Becker, S.P.¹, Eichhubl, P.¹, Laubach, S.E.¹, Reed, R.M.¹, Lander, R.H.², and Bodnar, R.J.³

¹Bureau of Economic Geology, The University of Texas at Austin, 10100 Burnet Road, Austin, TX 78758
²Geocosm LLC, 3311 San Mateo Drive, Austin, TX 78738
³Department of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, VA 24061

Abstract

Quartz cement bridges across opening-mode fractures of the Cretaceous Travis Peak Formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low-porosity sandstone. Incremental crack-seal fracture opening is inferred based on the banded structure of quartz cement bridges, consisting of up to 700 cement bands averaging ~5 µm in thickness as observed with SEM-cathodoluminescence. Crack-seal layers contain assemblages of aqueous two-phase fluid inclusions. Based on fluid inclusion microthermometry and Raman microprobe analyses, we determined that these inclusions contain methane-saturated brine trapped over temperatures ranging from ~130°C to ~154°C. Using textural cross-cutting relations of quartz growth increments to infer the sequence of cement growth, we reconstructed the fluid temperature and pore fluid pressure evolution during fracture opening. In combination with published burial evolution models, this reconstruction indicates that fracture opening started at ~48 Ma and above-hydrostatic pore fluid pressure conditions, and continued under steadily declining pore fluid pressure during partial exhumation until present times. Individual fractures opened over a ~48 m.y. time span at rates of 16-23 µm/m.y. These rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings.