Abstract

Since the 1920’s, approximately 379 reservoirs have been discovered in sandstones of the Delaware Mountain Group in West Texas and southeast New Mexico, with original oil in place (OOIP) exceeding 1,800 MMbbl. The 102 largest Delaware sandstone reservoirs in Texas (individual production >100,000 bbl) produced 226.8 MMbbl through December 1998. Greatest production is from reservoirs in the Bell Canyon Formation; the 63 largest Bell Canyon reservoirs in Texas have produced 178 MMbbl of oil. Low recovery efficiencies from Delaware sandstone fields (average 14%) are caused mainly by low solution-gas drive energy, high water production, and geologic heterogeneities affecting reservoir displacement.

Study of upper Bell Canyon sandstones exposed in outcrop has led to the development of a depositional model that can be used to guide reservoir characterization. Stratigraphic relationships indicate that the Bell Canyon sandstones were deposited by turbidity currents in a basinal system of channels and levees with attached lobes. Grain size is limited to fine sand and coarse silt. Siltstones provide the most important depositional heterogeneity within Bell Canyon reservoirs because of the grain size and permeability contrast between sandstones and siltstone facies. Siltstones occur (1) as widespread sheets that bound high-order cycles, (2) as discontinuous drapes along the base of channels or at the tops of sandstone beds, (3) interbedded with thin sandstones in levee deposits, and (4) overlying erosion surfaces associated with channel avulsion. Even thin siltstones can affect displacement operations in reservoirs. Because of the low permeability of siltstones, limited cross flow of fluids will occur between sandstones separated by siltstone. The geologic heterogeneity in Delaware sandstones should be taken into account in the design of enhanced oil recovery (EOR) projects.