Mineralized natural fractures (veins) cross-cut the New Albany Shale of the Illinois basin. They occur throughout the shale succession, are best developed in proximity to the Cincinnati Arch, and probably represent a seal breach by overpressured fluids in underlying strata. In size the veins range from a few inches to more than 30 feet in length, and they are mineralized with quartz, calcite and dolomite, and locally contain pockets of bitumen. They show variable morphologies, with some appearing non-deformed whereas others are folded and sheared and cause compactional deformation of the enclosing shale. Bitumen inclusions have been shown to come from a deeper source, and in this study we use fluid inclusion geochemistry and detailed petrography to investigate the source of the hydrocarbon-bearing fluids. Petrographic analysis shows that quartz is the earliest cement, that calcite and dolomite commonly fill spaces between quartz crystals, and that bitumen was deposited last. This paragenesis occurs in all veins throughout Indiana and Kentucky. Fluid inclusion analysis with a Linkam heating-cooling stage was used to determine temperature and salinity of mineralizing fluids. Detailed microscopy and SEM-CL was used to delineate single and two-phase primary, secondary and pseudo-secondary fluid inclusion assemblages. Fluid inclusions vary in size and show homogenization temperatures from 84.6° to 255.5° C, and salinities from 4.03 wt. % NaCl to 20.97 wt.% NaCl. These ranges in values overlap those of the Mississippi Valley Type fluids as well as estimated burial temperatures of the New Albany Shale, indicating fluid mixing or multiple fluid sources.