--> Calcite Vein Formation in the Utica-Point Pleasant Formations of the Appalachian Basin: Estimating Methane Density and Pressure Using Micro Laser Raman Spectroscopy, and Timing of Vein Formation Using Fluid Inclusion (FI) Microthermometry

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Calcite Vein Formation in the Utica-Point Pleasant Formations of the Appalachian Basin: Estimating Methane Density and Pressure Using Micro Laser Raman Spectroscopy, and Timing of Vein Formation Using Fluid Inclusion (FI) Microthermometry

Abstract

The Utica and Point Pleasant Fms. comprise over-mature Ordovician-age organic-rich shales, with reported VRE values of ~3 to 3.5%: In this paper, we present the results of a FI study on inclusions trapped in calcite from calcite veins that were sampled in core from the discovery well. Homogenization temperatures from primary 2 phase aqueous inclusions constrain their temperature of formation to ~134 °C (Utica Fm.) and ~142 °C (Point Pleasant Fm.), respectively. The timing of calcite vein formation was estimated by matching FI trapping temperatures with the burial history from a calibrated basin model. In addition, we have used “clumped” stable isotope thermometry, based on the equilibrium distribution of 12C/13C and 1H/2H in the mass 18 methane isotopologue (13CH3D), to constrain the temperature of formation of methane gas in the respective Fms. Measurements of Δ13CH3D on gas from the Marcellus and Utica shales yield methane formation temperatures of 147 +25 -22 °C and 160 +29 -25°C, respectively. The combination of burial history and FI formation temperatures shows that the analyzed methane formed at ~300 Ma and ~160 °C, possibly as a result of oil-to-gas cracking during peak burial. Methane formation clearly preceded the formation of the calcite veins which preserve lower formation temperatures (134 to 142 °C) that we infer occurred at ~ 25 Ma, coinciding with the onset of uplift in the Appalachian Basin. Using laser micro Raman spectroscopy we measured the characteristic peak shift for the C-H symmetric stretching in methane, from which we have determined the in situ density of methane. Liquid petroleum inclusions were also observed in some of the samples, and their pale blue fluorescence under UV light suggests that they are high API gas condensate liquids. According to our basin model, peak burial at ~250 Ma resulted in temperatures of ~225 °C. Exceptionally high methane gas pressures might be expected in an organic-rich source rock that has reached a thermal maturity of 3-3.5 % VRE. With inferred fluid pressure gradients in the source rock of ~1.3 to 1.4 psi/ft. (relative to present-day depth), seal integrity must be considered. If the basin model results are correct and the trapping of the methane inclusions is approximately coincident with the onset of uplift, the timing and cause of the calcite veins could be a manifestation of in situ fluid pressures exceeding the local fracture gradient.