MUDROCK CHEMOSTRATIGRAPHY: EXAMPLES FROM THE BONE SPRING FORMATION, PERMIAN BASIN, TEXAS

Mudrocks are difficult to characterize. Siliceous and calcareous mudrocks both may be dark colored, presumably due to organic carbon and dark minerals. Interpreting stratigraphic mineralogy in mudrock normally requires x-ray diffraction (XRD) analyses performed at close spacing. Application of x-ray fluorescence (XRF) and total organic carbon (TOC) analysis allows determination of elemental content over short sampling distances and, with supporting mineralogical data acquired by a few strategic XRD analyses, interpretation and semi-quantification of major mineral constituents.

Geochemical analysis was performed on two cores 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 in one core. XRD and minor-element data (Co, Cu, La, Mo, Nb, Ni, Pb, Rb, Sr, Th, U, V, Y, Zn, and Zr), δ¹³C_TOC, and δ¹⁵N were also generated from 17 pulverized samples in the same core. 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. δ¹³C_TOC variations (-28.9 to -25.5‰), with coincident mineralogical associations, may reflect sea-level changes where lower δ¹³C_TOC 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 elemental responses (e.g., Mo reflects reducing conditions attendant with lowstand basin circulation restriction), whereas major mineral changes reflect changes in depositional facies. 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 of sea-level phase, depositional facies development, and oceanographic chemistry.