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Vitrinite Reflectance Suppression: Fact or Fiction in Low Maturity Paleozoic Shales

Abstract

The suppression of vitrinite reflectance (Ro) is invoked to explain situations where this petrographic thermal maturity parameter has lower values than expected, for instance, in Paleozoic shales of the Appalachian and Illinois basins, where Ro sometimes is lower than in overlying Carboniferous coals. Recent work by the U.S. Geological Survey (USGS) suggests that these apparent thermal inversions may be due to misidentification of solid bitumen as vitrinite in shale. Both macerals occur as structure-less gray ‘blebs’ of low relief when dispersed in shale, and can only be conclusively distinguished when bitumen shows unambiguous void-filling textures, a criterion used in our study. To test ideas related to detecting ‘vitrinite reflectance suppression,’ the USGS is using hydrous pyrolysis experiments. Immature source rock and low-rank coal samples are pyrolyzed for 72 hr at temperatures (T) of 300-360°C and reflectance measurements are performed on run products. Similar to earlier hydrous pyrolysis results on samples of lignite (Huang, 1996, Org. Geochem. 24, 233-241) we observe a control by original hydrogen index (HI) on the evolution rate of Ro and solid bitumen reflectance (SBRo). In particular, the response to thermal stress of SBRo in high HI shale samples (n=7; avg. HI=554) is significantly lower than that of vitrinite in low HI coal samples (n=8; avg. HI 178) exposed to the same T. For example, average Ro increased by a factor of 1.91 in coals pyrolyzed at 300°C compared to a factor of 1.45 for SBRo in shales. At 320°C, average Ro increased by a factor of 2.27 vs. 1.87 for average SBRo. The gap in the response to thermal stress begins to diminish at higher T (>340°C), similar to extant observations from naturally matured geological samples where co-occurring SBRo is lower than Ro at low maturities (<1.0 Ro) and approximately equivalent at higher maturities. These results suggest caution is needed in the interpretation of Ro values in oil window shale plays, especially in the Middle Paleozoic, where solid bitumen is prevalent and easily misidentified as vitrinite, potentially leading to underestimation of in situ thermal maturity. Our future work will include experiments on coal and shale samples from the oil window with the goal to develop a hydrous pyrolysis discrimination test to distinguish bitumen from vitrinite, thereby enabling more accurate determinations of maturity in this important thermal regime.