Oil & Natural Gas Technology

The Ellenburger Group of the West Texas Permian Basin is part of a Lower Ordovician carbonate platform sequence that covers a large area of the United States. During the Early Ordovician, the Permian Basin area was located on the southwest edge of the Laurentia plate between 20° and 30° latitude. The equator crossed northern Canada, situating Texas in a tropical to subtropical latitude. The area of Texas was a shallow-water shelf, with deeper water conditions to the south where it bordered the Iapetus Ocean. Shallow-water carbonates were deposited on the shelf, and deep-water shales and carbonates were deposited on the slope and in the basin. The interior of the shelf produced restricted environments, whereas the outer shelf produced open-marine conditions. Diagenesis of the Ellenburger Group is complex, and the processes that produced the diagenesis spanned millions of years. Three major diagenetic processes strongly affected Ellenburger carbonates: (1) dolomitization, (2) karsting, and (3) tectonic fracturing. Pore networks in the Ellenburger are complex because of the amount of brecciation and fracturing associated with karsting. Networks can consist of any combination of the following pore types, depending on depth of burial: (1) matrix, (2) cavernous, (3) interclast, (4) crackle-/mosaic-breccia fractures, or (5) tectonic-related fractures.