From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, December 12, 2008
Fluid inclusion constraints on the opening history of synkinematically cemented fractures
Stephen P. Becker
Bureau of Economic Geology
Synkinematic cement bridges within fractures contain textural and chemical markers that record a history of crack-seal deformation and growth. Multiple generations of fluid inclusions are invariably trapped as these fractures open and seal in the presence of a fluid phase. These inclusions provide a unique opportunity to discover the instantaneous pressure-temperature-composition characteristics from a sample of the actual fluid or fluids present at the time of cement growth. We can then use cathodoluminescence petrography to determine the relative age of different generations of fluid inclusions, and by extension the thermal history of fracture opening. Thus, if we know an independent record of the thermal history of a system (for example, from burial history), this temperature record may be used to constrain the timing of fracture opening.
We investigated the history of fracture opening in “tight gas” reservoirs from both the East Texas and Piceance Basins. In the East Texas Travis Peak Formation, synkinematic fluid inclusion homogenization temperatures varied systematically from ~130°C increasing to ~150°C, and then decreasing to ~132°C. Methane compositions systematically varied with decreasing temperature from ~2700 to ~1700 ppm, indicating methane-saturated entrapment. According to available burial histories, this indicates fracture opening initiated at near maximum burial under above-hydrostatic pore fluid pressure conditions, and continued to present day hydrostatic conditions at average rates of 16-23 µm/m.y. for a total fracture opening history of 48 m.y.
In the Piceance basin, preliminary data from the SHCT core at the MWX site, and the Grand Valley Federal #2 core in the Grand Valley field similarly suggest long-lived fracture opening and cementation. The coexistence of gas and aqueous fluid inclusions within assemblages indicates trapping under gas-saturated conditions. Fluid inclusion homogenization temperatures in the SHCT core follow a sequence beginning at 145-150°C, increasing to 175-185°C, and then decreasing to ~158°C. In the Grand Valley #2 core, a similar temperature range and sequence is recorded beginning at 165-167°C, increasing to 170-172°C, and then decreasing to ~150°C. The coexistence of gas and aqueous fluid inclusions within assemblages indicates trapping under gas-saturated conditions. According to basin models, these temperature ranges suggest fracture opening histories over 30-35 m.y. However, long-lived maximum burial conditions introduce ambiguity into timing constraints, and thus these times should only be considered an absolute maximum for fracture opening.
These studies are providing insights into how and when fractures open and how long and under what circumstances they can stay open in the subsurface. Results also provide key input for evolving predictive models of fracture development.