Abstract

The Cutoff Formation is present over the entire northern Delaware Basin where historically it has been included in the ‘upper Bone Spring’ or ‘1^st Bone Spring carbonate”. Even today portions are called ‘Bone Spring shale’ or ‘Avalon shale”. The Cutoff in the basin and basin slope is equivalent to the lower (L 7, L 8, G 1 – G 4 HFS) San Andres Formation on the shelf. Shelfal Glorieta silici-clastics underlying the San Andres are considered equivalent to the 1^st Bone Spring sand. The true top of the Bone Spring Formation lies between the Cutoff Formation and the top of the 1^st Bone Spring sand but picking this top on wireline logs remains a problem.

The outcrop defined Williams Ranch and El Centro Members of the Cutoff Formation commonly can be recognized on wireline logs except near the basin margins where the El Centro facies onlap and interfinger with thick carbonate units. The basal Shumard Member cannot be confidently identified in the subsurface and if present its contact with the underlying Bone Spring Formation cannot be picked regionally with confidence using wireline log signature. This is not surprising because the Cutoff – Bone Spring contact even in outcrop is an unconformity separating similar lithology. Many if not most of the carbonate units in the Cutoff and upper Bone Spring formations are believed to consist of mass-transport deposits (MTDs) that interfinger with more normal basinal turbidites and suspension deposits including fallout (eolian dust?) and hemipelagic organic matter.

The Williams Ranch Member is predominately lime mudstone and wackestone and thins irregularly basinward. In outcrop, it is made up mostly of stacked mass-transport deposits (MTDs) that are mappable (Amerman, 2009). The underlying El Centro Member is characterized by laminated organic-rich silt-stone/shale and is currently an exploration target for oil and gas using horizontal drilling and fracturing. It is recognized on wireline logs by its high radioactivity and high resistivity. Near its top is the widespread but thin Avalon sand recognized on logs by moderate radioactivity and low resistivity. It is an important lowstand deposit useful as a time marker being only slightly younger than the Guadalupian – Cisuralian (Leonardian Stage) Series boundary.

To better understand the Cutoff – upper Bone Spring relationships and promote discussion our poster consists of selected regional cross sections generally using the Avalon sand marker as the datum. Several of the northern Delaware Basin cross sections attempt correlations onto the shelf margin. Several are tied to classic cross sections and studies published by the WTGS, RGS and others. Although each of our new wireline log displays include all of the Cutoff and Bone Spring sequences, we concentrate on the interval between the top of the Cutoff Formation and the top of the 1^st Bone Spring sand.