Abstract

An east-west-trending belt of lower San Andres oil fields extends 80 miles across southeastern New Mexico from the Pecos River near Roswell to the Texas-New Mexico border. These fields are along a porosity pinchout zone where porous carbonates grade laterally into bedded anhydrite and halite. The lower San Andres traps are associated with pre-Tertiary structural or stratigraphic traps. They have gas-solution reservoir drive and have tilted oil/water contacts. Oil and water production relationships from these fields are not consistent with present-day structure. These fields have been commonly interpreted to be hydrodynamic traps created by the eastern flow of fresh surface waters that enters the lower San Andres outcrops west of the Pecos River. There is no evidence, however, that surface water have moved through the lower San Andres in this area. This conclusion is supported by the fact that formation-water resistivities are uniform throughout the producing trend, there is no significant dissolution of carbonates or evaporites, and there is no increase in biogradation of oils adjacent to the lower San Andres outcrops. These fields are actually diagenetic traps created by porosity occlusion in the water column beneath the oil accumulations. The nearby evaporite beds provided the material for this occlusion of porosity. Hydrocarbons were originally trapped in pre-Tertiary structural and structural-stratigraphic traps. Bedded evaporites were effective barriers to both vertical and lateral hydrocarbon migration. Eastward tilting of the Northwest Shelf during the Tertiary opened these traps, but the oil remained in these structurally unfavorable position, because of the diagenetic sealing. The gas-solution drive in these reservoirs is a result of this sealing. The sequence of events leading to diagenetic entrapment include: 1) Triassic and Jurassic migration of hydrocarbons into broad, low-relief, post-San Andres structural and structural-stratigraphic traps; 2) rapid occlusion of porosity in the water column beneath oil reservoirs and; 3) Tertiary opening of closed structural traps.