Chemostratigraphy of Mudrocks: Bone Spring Formation, Delaware Basin, West Texas

Nance, Seay; Rowe, Harry

Mudrock successions defy easy visual characterization. Discriminating between siliceous and calcareous mudrocks is problematic because both may be dark colored, presumably from presence of organic carbon and dark minerals (e.g., pyrite). Documenting stratigraphic mineralogy profiles in mudrock requires expensive and time-intensive x-ray diffraction (XRD) analyses performed at close spacing. Application of x-ray fluorescence (XRF) and total organic carbon (TOC) analysis enables characterization of elemental abundances over short sampling distances and, with supporting mineralogical data acquired by relatively fewer strategic XRD analyses, interpretation and semi-quantification of key mineral constituents.

Geochemical analysis was performed at an average depth interval of 0.8 feet (374 analyses) on a 297-ft mudrock-dominated core from the lower Bone Spring Formation (Leonardian, Delaware Basin, Texas). A portable XRF scanner was used to generate the analytical suite of 13 elements (Mg, Al, Si, P, S. K, Ca, Ba, Ti, V, Cr, Mn, and Fe). TOC results were generated from 61 intervals. XRD and minor-element data (Co, Cu, La, Mo, Nb, Ni, Pb, Rb, Sr, Th, U, V, Y, Zn, and Zr), δ13CTOC, and δ15N were also generated from 17 pulverized samples. XRD data indicate that the succession includes carbonate- and siliciclastic-dominated facies. TOC ranges up to 5.6% and is directly related to clay abundance. δ13CTOC variations (-28.9 to -25.5‰), with coincident mineralogical associations, may reflect sea-level changes where lower δ13CTOC values mark lowstands. Elevated δ15N values (12.2 to 17.5‰) may record nutrient recycling, especially during lowstands.

The objective of this ongoing study is to characterize facies in terms of relative shifts in inorganic and organic constituents for facilitating correlation to geophysical well logs and developing a depositional/ oceanographic process model. Interpretation of sea-level variation is possible owing to anticipated mineralogical responses. TOC preservation appears to have been optimal under reducing conditions during sea-level lowstands. Mineralogy (interpreted from elemental data) combined with organic stable-isotope data enables modeling relationships between sea-level phase, depositional facies development, and oceanographic chemistry.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013