Reconstructing the growth of a fracture set using fluid inclusion microthermometry, El Alamar Formation (Triassic), NE Mexico
Abstract
Aperture-frequency data collected by scanline surveys of natural fractures in tight-gas sandstones and their outcrop analogs commonly follow a power-law aperture size scaling distribution that spans the aperture range from the microscale (apertures <0.1 mm) to the macroscale. The coefficient of the power-law equation represents the fracture intensity whereas the exponent reflects the relative abundance of large to small fractures. Comparing data sets collected in rock units from different tectonic regimes and burial settings, and from different structural positions within mesoscopic structures in the same rock unit, we find that the coefficient increases with increasing fracture strain but the exponent stays relatively constant. This suggests that microscopic fractures (microfractures) grow along with macroscopically visible fractures (macrofractures) throughout the evolution of the fracture set. To test this interpretation, we modeled the development of a power-law set of fractures consistent with the observation of incremental fracture opening through the crack-seal mechanism of repeated cycles of fracture opening and cementation. The simulation is an Excel routine that randomly distributes fracture opening increments among a set of active fractures. We compared the modeled sequence of cement increments and the resulting aperture size distribution against fracture scaling and cement timing data collected from the Triassic El Alamar Formation in NE Mexico. Cement timing was inferred through crosscutting relationships and comparison with fluid inclusion microthermometric data from other fracture sets that indicate that the fracture set used for this field validation formed during uplift and ambient cooling. Thus, opening temperature for this fracture set, serves as a proxy for fracture timing. The field scaling and fluid inclusion data are broadly consistent with the numerical simulation. New microfractures appear to have formed throughout the period of active fracture growth. Some microfractures grew to large sizes; some that formed early never reactivated. New microfractures formed amid dense arrays of pre-existing, actively growing fractures. The simulation does not account for fracture spatial arrangement, but the natural data show some correlation between fracture location and timing, suggesting that clusters of fractures may grow concurrently.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T21A2556H
- Keywords:
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- 1043 GEOCHEMISTRY / Fluid and melt inclusion geochemistry;
- 4440 NONLINEAR GEOPHYSICS / Fractals and multifractals;
- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8169 TECTONOPHYSICS / Sedimentary basin processes