Fluid Inclusion Constraints from the Opening History of Fractures in Tight Gas Sandstone Reservoirs

Stephen P. Becker¹, Stephen E. Laubach¹, Peter Eichhubl¹, Robert M. Reed¹, and Robert H. Lander^2
1Bureau of Economic Geology, University of Texas at Austin, Austin, TX
²Geocosm LLC, Austin, TX

Timing of fracture opening and sealing relative to hydrocarbon migration and reservoir charge is an essential, yet elusive, aspect of exploration and production in naturally fractured hydrocarbon reservoirs. Here, we develop a technique based on cathodoluminescence imaging and fluid inclusion analysis of cement bridges in partially sealed fractures to date fracture opening relative to the burial temperature history of the reservoir. Synkinematic cement bridges are a common attribute of natural fractures under reservoir conditions, acting as a natural proppant that preserves fracture porosity. A record of fracture opening is obtained by reconstructing the bridge growth history based on cathodoluminescence imaging, and detailed fluid inclusion temperature analyses of individual crack-seal cement layers. We currently apply this technique to core samples from tight gas sandstone reservoirs of the Cretaceous Travis Peak Formation in eastern Texas, and the Cretaceous Cozzette Sandstone of the Piceance basin in western Colorado. Preliminary data from the Travis Peak Formation suggest initiation of fracture opening at temperatures near maximum burial conditions approaching 150°C, with continued opening along a decreasing temperature trend to 130°C. These temperatures correspond to a maximum age of approximately 50Ma based on available burial history curves, and are consistent with fracture opening durations of tens of millions of years predicted by models of quartz bridge growth that are based on extrapolation of experimental quartz growth rates to sedimentary basin conditions.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas