Depositional Systems and Karst Geology of the Ellenburger Group (Lower Ordovician), Subsurface West Texas

The Ellenburger Group (Lower Ordovician) of Texas is a laterally extensive peritidal carbonate shelf sequence. It forms a major deep oil reservoir, having estimated reserves of 1.15 billion barrels of oil, and it also contains an estimated 2.2 billion barrels of oil equivalent. Despite its economic and geologic significance, comparatively little is known about the subsurface Ellenburger in West Texas; thus, this study presents a regional model of Ellenburger deposition and diagenesis.Six depositional systems, based on associations of lithologies and sedimentary structures observed in core, were recognized in the Ellenburger: (1) fan delta-marginal marine (litharenite); (2) lower tidal-flat. (mixed silliciclastic-carbonate packstone-grainstone); (3) high-energy restricted-shelf (ooid-peloid grainstone); (4) low-energy restricted-shelf (mottled mudstone); (5) upper tidal-flat (laminated mudstone); and (6) open shallow-water shelf (packstone-grainstone). The first two depositional systems record retrogradational sedimentation during initial transgression. The high-energy restricted-shelf system forms a laterally extensive sheet throughout most of Central and West Texas and represents relatively rapid and widespread flooding of the shelf. The latter three depositional systems record gradual progradational or aggradational sedimentation, or both. The open shallow-water shelf depositional system occurs as a broad fringe along the cratonward margin of the Ouachita foldbelt, and it represents the most open marine conditions on the shelf during middle to late Ellenburger sedimentation.Evidence of subaerial exposure and karst development is ubiquitous in Ellenburger carbonates. The most prolific karst event occurred prior to deposition of the Middle Ordovician Simpson Group associated with a global eustatic sea-level lowstand. This karst system is represented by fracture, mosaic, and chaotic breccias, and siliciclastic and carbonate infill sediments. Karst collapse breccias locally extend more than 600 ft (200 m) below the Ellenburger-Simpson contact, and laterally extensive phreatic cave systems developed between 30 and 300 ft (10 and 300 m)below this unconformity. Additional local karst development occurred in the Silurian-Devonian, Mississippian, and Pennsylvanian Periods.Diagenesis of the Ellenburger Group was dominated by three major styles of dolomitization. Very fine crystalline dolomite (5-20um) is restricted to tidal-flat facies and is interpreted to be a penecontemporaneous replacement fabric. Fine to medium crystalline dolomite (20-100 um), which is widespread in all facies, probably resulted from regionally extensive reflux processes operative during Ellenburger sedimentation. Coarse crystalline replacement mosaic dolomite and saddle dolomite cement formed in a burial setting after pre-Simpson karst formation and before Pennsylvanian faulting, uplift, and erosion. Other diagenetic events were karst-related dissolution episodes associated with repeated uplift and exposure of the Ellenburger platform and subsequent dedolomitization.The most common porosity type in Ellenburger reservoirs occurs in fractures and brecciated dolostones within paleokarst collapse zones. These porosity zones may be continuous from the upper Ellenburger erosion surface downward, or they may be represented by impermeable cave-infill sediments, resulting in vertical reservoir compartmentalization. Other porosity types are late, tectonically generated fracture porosity and vuggy and intercrystalline porosity produced during burial dolomitization, particularly in the high-energy restricted-shelf depositional system.
Charles Kerans

Kerans, Charles, 1990, Depositional Systems and Karst Geology of the Ellenburger Group (Lower Ordovician), Subsurface West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 193, 63 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation