Fracture-size scaling and stratigraphic controls on fracture intensity

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2002

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Abstract

Scaling techniques offer an opportunity to solve subsurface fracturesampling problems by extrapolating fracture properties from sub-millimeterscales to scales important for economic applications. Although extrapolation of fracture length and aperture distributions across observation scales is fraught with potential errors, sampling of opening-mode kinematic apertures along scanlines using new fracture-aperture measuring tools produces consistent power-law aperture distributions from the micron-scale to outcrop-scale. One-dimensional sampling avoids fracture connectivity issues inherent to traditional twodimensional length sampling methods. Sampling artifacts and mechanical layer effects can be diagnosed and accounted for, and extrapolation of power-law fracture intensities from the sub-millimeter scale up to the length scale of mechanical layers is feasible. Tests were performed in turbidite beds of the Ozona Sandstone, Texas, eolian Weber Formation sandstones, Colorado, and Lower Cretaceous carbonates of the Sierra Madre Oriental (SMO), Mexico. Outcrop studies in Weber Formation sandstones provided an opportunity to characterize well-exposed macrofracture systems as potential analogues for subsurface fractured reservoirs at Rangely Field. However, differences in stratigraphy and diagenetic history between surface and subsurface do not allow the direct extrapolation of these results to subsurface, reinforcing the idea that local data are necessary for fracture system characterization even in cases where long geologic time has passed between the time of sedimentation and the time of deformation that brought potential outcrop analogs to the surface. Another way to predict fracture properties in the subsurface is to analyze the relationships between fracture attributes and the geologic parameters of the rock volume that govern fracturing. Fracture-fill prediction using relative volumes of cement phases precipitated during and after fracture timing shows an empirical relationship with sedimentary facies in Weber Formation sandstones. Multivariate analysis of unbiased fracture intensity in SMO carbonates suggests that degree of dolomitization and position of a bed at the top of a stratigraphic cycle are the most important controls on fracture intensity in these rocks. Mud content has only a modest control on fracture intensity and bed thickness has the least control on fracture intensity, suggesting that published work concluding that fracture intensity is strongly governed by bed thickness may be biased by scale or sampling effects.

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