Investigation of prominent mineralized fractures in the middle to upper devonian mudstones of the Illinois basin: Implications for multi-episode fluid migration and seal breach

Mineralized natural fractures (i.e., veins) are abundantly observed to cross-cut the New Albany Shale of the Illinois Basin. Vein abundance varies laterally, showing an increase in proximity to the Cincinnati Arch, and reflects a seal breach by overpressured fluids that originated from underlying strata. In size, the veins range from several centimeters to more than 9 meters in length and 2-10 cm in width. They are mineralized with quartz, calcite, base metal sulfides, and are filled with bitumen. The veins show variable morphologies, with some appearing non-deformed whereas others are folded and cause differential compaction of the enclosing shale. Dominant fractures are oriented N30°E with conjugates oriented EW, consistent with the NE trending Wabash Valley fault system and the EW trending 38th parallel lineament. Evidence suggests these fractures may have been the conduit for mineralizing fluids from underlying strata. Bitumen inclusions have been shown to come from a deeper source, and in this study detailed petrography, trace elements, fluid inclusion and stable isotope geochemistry were used to investigate the source of the mineralizing fluids. Fluid inclusion work yielded homogenization temperatures from 99.5° to 257.9° C, and salinities from 2.74 to 20.97 wt.% NaCl. Both of these data ranges overlap with fluid inclusions observed in Mississippi Valley Type (MVT) deposits. The low end of the temperature and salinity range, however, coincides with estimated burial temperatures of the New Albany Shale. The observed range in salinities indicates fluid mixing and/or multiple fluid sources. Sulfur and oxygen isotope ratios are consistent with fluid mixing. The current data indicate a complex history of fluid migration and mixing during New Albany Shale deposition and later during hydrocarbon generation. The migrated hydrocarbons and the apparent connection to MVT deposits suggests that a sufficiently mature hydrocarbon-rich source rock exists downdip or in underlying strata (Schutter, 2015).