Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Processes and Facies Associations in Basin-Margin Strata of Structurally Confined Submarine Fans: Example from the Carboniferous Ross Sandstone (Ireland)
The Carboniferous Ross Sandstone of western Ireland was deposited in a structurally confined submarine basin. Two exposures contain strata deposited on the eastern lateral basin margin. One of these exposures, Ballybunnion, is documented in detail with lidar data and stratigraphic columns. The exposure contains three architectural associations. In ascending order they are: (A1) mudstone sheets and slumps, (A2) linked debrites and mudstone sheets, and (A3) sandy turbidite lobes. A regional cross section through the Ross Sandstone demonstrates that the vertical succession of architectural associations records laterally adjacent bathymetric features on the seafloor during deposition. Architectural associations A1 and A2 form a wedge of low net-to-gross strata that separates sandy basin-floor strata from the lateral basin margin.
Using data from the Ross Sandstone, two equations were defined that predict the geometry of the low net-to-gross wedge. The equations use three variables to solve for width (w) and trajectory of the basinward side of the low net-to-gross wedge (b). The two equations were applied to basin-margin strata in three northern Gulf of Mexico minibasins that contain comparable low net-to-gross wedges. The equations predicted "w"; and "b" within 20 percent of their measured values. The variables "w" and "b" also represent two important drilling parameters. "w" corresponds to the minimum distance a well bore should be positioned from the lateral basin margin to intersect sandy strata, and "b" corresponds to the deviation (from horizontal) of the well bore to follow the interface between of sandy and low net-to-gross strata.