EARLY DIAGENETIC PARTIAL OXIDATION OF ORGANIC-MATTER AND SULFIDES IN THE MIDDLE PENNSYLVANIAN (DESMOINESIAN) EXCELLO SHALE MEMBER OF THE FORT-SCOTT LIMESTONE AND EQUIVALENTS, NORTHERN MIDCONTINENT REGION, USA

A process of early diagenetic partial oxidation of organic matter and sulfides has altered the chemical composition of the Middle Pennsylvan ian Excello Shale Member of the Fort Scott Limestone and equivalents i n the northern Midcontinent region. This process was identified by com parison of organic carbon contents, Rock-Eval hydrogen indices, organi c carbon delta(13)C and element compositions of core and surface mine samples of the Excello Shale Member with analyses of three other under lying and overlying organic-matter-rich marine shales (offshore shale lithofacies) from southern Iowa, northern Missouri, eastern Kansas and northeastern Oklahoma. The end product of the partial oxidation proce ss is shale with relatively low contents of hydrogen-poor, C-13-enrich ed organic matter, lower contents of sulfur and sulfide-forming elemen ts, and relatively unchanged contents of phosphorus and many trace ele ments (e.g. Cr, Ni, and V). However, because of lower organic carbon c ontents, element/organic carbon ratios are greatly increased. The part ial oxidation process apparently took place during subaerial exposure of the overlying marine carbonate member (Blackjack Creek Member of th e Fort Scott Limestone) following a marine regression when meteoric wa ters percolated down to the level of the Excello muds allowing oxidati on of organic matter and sulfides. This hypothesis is supported by ear lier workers, who have identified meteoric carbonate cements within, a nd soil horizons at the top of the Blackjack Creek Member. The period of oxidation is constrained in that organic matter and sulfides in the Little Osage Shale Member of the Fort Scott Limestone and equivalents (immediately overlying the Blackjack Creek Member) appear unaltered. Similar alteration of other shales in the Middle and Upper Pennsylvani an sections may be local to regional in extent and would depend on the extent and duration of the marine regression and be influenced by loc al variations in permeability and topography. The partial oxidation pr ocess has likely led to a redistribution of sulfur and sulfide-forming elements into other organic-rich lithologies in the section. The alte red/oxidized shales are nongenerative with respect to hydrocarbon gene ration.