Clay smear seals and fault sealing potential of an exhumed growth fault, Rio Grande rift, New Mexico

Abstract

The Calabacillas fault, New Mexico, is an exhumed growth fault that provides insights on the processes of clay smear formation and the effectiveness of fault seal prediction for faults of this type in the subsurface. Exposed clay smears range from being continuous and having a taper geometry to being semicontinuous and segmented by secondary faults. In some cases, source beds are truncated at the fault and there as no attached clay smear. Detailed mapping of the fault zone shows that there is a veneer of clay gouge on the fault that is interrupted by multiple gaps that would reduce the ability of the fault to be an effective seal over geologic timescales. The presence of releasing dip relays in footwall source beds and the evolution of the dip relay during the growth of the fault zone primarily control the variability in clay smear type and continuity. Source bed plasticity, composition, and thickness play a secondary role. As the fault zone grows, the dip relay is breached, and the clay smear is progressively segmented by normal faults that translate it down the fault and eventually truncate it from its source. Smearing-type algorithms (CSP, for example) overpredict the likelihood of fault seal at the base of the source bed, because the clay smear is commonly detached from its source. A key threshold for fault seal prediction is the stage of fault growth at which the clay smear tapers become separated from their source beds. Below this threshold, smear-type algorithms work best. Above this threshold, abrasion-type algorithms work best.