Fluid Inclusion Study of Prominent Mineralized Fractures in the New Albany Shale, Kentucky, USA – Dissecting a Mississippian Seal Breach

Abstract

Mineralized natural fractures (veins) cross-cut the New Albany Shale of the Illinois and Appalachian basins. 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 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. Dominant fractures are oriented N70E with conjugates oriented EW, consistent with the NE trending Wabash Valley fault system and the EW trending 38th parallel lineament. These fault systems 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 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 fills the interstices between the quartz crystals, and that bitumen was deposited last. The latter 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 from 3 to 60 μm, show homogenization temperatures from 84.6° to 255.5° C, and salinities from 4.03 wt. % NaCl to 20.97 wt.% NaCl. The low end of the temperature range coincides with estimated burial temperatures of the New Albany Shale, whereas the higher end overlaps with fluid inclusion temperatures of Mississippi Valley Type (MVT) mineralization. The higher salinities likewise overlap with that seen in MVT mineralization. The observed range in salinities indicates the likelihood of fluid mixing or multiple fluid sources. The current data on paragenesis, fluid inclusions, and hydrocarbon distribution indicate a complex history of fluid migration following the New Albany Shale seal breach in the Mississippian.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014