Bruce W. Selleck , Colgate University
Colborne and Michaels continued work on the Utica Formation during the fall and spring of 2011-12 as their senior thesis projects. Their work clearly links the generation of Trenton-Black River HTD to the Utica Formation dewatering during rapid burial of the foreland basin sequence as a consequence of the Taconic Orogeny. In particular, their outcrop-scale studies reveal the importance of vertical fractures and ‘sand-dikes’ as conduits for vertical fluid transport between underlying Proterozoic basement and Utica Formation. The intervening Trenton-Black River units were similarly involved in fluid transport. Importantly, the fluid inclusion studies by Nicole MacDonald, supported by this grant during the summer of 2009, showed that basement-hosted veins are highly saline, where as Utica-hosted veins are relatively low salinity. The work of Colborne and Michaels demonstrates clear connection between these fluid reservoirs. Trenton-Black River HTD systems similarly show a range of fluid salinities, with mixed fluid compositions often proposed as the cause of dolomitization that formed HTD reservoirs. A number of veins sampled in the Mohawk Valley have aromatic hydrocarbons, and these were characterized using gas chromatography to demonstrate that petroleum maturation was ongoing during early vein development.
Colborne and Michaels presented “Fractures, Veins and Fluid Migration in the Utica Shale and Implications for Trenton-Black River Hydrothermal Dolomite Reservoirs, Northern Appalachian Basin, New York” at the March, 2012 Northeast Section GSA meetings, and “Fractures, Veins, Fluid Migration and Hydrocarbon Generation in the Utica Shale, Northern Appalachian Basin, New York” at the AAPG meeting in April, 2012.
Utica Formation – Geochemistry of Sulfide-rich Zones: During the summer of 2011, undergraduates Casey Portela, Michael Carbone and Robert Bickhart initiated study of sulfide-rich zones in the basal Utica Formation, using materials collected from the previous work of MacDonald, Colborne and Michaels. New materials were collected from outcrops in the Mohawk and Black River Valley areas of New York, and Carbone sub-sampled a Utica Formation core available on loan from the New York State Museum. The goal of work done during the summer of 2011 was to characterize the major and trace element chemistry of sulfide –rich zones to better understand what trace element signatures might be present in fluids that were expelled from the Utica during burial. In addition, the possibility of that some sulfide zones in the Utica are the result of sedimentary exhalative processes is also under study. Carbone and Bickhart are continuing this work as senior thesis projects in the current year. Bickhart has also worked with another Colgate undergraduate, Susannah Boote, to compare the geochemistry and mineralogy of sulfide-rich zones in the Utica Formation with similar zones in the Marcellus Formation. The similarity in the sedimentary facies of the sulfide-rich zones is striking, but trace element patterns are rather different. Boote, funded by Colgate University, has also assessed the geochemistry of water leachate from Marcellus Formation well cuttings (of interest given concerns about handling and disposal of sulfide-rich cuttings), and will present results with Bickhart at the NEGSA meetings in March, 2012.
Chemical Dating of Monazite and Xenotime in the Potsdam Formation: This work, funded in part by support from the New York State Energy Research and Development Authority, was undertaken in parallel with the Trenton-Black River studies to attempt to document the timing of major diagenetic fluid flow in the New York portion of the Appalachian Basin. Since transient fluid flow has been linked to the formation of HTD reservoirs, our approach has been to examine well-developed monazite overgrowths which can be dated using a specially-designed electron microprobe. This work has been carried out at the University of Massachusetts, Amherst.
It has been recognized for several decades that REE-phosphates (monazite and xenotime) can grow during diagenesis and low-grade metamorphism. An essential process for the growth of REE-bearing accessory phases at low-grade conditions is dissolution of detrital grains and precipitation of authigenic minerals, which involves pervasive fluid-rock interaction. The occurrence of low-grade REE-phosphate offers an unusual opportunity to date a fluid event.
Samples of the Potsdam Formation were derived from outcrop and core. The work also focuses on the textural and chemical relati nships of these REE-bearing accessory phases. Textural contrasts exist between rounded and fractured detrital monazite and zircon grains and new, subeuhedral REE-phosphate overgrowths. Chemically, authigenic monazite and xenotime overgrowths show enrichment in LREE and depletion in HREE, compared to detrital cores.
Samples in this study reveal four to five major overgrowth events between ca. 500 Ma (approximate depositional age of the Potsdam Formation) and ca. 200 Ma in both monazite and xenotime. These events broadly correlate with the major Paleozoic orogenic events as recorded in the Appalachian Mountains to the East (Taconic, Salinic, Acadian, Neo-Acadian and Alleghanian). We suspect that fluid transport, driven by orogenic loading, was responsible for dissolution of detrital monazite and zircon, and authigenic precipitation occurred with changes in fluid composition or temperature. The work done to date demonstrates the utility of the EMPA dating technique to resolve complex fluid-related growth history of REE-phosphate in relatively low-temperature diagenetic setttings. Unfortunately these results suggest that there have been multiple fluid flow episodes in the Appalachian foreland sequences, and do not permit establishment of a unique timing for HTD reservoir generation, but are permissive of Taconic or Salinic fluid events. This work was presented at the 2011 GSA annual meeting.