Barite in the Middle Devonian Marcellus Shale, Appalachian Basin: Occurrence, Petrography, Geochemistry and Its Implications
The Marcellus Shale is one of the most important gas shale plays in the world as horizontal drilling and hydraulic fracturing have been extensively utilized over the last decade. Sedimentary barite is ubiquitous in the Marcellus Shale and unusually high barium concentrations tend to occur in Marcellus flow back water, which can cause serious scale formation adversely impacting well performance and environmental concerns. This study investigates the characteristics of barite in the Marcellus and provides insights on its regional distribution and origin. Barite is regionally widespread in the Marcellus, focused immediately beneath or within the Cherry Valley Member. Barite-bearing zones range in thickness from centimeters to >3 meters and tend to be thicker in the east thinning westward. Barite occurs commonly as centimeter-scale, oval-shaped nodules, especially in central and western Pennsylvania. Eastward, barite is found as poorly-defined, incipient nodules and disseminated crystal aggregates. Barite in the Marcellus commonly occurs associated with pyrite nodules and carbonate concretions. Compaction-related fabrics indicate that barite formed as a product of early diagenesis. Barite crystals in nodules are typically finely crystalline (5 – 60 μm), becoming more coarsely crystalline (60 μm to >1 mm) eastward towards the Allegheny front. δ34S values determined from barite nodules range from 34.6 to 66.6 ‰. Barite deposits in shale are often considered indicative of low sedimentation rates and may denote a hiatus in sedimentation, which can cause dilute Ba2+ to be significantly enriched by microbial activities leading to barite precipitation. Our regional analysis of over 100 cored wells in the Marcellus documents regional variability in both barite crystal size and sulfur isotopic composition. Coarsely crystalline barite displays low δ34S values when compared to finely crystalline barite which have higher δ34S values. These trends in fabric and sulfur isotope composition are attributed to variable rates of microbial sulfate reduction related to changing sedimentation rates. The high δ34S values associated with the finely crystalline barite result from a lower sedimentation rate and commensurate enhanced microbial sulfate reduction. The Tioga Ash is considered as an important source of Ba2+ in the basin and those areas with thicker ash accumulation tend to show more barite nodules and higher barium concentration in the fracture stimulation flowback water.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015