Investigation of Prominent Mineralized Fractures in Middle to Upper Devonian Organic-Rich Mudstones of the Eastern U.S.: Implications for Multi-Sourced Fluid Migration During a Mississippian Seal Breach

Abstract

Mineralized natural fractures (i.e., veins) are abundantly observed to cross-cut and induce soft-sediment deformation in Middle to Upper Devonian mudstones (i.e., New Albany Shale and Ohio Shale) of the Illinois and Appalachian basins. Vein morphology and composition appear to vary laterally, are best developed in proximity to the Cincinnati Arch, and reflect a seal breach by overpressured fluids originating from underlying strata. In size, the veins range from several centimeters to more than 9 meters 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. Evidence suggests 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, Oxygen isotopes and detailed petrography to investigate the source of the hydrocarbon-bearing fluids. Petrographic analysis shows a West to East shift from calcite to quartz cement across the basin. Fluid inclusion analysis with a Linkam heating-cooling stage was used to determine temperature and salinity of mineralizing fluids. Detailed microscopy and a scanning electron microscope with Cathodoluminescence (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 89.5° C to 259.6° C, and salinities from 4.03 wt. % to 20.97 wt. % sodium chloride (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 fluid mixing and/or multiple fluid sources. The current data on paragenesis, fluid inclusions, Oxygen isotopes 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 #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015