FLUID INCLUSION ASSESSMENT OF SECONDARY QUARTZ MINERALIZATION NEAR HUNTINGDON, PA

Fluid inclusion studies of quartz collected from an open vein-fracture system crosscutting Mahantango Fm rocks just south of Huntingdon, PA reveal a complex history of fluid entrapment in the vein system. Results differ sharply from those reported by Curry, (2012) for fluid inclusions in samples collected less than two miles away in overlying strata of the Braillier Fm. Fluid inclusions observed within a euhedral quartz crystal were overwhelmingly dominated by secondary single-phase gas inclusions. However, this study focuses on a 2.5x0.6mm region of the specimen, which contains both primary aqueous and single-phase gas inclusions.

Aqueous inclusions in the sample show vapor-phase homogenization temperatures, T_h, ranging from 132.5 ^oC to 187.5 ^oC. This result contrasts with Curry’s data that documented significantly lower homogenization temperatures (<135 ^oC) consistent with maturation of the light oils seen by Curry. The range of temperatures for this sample suggests evolution of fluid temperature during the stationary growth of quartz. If a geothermal gradient of 25^oC/km is assumed, a median T_h value of 160 ^oC argues for vein mineralization at a depth of at least 6.4 km. Freezing point depression observed in these inclusions indicate fluid salinities at approximately 14 weight % NaCl equivalent, less saline than typical oilfield brines. Such result is consistent with the general absence of light oils in the inclusion assemblage, unlike Curry’s specimen.

Non-aqueous inclusions occur alongside the aqueous inclusions in varying morphologies and trace orientations suggesting primary entrapment of these gases. Homogenization studies on single-phase gas inclusions indicate entrapment of nearly pure methane, which was most likely sourced from the black organic-rich shale of the Marcellus Fm lying immediately beneath the rocks sampled in this occurrence, and suggest a local escapement of gas from the sedimentary basin rather than retention of methane within this normally tight reservoir rock. A single three-phase inclusion of aqueous and petroliferous liquids and hydrocarbon gas was observed in what appeared spatially and morphologically to be the same FIA as the gas inclusions. The uniqueness of this single fluid inclusion makes us reluctant to make definitive interpretations of this latter result.