Abstract

A detailed sedimentologic and chemostratigraphic study of 910 ft (277 m) of continuous core from a well in the eastern Delaware Basin indicates that mudstone beds in the unconformity-bounded Wolfcamp C, B, and A sequences recorded distinct geochemical character, highlighting changes in depositional and palaeoceanographic conditions in this part of the basin during the Early Permian.

Wolfcamp C is dominated by siliceous-argillaceous mudstone have low Zr, low authigenic U, and high S/Fe. These mudstone beds are interbedded with carbonate beds interpreted as sediment gravity flows. Wolfcamp C mudstone beds have low to good total organic carbon (TOC) (median = 2.4 wt%) and high clay and micas content. A thick (>50 ft [>15 m]) polymictic carbonate clast conglomerate (debris flow) marks the base of the Wolfcamp B. This conglomerate is overlain by siliceous and mixed siliceous-carbonate mudstone having high Ti/Al; these mudstone strata are interbedded with thin carbonate beds interpreted as hybrid event beds. TOC in Wolfcamp B mudstone is fair to very good (up to 4.8 wt%.). It contains low to fair authigenic U and intermediate Ni and V, suggesting oxic to suboxic conditions during deposition.

The base of the Wolfcamp A is characterized by a ∼15 ft (5 m) thick carbonate debris flow complex that grades upwards into silty siliceous mudstone and mixed siliceous-calcareous mudstone interbedded with thin carbonate beds deposited by turbidite currents and hemipelagic deposition. Wolfcamp A mudstone beds show high authigenic U, Si_excess, Zr, and Ti/Al. It also has thin ash beds.

We interpret that the ash beds within Wolfcamp A enhanced nutrient availability, increasing productivity, as indicated by its high TOC in mudstone (up to 8.4 wt%) as well as high Ni and V concentration. Although Wolfcamp A mudstone beds show high Zr (detrital related element), the siliciclastic sediments were transported deep into the basin by turbidite currents, but this did not hinder organic matter productivity and predominantly suboxic to anoxic conditions favored organic preservation.