Bob Loucks, Ph.D.
Senior Research Scientist
Bureau of Economic Geology
Jackson School of Geosciences
The University of Texas at Austin
A long, continuous core (1155 ft; 352 m) from the Ellenburger Group in Upton County, Texas allowed for a detailed reservoir description of depositional setting, lithofacies, pore networks, and impact of subsequent karst processes on the reservoir geology. This furthers the understanding of the origin and characteristics of pore types relevant for waste-water disposal and CO2 storage in the Ellenburger Group. The Lower Ordovician Ellenburger Group in West Texas is a thick dolomite sequence that was deposited on a broad shelf over a 16 m.y. time period in the paleo-Tobosa Basin. The lower two thirds of the cored section in the area of investigation is dominated by microbial thrombolite complexes. Interbeds of oolites and evaporites are associated with the complexes and may represent small-scale fluctuations in relative sea level. In younger intervals, the lithofacies transition into a peritidal complex composed of stromatolites and laminated mudstones, some displaying relic evaporite nodules. The lithofacies stacking pattern is similar to the regional established stacking pattern where the most open-marine lithofacies are at the base of the stratigraphic section and the most restricted lithofacies are at the top of the stratigraphic section. This consistent stacking pattern suggests a stable structural setting for millions of years where constant environmental conditions existed. Following deposition, lithification, and dolomitization of the Ellenburger Group, the section underwent intense meteoric karsting (epigenic) during Late Middle Ordovician Sauk-Tippecanoe megasequence as a result of long-term exposure. The meteoric karsting affected the upper 700 ft (215 m) of the Ellenburger Group in the studied core. The pore networks in the core are composed of vuggy (small cavities) pores in the thrombolite lithofacies and crackle and mosaic breccia fracture pores, and chaotic breccia interclast pores in karst-affected intervals. Porosity in the core is low (estimated <5%) in the Ellenburger section, whereas permeability may have been relatively high (estimated hundreds of millidarcys). Loucks and Kerans (2019) presented data at the field-reservoir-quality level from a NRG Associates database that shows porosities in shallower reservoirs (less than 8000 ft [2500 m]) generally range between 3% to 16% and permeabilities generally range between 1 md to 100 md. Deeper reservoirs greater than 8000 ft (2500 m) have porosities generally between 2% and 5%; permeabilities vary greatly from 1 md to 100 md, and at depths below 15,000 ft (4600 m), the permeability range condenses to 2 to 5 md. These pore systems support hydrocarbon production and now are being evaluated as a known waste-water disposal unit and potential for CO2 storage.