Mapped Quartz Cement Growth History in Opening-Mode Fractures from the La Boca Formation, Northeast Mexico

Meghan Playton¹, Stephen E. Laubach², and Robert H. Lander^3
1Chevron, Houston, TX
²Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX
³Geocosm, Austin, TX

Sandstone outcrops of the Triassic to Lower Jurassic La Boca Formation offer unusually well preserved examples of cements within opening-mode fractures (commonly referred to as veins or joints). Although fractures in these outcrops that can be attributed to subsurface deformation contain quartz cement, some retain open pore space where others are sealed. The preserved quartz cement within otherwise open fractures contains crack-seal textures that record fracture opening history.

Here I present six examples of mapped quartz cemented fractures to record opening history, observe trends in cement growth, and roughly estimate the fracture opening rate based on fluid inclusion data and empirical growth rates for quartz cement. CL imaging of quartz cement reveals crack-seal textures that show cement precipitation while fractures are opening. Temperatures ranging from 92°C-135°C were extracted from fluid inclusions in two different fractures. Assuming from previous studies that quartz growth is dependent upon surface area and temperatures, I used the maps of crack-seal increments to calculate minimum opening history rates (quartz only) of fractures in sandstones. Although temperature varies during burial, I assumed two temperature cases to represent the extent of burial (up to 6 km) for these sandstones. At 110°C, the quartz growth rate is 20 µm/ma. At 200°C the growth rate is 400 µm/ma. Based on the fracture size and quartz morphology as defined by the mapped structures, time for quartz growth ranges from 900,000 to 166 ma.

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