The Sand Wash Basin has yielded some of the best Niobrara oil production in Colorado, producing from vertical, unstimulated well bores in heavily fractured reservoirs along anticlinal traps at depths of 4,000 to 9,000 feet on the southeast side of the basin. Several wells have produced in excess of 1 MM BOE from the Niobrara section, approximately 1,000 - 1,500 feet thick. The Niobrara formation exists throughout the Sand Wash Basin to depths of 20,000 feet, occupying two depo-centers east and west. USGS investigations reported vitrinite reflectance values, suggesting that the eastern half of the basin may be prospective as a shale resource play (Finn and Johnson, 2005).
A statistical fracture analysis approach was employed to identify high-fracture areas and fault zones from linear geomorphic elements interpreted from satellite imagery. These results were combined with structural observations from gravity and aeromagnetic data, identifying a northeast-trending structural corridor along the southeastern basin flank coincident with historic Niobrara production. This structural fairway represents the Soda Creek/Fish Creek Shear Zone, a deep-seated paleotectonic fault that trends northeastward through the Park Range continuing into North Park Basin. This same shear zone hosts a number of known hot spring locations, including those found near Craig and Steamboat Springs. This prompted investigation of unusually hot bore-hole temperatures and geothermal gradient for the basin.
High geothermal gradients (>2.2 deg F/100 feet) within the Niobrara formation are observed along the shallow southeast flank of the basin (<6,000 feet) indicated by Finn (2005). This is unusual since increased temperatures are expected to occur within more deeply buried Niobrara, especially at the eastern basin depo-center within the proposed oil window. Night-time thermal infrared satellite imagery was modeled to identify unexplained warm surface exposures, coinciding with known hot springs and predicting warm exposures along the shear zone corridor. It is proposed that these warm exposures reflect elevated subsurface temperatures.
Together, structural analysis and geothermal investigation have proposed a new model for a Niobrara resource play in the Sand Wash Basin. The shear zone corridor is not only the source of mechanical fracturing and folding by reactivated tectonic movement but also provides the conduit for super-heated, acidic brines and fluids from depth, invading the calcareous Niobrara and enhancing permeability through chemical means. The model also suggests that depth of burial may not be the only factor effecting hydrocarbon maturation and migration in this area. Finally, a Niobrara resource fairway is proposed for the southeastern flank of the basin characterized by high fracture areas, interpreted faulting, drag folding, high subsurface geothermal gradient, unusually warm surface exposures, and hot spring development.
URTeC 1618611
