Abstract

The lower Guadalupian Brushy Canyon Formation has become an important exploration target in the Delaware Basin and has helped to revitalize exploration in the southeastern part of New Mexico and west Texas. The Brushy Canyon is the lowermost member of the Delaware Mountain Group and is generally believed to have been deposited in deep water.

The Brushy Canyon Formation in the Hat Mesa Delaware field is a subarkosic, slightly shaly sandstone. The grain size of the reservoir varies from silt (Mz=0.05mm) to very fine sand (Mz=0.11mm). This results in high irreducible water saturations (Swirr) as well as low permeabilities. Illite and mixed layer clays occur throughout the reservoir both as pore lining and intergranular pore fillings. Water productive as well as oil productive formations have high residual oil saturations as a result of very low permeabilities in the invaded zones. All of the aforementioned characteristics present interpretive difficulties to the log analyst.

Cross-plotting techniques are utilized to discriminate between productive and non-productive intervals as well as define critical values of water saturation (Sw), porosity (ø), and permeability (k). A porosity (ø_core) versus absolute permeability (ka_core) cross-plot was used to establish a porosity cutoff of 15% for zones with a permeability of >1 md.

Productive intervals are characterized by a water saturation (Sw) of <60%, effective porosity (ø_e) of >15%, and a volume of clay (V_d) <15%. In addition it was noted that of the productive zones, the very fine-grained sands formed much better reservoirs than the siltstones.