Abstract

Bone Spring (Leonardian) mudrock successions in Delaware Basin vary between silica- and carbonate-rich facies marking depositional responses to sea-level changes. Increased Mo, δ15N, and total organic carbon (TOC) record reduced oxygenation during sea-level lowstands. In a 91.4-m (300 ft) core from Sun No. 1 Houssels well and a 39.6-m (130 ft) core from Shell No. 1 Marsden well (Reeves Co., Texas), lower Bone Spring cycles comprised siliceous-calcareous mudrock couplets. The Marsden core was correlative to the upper part of the Houssels core. Cyclicity was interpreted from mineralogical calculations at numerous core horizons using energy-dispersive X-ray fluorescence (ED-XRF) measurements of elemental content. Elemental abundances enabled predicting mineralogy based on the stoichiometric relationships between elements and dominant minerals (calcite and quartz), and from average values of Si and K in published illite analyses. The average sample spacing for ED-XRF measurements was 0.24 m (0.8 ft) for the Houssels core and 0.15 m (0.5 ft) for the Marsden core. The measurements and interpretations provided: (1) information regarding comparisons in stratigraphic and geographic development of facies over the 10-km (6.2-mi) distance between wells and (2) guidance for petrographic, scanning electron micrograph, and organic matter analyses that address processes that produced different facies. Greater carbonate abundance in the Houssels core reflected the proximity to carbonate shelves on the Central Basin Platform (CBP); the greater siliciclastic abundance in the Marsden core was thought to reflect the greater distance from the CBP. Redox element (S and Mo) and TOC concentrations were greater, and δ15N values were higher in some siliciclastic-dominated intervals, suggesting that anoxia characterized sea-level lowstands when normal marine water flowing from the Panthalassa Ocean was inhibited through narrow interbasin channels or over sills. Carbon isotope (δ13CTOC) variations suggested changes in organic matter characteristics, whereby greater C13-depleted organic matter was deposited during sea-level lowstands, marking: (1) reduction of shallow-marine sources or (2) increased terrigenous organic carbon contributions.

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