Using Hierarchical Cluster Analysis to Improve Facies Definitions in Permian Mudrocks (Wolfcamp and Lower Leonard), Midland Basin, Texas

Abstract

Mudrocks are notoriously difficult to describe. XRF analysis with calibrated hand-held instruments gives quantitative elemental results that provide insight into mineralogical composition of these fine-grained, dark rocks, if XRF results are supplemented by mineralogical analysis (XRD). When properly interpreted, XRF data can improve facies definitions, but understanding the abundance of elemental data (20+ major and trace elements for every data point) is problematic. The object of this study was an 852-ft continuous core through a sequence of interbedded basinal hemipelagic and sediment gravity flow deposits. Facies delineation based on core description was refined using iterative hierarchical cluster analysis, a technique that treats the rock as a whole, rather than analyzing individual elements or element ratios (e.g. Ca, Al, Si/Al, Si/Ti). Elements were interpreted as proxies for productivity (Ni, Zn, V), reducing conditions (Mo, U), detrital deposition (Si, Al, Ti, Zr, Rb), carbonate deposition (Ca, Mg, Sr), phosphate enrichment (P, Y), and sulfur enrichment (S). The most significant cluster-defining elements were determined by applying analysis of variance and a partitioning index to elements in each cluster. This approach produced chemofacies (e.g. high-detrital siliceous mudrock) that cannot be ascertained as rapidly or as quantitatively by other methods, delineated a previously-unrecognized geochemical boundary between the Wolfcamp and lower Leonard (sulfur-enriched mudrocks below vs. high-redox/high productivity mudrocks above), and revealed sub-meter-scale cyclicity of chemofacies that is not otherwise apparent. Calibrated XRF data subjected to cluster analysis provide finely-detailed, core-based, geochemical ‘ground truth’ that is not available by any other means. This technique is a valuable supplement to traditional description of lithofacies based on depositional features seen in core.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016