HISTORY OF FRACTURE OPENING, TEMPERATURE, AND FLUID PRESSURE IN THE MARCELLUS SHALE, WEST-CENTRAL PENNSYLVANIA VALLEY AND RIDGE

Fluid inclusion microthermometry of vein minerals provide information on the fluid history: trapping conditions (PT), composition, and sequence. Understanding the fluid history is important in establishing the deformational history and hydrocarbon generation and migration. Veins were sampled from the Marcellus shale near the northern plunge-out of the Broadtop synclinorium.

The earliest-formed vein minerals are within northeast-striking veins and consist of blocky calcite and dolomite, and include light-brown tinted hydrocarbon inclusions. Oxygen and carbon stable isotope data of the carbonate minerals are similar to those in the undeformed Marcellus of the Appalachian Plateau province, suggesting precipitation from locally-derived fluids. Later northwest-striking veins contain calcite with CH₄ inclusions that indicate trapping at 95°-130°C and 102-118 MPa (3.9 - 4.1 km). Stable isotope values of these later veins have a wide range, with oxygen values ranging from 17.0 - 23.7 per mil and carbon ranging from -9.0 - 0.1 per mil indicating an opening of the fluid system to fluids from underlying carbonate rocks.

Large colorless quartz crystals (<0.5 - >3 cm) are found along later northwest-striking veins. The crystals occur singly and as clusters. Many of the crystals exhibit multiple growth zones as defined by CH₄±CO₂ and aqueous fluid inclusions. Detailed investigation of inclusions within one large crystal exhibit a complex pressure-temperature history. The core of the crystal contains aqueous brine inclusions that homogenize at ~150°C and have a 15.2 wt. % equiv. salinity, while the outer rim of the crystal contains inclusions that homogenize at between 184° and 204°C and have 11.2-13.1 wt. % equiv. salinity. CH₄±CO₂ from the core to the rim of the crystal show fluctuating fluid pressures with each growth zone. For example, from core to rim, minimum CH₄±CO₂ inclusion homogenization values have the sequence -105°C, -97°C, -102°C, -107°C, -93°C, -104°C, -98°C, -86°C.