Summary

Natural fractures in tightly cemented rocks may enhance the permeability of the reservoir, and provide gas migration pathways during gas charge. Thus, knowledge of the opening and cementation history of natural fractures is essential for understanding migration and the development of localized, enhanced productivity. Knowledge of fracture propagation rates is also fundamental to understanding deformation of the Earth. Reconstructions of partially cemented natural fractures from tight-gas sandstones suggest that some fractures open over tens of millions of years. The duration of fracture opening is obtained by reconstructing the fracture opening history through textural mapping using SEM-CL imaging of quartz fracture cement bridges and by microthermometry of tens to hundreds of fluid inclusion assemblages trapped in these cement bridges. These bridges formed by repeated fracture opening and cementation (crack-seal), trapping fluid inclusions during fracture opening and thus record the temperature, pressure and fluid composition history during fracture opening. The timing and rate of fracture opening can then be reconstructed by relating fluid inclusion temperatures to the known burial history of the formation. Previous studies reported fracture widening histories; here for the first time we report fracture widening and lengthening histories.

Fluid inclusions trapped in syn- or postkinematic fracture cements obtained from cores in the tight-gas sandstones of the Cretaceous Mesaverde Group in the Piceance Basin, Colorado, and the Travis Peak Formation in the East Texas Basin indicate a thermal history varying from ~140°C to ~188°C to ~160°C, and from ~130°C to ~154°C to ~134°C, respectively. Calculations based on fluid inclusion microthermometry, Raman spectroscopy, and equation of state modeling suggest fracture opening under significant pore-fluid overpressures in the reservoirs. Models of gas and oil generation in the basins suggest that fracture opening and elevated pore-fluid pressures coincided with maximum gas generation in the source rocks within and underlying the Mesaverde and Travis Peak reservoirs. These correlations strongly suggest that protracted growth of the pervasive fracture system was the consequence of gas maturation and reservoir charge, and that fracture opening lasted for ~35 m.y. in the Piceance Basin, and ~48 m.y. in the East Texas Basin.

URTeC 1582124

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