An integrated paleomagnetic, rock magnetic, and geochemical study of the Marcellus shale in the Valley and Ridge province in Pennsylvania and West Virginia

Abstract

An integrated paleomagnetic, rock magnetic, and diagenetic study of the Devonian Marcellus Subgroup from threefolds in the Valley and Ridge province in Pennsylvania and West Virginia indicates that the unit contains an intermediate temperature chemical remanent magnetization (CRM) with south-southeast declinations and shallow negative inclinations residing in pyrrhotite and a CRM with more southerly declinations and shallow positive inclinations residing in magnetite. Rock magnetic results confirm the presence of pyrrhotite and magnetite. Tilt tests indicate that pyrrhotite and magnetite CRMs are syntilting to posttilting, and paleopoles for both CRMs are similar and plot on the Permian part of the apparent polar wander path for North America. The Marcellus Subgroup has a complex paragenetic sequence which includes bitumen deposition and vein formation. The magnetite and pyrrhotite CRMs formed in the Permian, probably due to burial diagenetic processes and not orogenic fluids. The base of the unit contains the highest total organic content (TOC) values and highest magnetic intensities, both of which decrease up section. This connection between TOC and magnetic intensity suggests that the magnetite formed as a result of the maturation of the organic matter although more work is needed to test this hypothesis. The pyrrhotite may have formed as a result of thermochemical sulfate reduction involving organic matter during the production of methane in this gas reservoir. The similar Permian CRM ages may be explained by thrusting which resulted in rapid burial of the Marcellus Subgroup as it passed through the oil window and into the gas window quickly producing magnetite and pyrrhotite, respectively.

Thermal Alteration of Pyrite to Pyrrhotite During Earthquakes: New Evidence of Seismic Slip in the Rock Record

“…It is known that pyrrhotite may form by prolonged heating pathways, such as reaction of magnetite and pyrite at low‐temperature (<200°C) as a result of burial diagenesis and anchimetamorphism (Gillett, ). It may also form by thermochemical sulfate reduction (Manning & Elmore, ). The magnetic susceptibility of magnetite is much higher (~20 times) than that of pyrrhotite (Peters & Dekkers, ).…”

Magnetic Anisotropy in Silurian Gas‐Bearing Shale Rocks From the Pomerania Region (Northern Poland)

“…This interpretation is consistent also with literature describing the early diagenetic formation of similar calcareous concretions. A high quantity of calcium carbonate dissolved in the water and suboxic conditions allowed calcareous concretions to precipitate in the newly deposited sediment (see Lash, ; Lash & Blood, , ; Manning & Elmore, ; Oertel & Curtis, ). In such calcareous concretions, the early postdepositional environmental chemistry of the strata and very early compactional fabrics are commonly preserved (Day‐Stirrat, Loucks, et al, ; Oertel & Curtis, ).…”

A Diagenetic Study of the Wolfcamp Shale, Midland Basin, West Texas

“…At the University of Oklahoma, the paragenesis of several North American shales from different depositional basins and tectonic settings were compared. Paragenetic sequences were constructed for the Marcellus Shale (Devonian), Woodford Shale (Devonian/Mississippian), Barnett Shale (Mississippian), and Haynesville Shale (Jurassic) (Dennie et al, 2012;Benton, 2013;Roberts, 2014;Steullet, 2014;Manning, 2015) in order to compare authigenic mineralogy and also timing of diagenetic events (Figure 16). Authigenic minerals, such as, calcite, dolomite, quartz, barite, celestine, anhydrite, sphalerite, and albite are found in all five shale units.…”