ABSTRACT
Exploration in Permian (Guadalupian) deep-water sandstones of the Delaware Mountain Group, west Texas and southeast New Mexico, represents a success story of the 1990s derived from reevaluation of reservoirs previously deemed uneconomical. Recent discoveries have concentrated on the Brushy Canyon in New Mexico and, to a lesser extent, the Cherry Canyon in Texas. Brushy Canyon reservoirs in particular previously were overlooked due to indications of poor reservoir quality from log and well test data; however, oil shows observed on mud logs across the northern Delaware basin led to new completion efforts in the late 1980s and 1990s using gel-sand fracture stimulations. Productive reservoirs are very fine to fine-grained arkosic to subarkosic sandstones with porosities of 12–25% and permeabilities typically of 1–5 md. Better reservoir quality is concentrated in massive channel sandstones variably interpreted as deposited by turbidity or saline density currents. Significant clay content, lamination, and close interbedding between oil- and water-bearing units make log analysis and reserve estimates problematic. As a result, the mud log remains the cheapest, most practical indicator of pay. Reservoir sandstones can be divided into a series of major productive trends related to proximal/slope and more distal/basin-floor depositional settings. Well productivity is variable within each trend, but primary recovery rarely exceeds 10%. Options for enhanced recovery include pressure maintenance, waterflooding, and carbon dioxide flooding. Early indications suggest that carbon dioxide flooding may be most appropriate in these low-permeability, clay-bearing reservoirs.
