PETROGRAPHY AND GEOCHEMISTRY OF HEAVY MINERALS IN USA SHALE PLAYS

Heavy (≥ 2.9 g/cc) accessory (usually ≤1% wt.%) minerals have been studied intensively in sandstone for more than 100 years but much less so in shale. This is because their finer, silt and clay (<63 micrometers) sized grains have been more difficult to image and analyze for chemical and isotopic composition and age. Both detrital and authigenic heavy minerals have been described in shale. The detrital grains are useful for determining provenance and stratigraphic correlation of clastic units in that heavy minerals tend to be more distinctive of source regions than framework minerals. Authigenic grains are useful for tracing the sources and sequence of basinal fluid migration during early and late diagenesis.

Heavy minerals in polished thin sections of samples from four well-known shale plays in the United States (Marcellus, Utica – Point Pleasant, Bakken, Barnett) were identified and imaged using automated scanning electron microscopy, and analyzed for chemical composition by electron microprobe. The Marcellus sample is an argillaceous siliceous mudstone; the Utica – Point Pleasant sample is a calcareous mudstone; the Barnett sample is a siliceous calcareous mudstone; and the Bakken sample is a siliceous organic mudstone.

Heavy minerals in all samples are zircon, tourmaline, rutile (titania minerals), and apatite (calcium-phosphate minerals). Detrital zircon is present dominantly as fragments, suggesting mechanical fracturing during long-lived or intensive processing during erosion and transport. Some secondary zircon containing inclusions and pores forms as overgrowths on detrital zircon and may represent fluid-altered or -precipitated grains. Tourmaline forms subangular to subrounded, unzoned detrital grain fragments (some with quartz inclusions) with chemical compositions indicating a mixed magmatic and metamorphic provenance. Titania minerals form detrital and authigenic grains, the latter with significant Si, Al, Ca, Fe, and Cr. Ca-phosphate is present mostly as authigenic grains, some with compositions of carbonate or sulfate fluorapatite.

Chemical and isotopic compositions of these heavy minerals hold significant, largely untapped potential for understanding the distribution and diagenesis of subsurface shale units in exploration and production drilling programs.