Petroleum potential for the Gothic Shale, Paradox Formation in the Ute Mountain Ute Reservation, Colorado and New Mexico

Abstract

The Gothic shale of the Middle Pennsylvanian Paradox Formation, Hermosa Group in the Paradox Basin of Utah and Colorado is an excellent organic-rich source rock. The organic rich Gothic shale, along with the Chimney Rock, Hovenweep and Cane Creek shales, are thought to be the source for the oil and gas in the Paradox Basin. These source rock units have provided the oil for many conventional fields in the basin. The basin contains prolific oil fields, such as Aneth, which has produced over 440 million barrels of oil. The fields also have produced approximately 1TCF of gas. The Ute Mountain Ute Reservation (UMUR) is located in the northwestern portion of New Mexico and southwestern corner of Colorado. Thermal maturity increases from southwest to northeast for the Gothic shale in the Paradox Basin. The basin has a uniform heat flow in the interior of the Colorado Plateau which may indicate the lack of significant crustal thermal sources. The tectonic stability of the Colorado Plateau may have also imparted a high degree of thermal stability. The UMUR lies within an unconventional continuous-type oil and gas play of the Paradox Basin where the Gothic shale has total organic carbon (TOC) contents ranging from 0.72-2.05 wt. %. The Gothic shale contains oil prone Type II, Type II-III mix, and gas prone Type III kerogens. Measured vitrinite reflectance (Ro) values of 1.17% and 1.32% were observed from Gothic shale samples of 5988 ft (8-21 Antelope well) and 6141.3 ft (34-13 Grouse well) in the UMUR area indicating maturity equivalent to the condensate-wet gas zone. The measured Ro values increases to 1.49%, on the western portion of the UMUR, indicating maturity here that is in the dry gas window. The Ro value of 1.49% was observed at a depth of 8771.5 ft. in well Ute Mountain 44-34. The northern border of the UMUR is less than fifteen miles south of Bill Barrett Corporation's (BBC), Denver, CO, successful Gothic Koskie wells (designated the Pedro Field). The BBC is currently producing gas from horizontal wells completed in the Gothic shale. The Gothic shale here contains good gas content; acceptable TOC, extremely brittle rock and an overpressured reservoir have contributed to the excellent development of a producing area. Cumulative gas production rates have ranged from 1.5 to 4.9 MMCF/D (June, 2010) with some associated condensate. The cumulative production is 1.16 BCF June 2010. The Gothic shale has many similarities to other successful gas shales. Proper completion is the key. A small fracture stimulation on a tight rock yields a small gas well. However, a larger fracture stimulation would increase the gas production rates. Currently BBC is working on fracture stimulation design and evaluating the overall commerciality of the play. Recent advances in technology and horizontal drilling now allow production from these low permeability shales, which once were thought to be too "tight" to produce. Comparisons between mapped Gothic shale thickness, programmed pyrolysis and geochemical data in the UMUR and BBC exploration areas indicate these prospective shales extend southward from the BBC shale play area to the UMUR area.