Depositional Facies and Diagenetic History: Keys to Reservoir Porosity, Quality and Performance, East Vacuum Grayburg-San Andres Unit, Lea County, New Mexico

Abstract The East Vacuum Grayburg-San Andres Unit (EVGSAU) is located in the Permian Basin, Lea County, New Mexico. The EVGSAU was unitized in 1978 and has been under waterflood operations since 1979. C02/water injection (WAG injection) commenced in 1986. An interdisciplinary reservoir study was initiated in 1994 to integrate the unit's geological character and reservoir performance in order to optimize the C02/waterflood. This paper examines the sedimentologic and stratigraphic framework, lithologic character, pore system, quality and continuity, and architecture and internal geometry of the reservoir and their application to safeguarding prime C02 flood performance within the reservoir. The Permian-aged San Andres Formation is composed of two major, eustatically controlled, upward-shallowing carbonate ramp sequences that contain multiple, anhydritic, dolomitized successions in which subtidal grain-rich lithofacies grade upward into peritidal, mud-rich carbonates and siliciclastics. A siliciclastic, lowstand tidal flat unit, the Lovington sand, caps the lower sequence and separates the formation into the Lower and Upper San Andres. Grainstones and packstones deposited as high-energy shoals and bars are the primary reservoir lithologies. Diagenetic processes critical to the development of good reservoir porosity and quality are; (1) early dolomitization of the carbonate section, (2) intense dissolution and alteration of anhydrite to gypsum, and (3) dissolution of dolomite. Four correlatable subtidal grain-rich intervals constitute the chief flow units and the only continuous waterfloodable intervals across the field. Relationships between the distribution of the subtidal grain-rich lithofacies, porosity content, anhydrite and gypsum abundances, and structural position exhibit considerable potential for mapping reservoir flow units and for defining reservoir quality within flow units. This geological model provided the architectural basis for the reservoir simulation pattern model used to evaluate reservoir performance and to modify EVGSAU operating plans to ensure optimum flooding. The geological model helped determine; 1) Upper San Andres vs. Lower San Andres performance, 2) infill spacing within the Upper and Lower San Andres, 3) the potential for horizontal injection in the Upper and Lower San Andres, and 4) repressurization volumes required for the Upper San Andres.