Geometry, Depositional Processes, and Facies Associations of Basin-Margin Strata, Carboniferous Ross Sandstone (Ireland): Applications to Reservoirs located on the Margins of Structurally Confined Fans

The Ross Sandstone, Ireland was deposited in a structurally confined submarine basin. The Ballybunnion exposure contains strata deposited on the margin of the basin. The exposure is divided into three units. The lower unit contains laminated shale with phosphate and siderite nodules, structureless siltstone, convolute bedding/slumps, locally contorted shale, and siltstone turbidites. The middle unit contains co-genetic debrite-turbidite beds, siltstone turbidites, and structureless siltstone. The co-genetic debrite-turbidite beds contain evidence that both fluid turbulence and matrix strength were operating simultaneously during transport of the flows that created them. The upper unit contains thin-bedded sandy turbidites, amalgamated sandy turbidites, siltstone turbidites, structureless siltstone, and laminated shale.

Basin margin strata at Ballybunnion are coeval to sandy turbidite channels and lobes deposited on the basin floor. A cross section through the Ross Sandstone reveals a wedge of low net:gross, poor reservoir quality strata that physically separates sandy, basin-floor strata from the basin margin. The wedge of strata is referred to as the transition zone. The transition zone is composed of co-genetic debrite-turbidite beds, structureless siltstone, slumps, locally contorted shale, and laminated shale. Using data from the Ross Sandstone, two equations are defined that predict the size and shape of the transition zone. The equations use three variables to solve for width (w) and trajectory of the basinward side of the low net-to-gross wedge (beta).

Seismic examples from Gulf of Mexico minibasins reveal a wedge of transition zone sediments, similar to that in the Ross Sandstone. The variables “w” and “beta” can be used in subsurface basins to define two important drilling parameters. “W” corresponds to the minimum distance a well bore should be positioned from the basin margin to intersect sandy strata, and “beta” corresponds to the deviation the well bore needs in order to continue to intersect sandy strata.