Abstract
Detailed investigations of Morrow Sandstones in Cimarron County, Oklahoma, provide a bird’s-eye view of problems encountered during regional exploration, production, and completion practices over a wide area from New Mexico to Kansas. The generally poor lateral and vertical control of sand distribution can be improved through a detailed knowledge of environments of deposition. Frequent formation damage because of poor completion procedures can be largely prevented through an understanding of Morrow sandstone diagenesis.
Morrow sandstones in Cimarron County form two distinct reservoirs. Type 1 reservoirs are thick (10 to 50 ft or 3 to 15 m), porous (20 to 23%), permeable (47 to 236 md), and very coarse grained (0.83 mm). These reservoirs are fan-shaped, being less than 1 mi (1.6 km) wide at the apex (on the northwest) and 4 mi (6.4 km) wide at the southeastern edge. Sand thickness is greatest at the center of the body. These sands were deposited in the estuarine portions of a braided fluvial system.
Type 2 reservoirs are thin (generally less than 20 ft or 6 m), have relatively low porosities (4 to 20%) and permeabilities (3 to 100 md), and are fine grained (0.24 mm). These reservoirs are discontinuous, lenticular, elongate units which trend at approximately right angles (NE-SW) to the Type 1 reservoirs. Maximum width is 1 mi (1.6 km); maximum length is of the order of several miles. These sands were deposited in lower tidal-flat and shallow, offshore-marine environments, as beaches and bars.
Once the reservoir has been discovered, it is vital that completion practices be tailored to the specific rock composition. Failure to do this may result in serious formation damage, and the bypassing of potential production. Problems characteristic of these sands include: (1) a migration of fines, (2) extreme acid sensitivity, and (3) possible water sensitivity. The sand composition may often require a multistage acid job with KCl flushes. Hydrofluoric acid should not be used unless the detailed sand composition has been determined by thin-section analysis.
