Detection of natural weathering of Upper McAlester coal and Woodford Shale, Oklahoma, U.S.A.

Natural weathering refers to the natural oxidation of organic and inorganic matter in rocks at the surface and in the shallow subsurface. The objective of this study is to apply routine geochemical analyses (including organic petrography) to coal and shale samples, extending from the weathered to unaltered zones, to illustrate the effects of weathering on organic matter in coal and shale. Some significant observations from the Upper McAlester coal and the Woodford Shale are: (1) a sulfate index (SI), using sulfur-form analysis data that reveals the transformation of pyrite to sulfate minerals, is a very sensitive weathering indicator; it increased in weathered samples; (2) Rock-Eval pyrolysis data, such as hydrogen index (HI) and oxygen index (OI), are very sensitive to weathering; OI/HI increased in weathered samples; (3) vitrinite reflectance decreased in weathered samples; and (4) petrographic signs of weathering include microfractures, pitted surface, dark reaction rims, and high relief. Sulfate index and petrographic signs of weathering should be used on surface and shallow subsurface rock samples as an indication of the extent of weathering; the validity of other analyses can then be evaluated accordingly.