DEFORMATION AND FLUID HISTORY OF THE MARCELLUS SHALE IN THE PENNSYLVANIA VALLEY AND RIDGE: EVIDENCE FROM FLUID INCLUSION MICROTHERMOMETRY OF VEIN MINERALS

Fluid inclusion microthermometry of vein minerals performed on four Marcellus shale cores from widely different structural settings in the Pennsylvania Valley & Ridge province provide information on the fluid history: trapping conditions (PT), composition, and sequence. Each core exhibits a unique fluid and deformation history indicating a strong relationship to structural position.

Veins in the Roy Adams core from the western margin of the Anthracite belt exhibit complex vein mineral paragenetic sequences of calcite, dolomite, and quartz. Early calcite contains only bitumen and CH₄ inclusions trapped at 65°-90°C at 51-58MPa (1.9-2.0 km). Quartz contains coeval CH₄±CO₂ and aqueous inclusions (15-23 wt. % NaCl equiv.). Early quartz was formed at 120°-170°C and 182-218 MPa (7.0-8.4 km); later quartz was formed at 170°-215°C and 150-163 MPa (5.8-6.3 km). The two different trapping depths recorded by the quartz are interpreted to represent trapping before folding, and then late during or after folding, with syn-folding erosion removing overburden. The late high temperature fluids may indicate opening of the system by fracturing and the influx of deep basinal fluids.

Veins in the Bilger core from a syncline just east of the Broadtop Synclinorium exhibit two to three stages of vein minerals (calcite, dolomite, quartz). Early calcite contains only bitumen inclusions. Quartz contains coeval CH₄±CO₂ and aqueous inclusions (10-13 wt. % NaCl equiv.) trapped at 190°-210°C and 130-148 MPa (4.6-5.7 km). The high temperature brines may signify opening of the system by fracturing and the influx of deep fluids.

Veins in the Game Lands core from a syncline in the eastern Valley & Ridge province exhibit two to three vein minerals stages (calcite, barite, pyrite). Early calcite and barite contains only bitumen inclusions, while later calcite contains CH₄ and liquid hydrocarbon inclusions. CH₄ inclusions were trapped at 75°-125°C and 62-92 MPa (2.4-3.5 km). Liquid hydrocarbon inclusions in a bed-parallel vein have a minimum trapping temperature of 125°C (~ 5.4 km).

Veins in the Bald Eagle core from the Allegheny Front in central Pennsylvania contain one or two stages of calcite and/or dolomite. Both mineral contain only bitumen and rare liquid hydrocarbon inclusions. No trapping temperature or depth could be determined.