Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
West Texas Geological Society Fall Symposium, Midland, Texas, October 25-26, 2001
Chert Reservoir Development in the Devonian of West Texas: Contrasts Between Proximal and Distal Settings
Stephen C. Ruppel1, and Roger J. Barnaby2
The lower Devonian Thirtyone Formation of West Texas and New Mexico is the largest chert reservoir succession in the world, having produced more than 750 million barrels of oil. As much as 650 million barrels of additional mobile oil remains in these reservoirs, making this play an important target for further exploitation.
All Thirtyone chert reservoirs have much in common, including relatively high porosities and low permeabilities and a bimodal pore distribution containing abundant microcrystalline pores. However, distinct differences in depositional geometries and styles of reservoir heterogeneity are apparent between reservoirs in proximal and distal settings. Proximal reservoirs are composed of a single, thick (up to 100 ft), sheetlike chert unit that extends for hundreds of square miles. Heterogeneity in these reservoirs, which were deposited on a gently sloping outer platform during regional transgression, is a function of faulting, fracturing, and dissolution of associated carbonate along unconformities.
By contrast, distal reservoir successions comprise thin, vertically stacked and laterally discontinuous chert intervals whose origin is a function of transport and deposition of siliceous sediments as debris flows and turbidites. Flow units in these reservoirs are thin (10-20 ft), spatially limited in size, and separated vertically and laterally from one another by low-permeability mud-rich, siliceous sediments and hemipelagic deposits. Flow unit distribution is the result of both paleotopography and sea-level cyclicity. Cherts are most abundant in transgressive and early highstand legs of sea-level rise-fall cycles and display offset stacking suggestive of topographically controlled reciprocal sedimentation. Faults and fractures appear to be less significant contributors to reservoir heterogeneity in distal reservoirs.
of Economic Geology, The University of Texas at Austin
C. Ruppel, Research Scientist