CHARACTERIZATION OF BEDDING-PARALLEL FRACTURES IN SHALE - MORPHOLOGY, SIZE DISTRIBUTION AND SPATIAL ORGANIZATION

Natural fracture systems are important components of production in shale gas reservoirs as they may contribute to permeability of the reservoir, or they may reactivate during hydraulic fracture treatment. However, little is known about their size scaling and spatial distribution. Bedding-parallel, calcite-filled fractures are common in shale formations. Understanding the aperture-size scaling and spatial organization of bedding-parallel fractures can improve modeling of the combined fracture network, both hydraulic and natural. A total of ten fracture data sets were collected from the Vaca Muerta (7), Marcellus (2) and Wolfcamp (1) shale formations. Bedding-parallel fracture attributes including strike, dip, aperture size, spacing, length and texture, were collected from outcrops of the Vaca Muerta Formation in the Neuquén Basin, Argentina. Ten sets of scanline data were collected from the Vaca Muerta, Marcellus, and Wolfcamp formations through direct core measurements and photographic panels of slabbed core. Nine out of ten datasets follow a negative exponential distribution. Fracture attributes such as intensity and size range are different in the three studied shales. Even within the same shale formation, fracture intensity and size range can be variable. Aperture size ranges of bedding-parallel and vertical fractures in these shales are comparable as are fracture intensities for the Marcellus examples. Bedding-parallel fractures, however, have higher intensities than vertical fractures in the Vaca Muerta examples. The relationship between fracture spatial organization and stratigraphy and mechanical interfaces within the host rock is also investigated, with preliminary results suggesting that bed-parallel fractures are more intense in organic-rich layers in some cases, but not in others.