FRACTURE MINERALIZATION AND PALEOFLUID HISTORY OF THE MARCELLUS SHALE: WEST VIRGINIA APPALACHIAN PLATEAU

In order to investigate the paleofluid history of the Marcellus shale we examined mineral-filled fractures from four cores in the eastern Appalachian Plateau of West Virginia. The cores are: WV-6 from Monongalia County, sampling the 2185 – 2289 m depth interval; WV-10 from Upshur County, sampling the 2173 – 2183 m interval; Armstrong #1 from Taylor County sampling the 2230 – 2372 m interval; and Goff # 1 from Harrison County sampling the 2147 – 2205 m interval. The interval sampled by all cores is the organic-rich black shale of the Middle Devonian Marcellus Fm.

The veins are subvertical and bedding-parallel, along with en-echelon vein groups. The mineral veins examined are all calcite veins with blocky to elongate blocky calcite as well as fibrous calcite. There is also common bitumen between the crystals. Several veins in each core also contain uncommon pyrite. In WV-6 several veins contain bladed or fibrous barite, while in WV-10 there is uncommon early blocky dolomite.

All cores contain single-phase inclusions that are interpreted as CH₄±HHC (HHC=higher hydrocarbons) along with uncommon CH₄ inclusions (homogenization temperatures <-82 °C. Dark, single-phase degraded hydrocarbon inclusions are ubiquitous. For the Armstrong #1 core, homogenization values range from –70 to –50 °C and –110 to –86 °C. Rare pale-blue fluorescent inclusions are also present in these veins indicating liquid hydrocarbons. For the Goff #1 core, homogenization values range from –70 to –50 ° C. For the WV-6 core, homogenization values range from –74.6 to –1.6 °C and –82.3 to –100.5 °C. For the WV-10 core homogenization values range from –45.2 to –43.1 ° C. Only the WV-6 core contains aqueous fluid inclusions. These occur in late equigranular barite and homogenize between 69.0 to 80.4 °C with ice melting at -28.5 to -19.0 (21.7 to 28.7 wt. % NaCl equiv. salinity).

In summary, all cores exhibit similar thermal maturity. Uncommon CH₄ inclusions indicate maximum burial depths of 3.5 to 4.2 km, and possibly up to 5.5 to 6.6 km. Bed-parallel veins are present in all cores and suggest high pore-fluid pressure, at or near lithostatic.