shales. The aperture size ranges from < 0.1 mm to ∼10 cm. Intensity (fractures/m) of bed-parallel fractures in some locations is comparable to the intensity of vertical fractures, while in others it is 3-4 orders of magnitude greater. Lateral extent is variable, but broadly proportional to thickness (max extent > 32 m). Some fractures are segmented, and others are continuous without a break for 10s of meters. Fibrous cement fills (beef) are typically of calcite or sulphates, sometimes with hydrocarbon inclusions. Many fractures show multiple phases of growth, with the kinematics of each deformation phase indicated by the directions of fibers. The spatial organization of bed-parallel fractures is examined in relation to composition and layering, fracture mechanism, structural and tectonic context, and mechanics of layered sequences. Different mechanisms may be responsible for fracture generation and more than one mechanism may have operated in the history of a given shale. We attempt to use stable isotope geochemistry and fluid inclusion analysis to constrain conditions of formation, and thereby narrow down the possible mechanisms for each case. This approach, along with tying the occurrence to lithotype, will allow us to predict bedding-parallel fracture occurrences.