Fracturing in basinal siliciclastic deposits of the Wolfcampian Alta Formation, Marfa Basin, west Texas

Abstract

The Wolfcampian Alta Formation represents 1700 m of deep-water siliciclastic deposits exposed in the Marfa Basin, the southwestern sub-basin of the Permian Basin complex of west Texas. These exposures are important outcrop analogs for the highly productive Wolfcamp Shale oil and gas reservoir of the Delaware and Midland Basins because they are of similar age, lithologies, and depositional environments. We present preliminary field data from outcrops of the Alta Formation in the southeast part of the Chinati Mountains, including lithostratigraphy, fracture characterization, and mineralogical analyses. Mesostructural deformation fabrics are dominated by up to four systematic sets of bed-perpendicular opening- mode fractures but also include rare bed-parallel opening-mode veins (beef), and occasional normal faults and thrust faults. Opening-mode fractures are generally bed-restricted and are interpreted to record a complex history reflecting changing extension direction at the time of fracturing in these sandstone and shale strata. Fracture dimensions mapped in a sandstone bedding pavement exposure show that length (parallel to bedding) to height (perpendicular to bedding) ratios for opening-mode fractures range from 0.13 to 38.56, with an average aspect ratio for all mapped opening-mode fractures of 4.84. Scanline surveys of a systematic NE-striking opening-mode fracture set show that fracture spacing is strongly correlated with mineralogy in both sandstone and shale lithologies, with a strong positive correlation for fracture spacing vs. clay content, and very strong negative correlations for fracture spacing vs. quartz, quartz + feldspar, and brittleness index. Bed thickness vs. fracture spacing data from scanlines show marked differences between sandstone and shale beds, with a very strong positive correlation for sandstone beds, a weak negative correlation for shale beds, and a very weak positive correlation – i.e. no correlation – for combined data. These results suggest that composition exerts a first-order control on opening-mode fracture abundance, and that bed thickness is likely a subordinate, or less important, controlling factor. These relationships can potentially be leveraged for mineralogy-based subsurface fracture prediction in comparable siliciclastic deposits.