STRATIGRAPHIC VARIATIONS IN THE DEPOSITIONAL ENVIRONMENT AND THE THERMAL MATURITY OF THE PENNSYLVANIAN TURNER MINE SHALE, ILLINOIS BASIN, INDIANA, USA

The Turner Mine Shale directly overlies the Springfield Coal Member in the Illinois Basin, and, together with the Springfield Coal, represent a potential gas resource, as yet untapped. However, there is a sparsity of research on the paleoenvironmental conditions associated with deposition of Pennsylvanian shale units in the Illinois Basin and few studies that examine the links between organic biomarker indices and inorganic sulfur analyses. Thus, the purpose of this study is to employ integrated geochemical analyses to: (i) evaluate the evolutionary succession of depositional environments represented by the stratigraphic sequence comprising the Springfield Coal, the overlying Turner Mine Shale, and the grey shale above the Turner Mine Shale, (ii) interpret the geochemical characteristics of the rock extracts, and (iii) investigate and assess the maturity characteristics of the sequence reflected by distinct biomarkers indices.

Results from organic petrography and biomarker analyses suggest that the Turner Mine Shale and the overlying grey shale were deposited in a deep marine setting with their organic matter (OM) dominantly sourced from marine algae. By contrast, the overlying grey shale interval contains higher concentrations of 20R C₂₉ steranes that suggest an increased input of terrestrial OM. Moreover, results from OM analyses of the Springfield Coal suggest, as expected, that its primary source is terrestrial vegetation. Yet the extremely low values for the sulfur isotope composition of extracted sulfide (δ³⁴S_CRS) in the Turner Mine Shale are indicative of greater variability in the depositional paleoenvironment than other intervals examined in this study.

Surprisingly, samples from the stratigraphic sequence taken at similar depths and representing comparable thermal history exhibit marked variability in their maturity indices. This disparity contravenes the established paradigm that the thermal maturity of sedimentary rocks progresses with depth associated with increasing temperature and time of burial and prompts evaluation of the various scenarios related to OM sources or matrix effects that could explain these observations.