Geologic comparison of the oil window and dry gas window within the Tow Creek member of the Niobrara Formation in the Piceance Basin of northwestern Colorado, A

Abstract

The Piceance Basin of northwest Colorado has historically produced significant amounts of oil and natural gas from shallow Upper Cretaceous Mesa Verde Group reservoirs. In the past decade a deeper target has earned attention within the Mancos Shale interval. The Niobrara Formation is a proven resource in the highly explored Denver Basin. More recently it has become a target in western basins, such as, the Sand Wash, San Juan, and Piceance Basins of the Rocky Mountains. Although compositionally the Niobrara differs within the Piceance Basin, production from the interval has taken place in large quantities. Varying organic thermal maturities exist in the same stratigraphic interval across the basin. This is seen with the production of oil in the north and the production of dry gas in the south. This study analyzes the strata in each of these locations in an attempt to better understand why differing levels of maturity exist within the same stratigraphic interval and what factors are affected by this differing maturity. Through the use of core and thin section descriptions, six fairly similar lithological facies with slightly differing depositional environments were determined. Bulk mineralogy shows a major difference from the Denver Basin, with an introduction of a significant amount of siliciclastic material. However, within the Piceance, both the northern and southern Niobrara strata contain a very similar make up consisting of a near even mix of siliciclastic, carbonate, and clay material. This mixture classifies the strata as a mixed mudstone. Source rock analysis confirms the two different maturity levels within the basin and highlights the good to very good amount of total organic carbon present. Subsurface mapping correlates the strata across the basin and shows a thickening trend within each member of the Niobrara to the northwest. Fracture analysis through the use of borehole image logs, shows a structurally complex basin that causes sporadic quantities and orientations of natural fractures. FE-SEM analysis demonstrates a dramatic increase in organic microporosity with increased thermal maturation of the Niobrara source rock. This study helps to improve the understanding of two viable hydrocarbon plays within the Piceance and adjacent basins. As the demand for hydrocarbons continues to increase, more exploration into this type of dynamic unconventional petroleum system is necessary. This study has brought to light the main features contained within this petroleum system. Future work will help to better narrow the target of the location of sweet spots and how to predict the presence and effect of key features such as natural fractures.