PALEOREDOX ANALYSIS OF TWO CORES THROUGH THE MARCELLUS SHALE, HARRISON AND WETZEL COUNTIES, WEST VIRGINIA

The Appalachian basin contains a number of organic-rich shale units that have been recognized as important for natural gas production. In particular, the Middle Devonian Marcellus Shale is one of the largest unconventional natural gas plays, accounting for 40% of shale gas in the U.S. with production estimates reaching up to 15 billion cubic feet of natural gas per day. The Marcellus shale has been heavily researched, but questions remain about the depositional environment and, in particular, the extent to which the basin was restricted.

A high-resolution geochemical study was completed on a total of 145 feet of core from two separate wells in Harrison and Wetzel counties, West Virginia. We use iron speciation, total organic carbon (TOC), δ13Corg, redox-sensitive trace elements, and major and minor elemental analyses to explore bottom-water redox conditions in the Marcellus shale. Preliminary data suggests that both Marcellus shale cores were deposited in predominantly anoxic and often euxinic conditions, consistent with previous research based on pyrite framboid distribution and trace element enrichment. Comparative results indicate that the depositional environments between the cores are markedly different. Iron speciation data from the Harrison County Marcellus interval shows strong signals of anoxia and euxinia, as well as a depletion of molybdenum and uranium abundances through time, perhaps indicating increasingly restricted conditions. Similar data from the Wetzel County Marcellus interval suggests little restriction and only intermittent euxinia, but maintains an anoxic signal throughout.

It has been suggested that in addition to thermal maturity, spatial factors, including basin restriction, depositional environment and sedimentology play a key part in unconventional source rock and reservoir quality. The wells used in this study were spaced only 36 miles apart and such variable results were unexpected. Ultimately, we seek to further investigate how redox conditions in particular correlate with typical indicators used in natural gas exploration and production to increase the efficiency and accuracy with which basins and formations are evaluated.