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Unocal Corporation, Sugar Land, TX 77478, USA
Dolomite and dolomite reservoirs are common in Ordovician and middle to upper Permian carbonates in the ‘Permian Basin’ area of the SW United States. Scattered, but significant, dolomite reservoirs also occur in the Silurian and Devonian. In the middle to upper Permian, platform and shelf-top carbonates were dolomitized, while limestones in slope and basinal environments were not. Most dolomitization of Ordovician and Permian carbonates occurred in evaporated seawater shortly after deposition (reflux dolomitization). Most dolomitizing brines apparently formed in tidal flat and restricted lagoonal environments in the platform and shelf interiors, and on depositional highs near shelf and platform margins.
More than 26 billion barrels of oil have been produced from the Permian Basin, with most of that oil coming from Palaeozoic (mainly Permian) dolomites. Permian Basin dolomites have very heterogeneous porosity and permeability on a wide range of scales. Field-scale (km-scale) variations in porosity are commonly related to position in the original dolomitizing system. Porosity generally increases away from the apparent source of the dolomitizing brines because a greater volume of dolomite was precipitated in proximal parts of the dolomitizing system than in the distal parts; hence, porosity is greater in dolomites in the basinward parts of fields.
Most Permian Basin dolomite reservoirs are structural traps with stratigraphic enhancement of closure by loss of porosity and permeability towards the shelf or platform interior. Many traps were formed by compactional drape over the same features that created highs during deposition. Hence, the structurally highest parts of many fields have the poorest porosity and permeability because they coincide with proximal parts of the original dolomitizing system. The most porous, permeable and productive dolomites are on the basinward flanks of structures, and often near the oil-water contact.
Dolomite reservoirs in the Permian Basin are quite variable. Ultimate oil recoveries from these fields range from <1000 barrels to 2 billion barrels, with the largest fields in shallow middle Permian (San Andres/Grayburg) reservoirs. Reservoir depths range from 1500 to more than 14 000 ft (500–4300 m). Average porosities for fields are 1–21%, with porosities generally decreasing with depth. Average permeabilities are 1–1000 mD. Many deeper reservoirs have high permeability related to fractures in karstified Ordovician reservoirs. Recovery efficiencies are 10–65% of the original oil in place, with higher recovery efficiency associated with larger pores and higher permeabilities.
