Are laminated siltstones part of the same lobe system? An architectural and geochemical test using 1D facies successions in the Bone Spring Sands/Silts and Wolfcamp X & Y, Delaware Basin

The lowermost Bone Spring Formation, Third Bone Spring Sand/Silt, in the Delaware Basin records submarine lobe deposits formed where turbidity currents and hybrid-event beds (HEBs) exit channel confinement and decelerate, reducing transport capacity. Because deposition reflects the coupled effects of flow dynamics and seafloor topography, these lobes develop complex facies mosaics that are difficult to predict, especially in lobe-fringe settings.

The Wolfcamp X and Y sand/silt intervals overlying Wolfcamp A, together with the lower, middle, and upper lobe complexes of the Third Bone Spring Sand/Silt, were deposited during relative sea-level fall. We analyze cores and compare gravity-flow sandstones with laminated to anastomosing siltstones. Facies successions are used to quantify spatial heterogeneity and hydraulic energy gradients through changes in stacking patterns and the relative proportions of structureless versus structured sandstone/siltstone, laminated/anastomosing siltstone, and mud. To evaluate whether laminated siltstones are part of the lobe deposits, we also apply geochemical tools (XRF-derived elemental proxies paired with XRD mineralogy) to test whether laminated siltstones share an extrabasinal, hydrodynamic signature with lobe-axis deposits, rather than reflecting hemipelagic or seasonal sediment input.

Results show that lobe axis and off-axis deposits form the core of each lobe and are dominated by structureless and structured sandstones; sandstone content decreases systematically toward the fringe and is lowest in distal-fringe settings, an undercharacterized subenvironment despite its importance for unconventional opportunities. Individual beds and packages range from 1 to 190.25 inches thick. Upsection, the apparent slope of winnowed turbidite deposits decreases from the Wolfcamp X interval through the lower, middle, and upper lobe complexes, suggesting reduced hydraulic energy gradients and/or increasing confinement.

Clean sandstone comprises ~90% of the thickest Wolfcamp X and lower-lobe sections, declining to ~80% in the middle lobe and ~75% in the upper lobe. These structureless sandstones exhibit the best reservoir quality (average porosity ~12%). Structured sandstones with rafted mud clasts and dewatering pipes are more heterogeneous and lower porosity. The core also contains 42 laminated to anastomosing siltstone–sandstone beds (1–47 inches thick); mud content within laminated to anastomosing siltstone increases upward, and several units are fully bioturbated.