Permeability Architecture of the Hidden Valley Fault Zone in the Glen Rose Formation: Canyon Lake Gorge, Comal County, Texas

Abstract

The permeability architecture and sealing potential of faults in carbonate rocks are fundamentally important to hydrocarbon reservoir performance, but remain poorly understood. Characterization of geologic structure and secondary porosity in faulted carbonate rocks in central Texas elucidates fault zone permeability characteristics in a region where studies of carbonate aquifers have demonstrated that faults may serve as either barriers or conduits with respect to subsurface fluid movement.

The Canyon Lake Spillway Gorge near New Braunfels, Texas exposes over 800 m of the “seismic-scale” Hidden Valley normal fault and 108 m of vertical stratigraphic section of the Upper and Lower Glen Rose Formation. Numerous year-round seeps and springs discharge from the fractured carbonate strata and are controlled directly by the Hidden Valley fault zone and secondary porosity features within the fault damage zone. Water chemistry and quality analyses show that flow to the springs and seeps is driven largely by seepage from Canyon Lake with minimal additional flow from the Trinity aquifer (Glen Rose limestone). High-permeability discharge and recharge features in the Gorge are localized in areas of enhanced, secondary porosity in excess of the matrix porosity, which was measured in limestone beds using point counting to be 0.1 % to 7.6 % (average 1.9 %). Secondary porosity types include: (i) extension fractures, (ii) faults, (iii) dissolution-enhanced fractures and faults, and (iv) vuggy porosity formed by dissolution of sedimentary burrows. Dye tracer studies indicated rapid fault-parallel groundwater flow and in some cases little or no mixing over distances of hundreds of meters from recharge to discharge points along the Hidden Valley fault zone. This lack of mixing or dilution along with high flow rates over long distances indicates discrete or isolated conduit flow through secondary porosity. The porosity and permeability network in this system is quite complex, and here we examine components of this network.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018