AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Chemostratigraphic Studies of the Marcellus Shale by Handheld X-ray Fluorescence (HHXRF) Analysis
(1) Thermo Fisher Scientific, Houston, TX.
(2) Shell Exploration & Production Co., Houston, TX.
Handheld X-ray fluorescence is a powerful tool for correlating rock geochemistry to stratigraphy. Chemostratigraphic techniques employ major, minor and trace element abundances and ratios to correlate elemental chemistry to mineralogy and rock properties. Trace metals (e.g. V, Cr, Ni, and Mo) have been shown to be proxies for total organic carbon (TOC) in mudrocks. A HHXRF equipped with a large area (25 mm2) silicon drift detector (SDD) was compared with independent laboratory results obtained by ICP-MS on 160 sedimentary rock samples of mixed lithology. The results show typical correlations (R2) > 0.90 and repeatability < 5 % relative standard deviation (RSD) for most major, minor and trace elements from Mg to U.
The Marcellus Formation of the Appalachian Basin is a sequence of black shale with minor limestone interbeds of middle Devonian age. Cores were analyzed in situ and analysis times were 30 seconds each for the low, main and high energy filters and 60 seconds on the light energy filter for a total counting time of 150 seconds. The top and base of the Marcellus is indicated by sharp contacts in the amount of Stot present. The lower members within the Marcellus can be identified by increased Si/Al, Ti/Al and Ca/Al ratios. Carbonate concretions display high Ca/Mg and increased Sr, but vary in clay content as inferred by K/Al. The formation is enriched relative to the surrounding stratigraphy in Cu, Zn, S, Mo, Ni, V and Cr. The lower members of the Marcellus have distinctly higher amounts of Mo, Ni, V and Cr indicating deposition in a more reduced setting relative to the upper Marcellus and higher TOC content.
Relative shifts in elemental ratios can be used to quickly note the changing mineralogy in a formation at the logging trailer, in the core shed or at the outcrop. This case study demonstrates that handheld XRF can be used to rapidly generate dense geochemical data sets that can accurately define stratigraphic intervals within targeted formations.