Fluid Inclusion Evidence of Overpressure throughout a ~42 M.Y. History of Fracture Opening In the Travis Peak Formation, East Texas Basin

Fluid inclusion assemblages (FIAs) hosted in fracture-bridging synkinematic quartz cements record the PTX properties of fluids present during fracture opening. We have previously demonstrated that a systematic temperature decrease with the relative age of FIAs hosted in a synkinematic quartz cement bridge from the Travis Peak formation in the East Texas basin may be used to date fracture formation from ~42 Ma to the near-present day. Here, we present new data on the composition of these FIAs that suggests a pressure decrease with time. Laser Raman spectroscopy was used to estimate the methane concentration in FIAs of known relative age in a synkinematic quartz bridge sampled from a largely open fracture in core at a depth of 3 km. The methane concentration decreases systematically with temperature (and by extension, time), from ~3300 ppm at 151°C to ~1900 ppm at 134°C, suggesting a PT path that traversed the two-phase field in the ternary H₂O-NaCl-CH₄ system. Calculated trapping pressures for these inclusions range from lithostatic pressure at maximum burial of 3.5 km when fracture formation initiated, down to hydrostatic pressure near the modern day burial depth of 3.0 km. We propose a model in which reservoir charge occurs near maximum burial conditions at ~42 Ma, increasing the pore fluid pressure to lithostatic conditions and providing an impetus for fractures to form. During uplift and cooling from ~42 Ma to the present, methane continuously leaks out of the system at a gas-water contact, maintaining methane saturation of pore fluids below the contact while fracture apertures repeatedly widen due to overpressure driven crack tip propagation. The pressure eventually decreases to hydrostatic conditions near to the present day, and fracture propagation ceases.