Geological-Engineering Study of Cato (San Andres, Permian) Field, Chaves County, New Mexico: ABSTRACT

Abstract

Cato field in Chaves County, New Mexico, produces 25° gravity oil from two Permian San Andres dolomite reservoirs which are separated by 20 ft of impermeable anhydrite. Average depth to pay is 3,500 ft and each zone is about 50 ft thick. Although the average porosity of the upper zone generally is less than that of the lower zone, 70% of the field reserves are in the upper zone. Dolomite beds of the upper zone (main pay) differ from those of the lower zone in that they are darker, more coarsely crystalline, have larger connective pores, a much lower water saturation, and although generally less porous, have much better permeability. The environment of deposition apparently was one in which shoal oolite was deposited with laterally equivalent lagoonal carbonate mud and open-marine fossiliferous mud. The net pay of this zone has large intergranular pores, coarse intercrystalline pores, and cavernous dolomite with solution channels and large vugs. Dolomite beds of the lower zone are believed to be an evaporitic lagoonal deposit. They are apparently of primary and early diagenetic origin. Generally, they are light-colored, very finely crystalline, anhydritic dolomite. Very fine intercrystalline pores and fine moldic pores are prevalent with lesser amounts of interparticle pores. Mercury-injection capillary-pressure data, porosity-permeability plots, and thin-section studies were analyzed together with reservoir performance. Results indicate that the pay of this facies has a much higher porosity cut off than the upper zone (main pay) because of extremely fine pore connections. Detailed core studies of intensity were compared with performance data and indicate that the San Andres of Cato field is not a fracture reservoir. Joint type fractures are typically only a few inches in length and do not form a self-connecting, petroleum-permeable system. They do serve as connective links between zones of matrix permeability. Lithologic studies, production interference test studies, and laboratory analyses indicate that well spacing greater than 40 acres would make this type reservoir more attractive economically. Cores and good samples along with a good logging program are a necessity in San Andres Formation evaluation. End_of_Article - Last_Page 751------------