Application of pXRF elemental analysis and organic petrography in correlation of Pennsylvanian strata: An example from the Indiana part of the Illinois Basin, USA

Abstract

The principal purpose of this research is to investigate whether elemental chemistry obtained by a portable X-ray fluorescence (pXRF) analyzer can aid in correlating lithostratigraphic units within the Pennsylvanian coal-bearing sequence of the eastern part of the Illinois Basin in Indiana. The study is based on data collected from four boreholes: Indiana Geological and Water Survey wells SDH-377 in Daviess County, SDH-379 in Pike County, SDH-383 in Posey County, and SDH-43 in Warrick County. The organic-matter-rich marine black shales and limestones are our special targets because they may represent valuable marker beds. Highly elevated U and Mo concentrations can be utilized to clearly identify the stratigraphic positions of the organic-matter-rich marine black shales in the succession from the oldest to the youngest: Sh1, Sh2, Sh3, and Sh4. Increased concentrations of both U and Mo provide further evidence for the deposition of these shales in anoxic environments. Variations in Ca and Mg concentrations aid in identifying limestone and dolomitic limestone horizons through the interval. Although the positions of marine shales and limestone strata were clearly defined based on pXRF data, identifying specific shales or limestones was problematic because of the wide elemental variations that exist within individual horizons. To help overcome this difficulty, reflected light microscopy was used as complementary technique on shale samples. These shales represent early oil window maturity(vitrinite reflectance 0.56 to 0.58%) and their organic matter is dominated by low-reflectance amorphous organic matter (AOM) and high-reflectance micrinized AOM. Well-preserved alginite, vitrinite, inertinite, and solid bitumen are rare. Our results show that proportions of low-reflectance AOM and micrinized AOM and framboidal pyrite content are promising characteristics that may help to identify individual shales. Specifically, Sh1 and Sh4 are dominated by low-reflectance AOM, whereas the other two shales have micrinized AOM as the dominant component. Sh2 and Sh4 have consistently high framboidal pyrite content.