Approximately 153 billion bbl of in-place oil have been discovered in Texas oil reservoirs. Characterization of 500 of the largest of these reservoirs (those that have cumulative oil productions of more than 10 million bbl) on the basis of geologic and engineering parameters, facilitates the grouping of Texas oil reservoirs into families or "plays" of similar reservoir geology and common engineering and production attributes. Basic data for each reservoir were tabulated from information in the hearing files of the Texas Railroad Commission and other public sources. Thirty variables were examined for each reservoir. Oil plays were characterized in terms of: (a) reservoir genesis, (b) petrophysical properties of the reservoir, (c) trapping mechanism, (d) drive mechanism, (e fluid properties, (f) volume of in-place oil, (g) recoverable reserves, (h) calculated oil recovery efficiency, and (i)

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reservoir management practices and conventional well spacing. Most of the major Texas oil reservoirs can be grouped into 48 geological plays which account for 71% (32 billion bbl) of all Texas oil production.

Twenty-one of the plays are located in a belt along the Texas Gulf coastal plain and in the East Texas basin. Mesozoic and Cenozoic sandstone reservoirs deposited in fluvial-deltaic and strandplain systems dominate over fluvial sandstones and carbonate reservoirs. The remaining 27 plays extend westward from north-central to west-central Texas. Dolomite is the prevalent reservoir lithology with sandstone and reef-associated limestone being more abundant than chert, conglomerate, and nonreef associated limestone. Reservoir genesis in the north and west Texas plays is diverse and includes a spectrum of clastic depositional environments from fan and fan delta to slope and basin systems. Carbonate reservoirs have been interpreted as open and restricted shelf deposits, platform margin-assoc ated banks and reefs, and deeper water atoll and pinnacle reef systems. Unconformity-related reservoirs in west and east Texas, such as the prolific East Texas field, are grouped into two plays regardless of the depositional history of the reservoir. Recovery efficiencies of the Paleozoic north and west Texas plays are considerably lower than those of the coastal plain and east Texas plays.

The effects of drive mechanism, lithology, permeability, API gravity, and viscosity on reservoir performance are well known. An additional important control on recovery efficiency that has been emphasized by this study is reservoir genesis. Although productivity can be modified by extremes in permeability or hydrocarbon character, it otherwise follows predictable trends based on the known geologic complexity and heterogeneity of the depositional system of the reservoir.

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