Abstract

The Seven Rivers Formation is an oil and gas reservoir in many fields across the northern shelf of the Delaware Basin. The largest Seven Rivers reservoir, Grayburg Jackson Pool (formerly Fren Pool), has yielded more than 5.4 MMBO and 1.6 BCF of associated gas. Grayburg Jackson and other fields that overlie the Artesia-Vacuum Abo reef trend mark the northernmost significant Seven Rivers production where porous dolomite stringers pinch out landward into bedded anhydrite.

Two wells were cored and thin sectioned to study these thin (< 4 ft) dolomite reservoir beds. The cores demonstrate that the upper Seven Rivers is comprised of massive to bedded nodular anhydrite (majority); non-reservoir, algal laminated, fenestral, dolomitized boundstone/mudstone; and dolomitized grainstone/packstone reservoir rocks. Petrography reveals complete dolomitization of carbonate units, abundant anhydrite cements in the laminated facies, and excellent porosity preservation in the higher energy facies. These lithofacies represent depositional environments that range from supratidal sabkha to intertidal mud flat and tidal channel. The grainstone/packstone facies are the primary contributors to production having porosity ranging from 10 to 28.5% and permeability ranging from 0.1 to 35 md. Well log-derived pore volume mapping demonstrates that the higher energy facies are related to shore-perpendicular porosity zones suggestive of tidal channels.