Integrated seismic and geologic analysis of Waddle Creek field in southeastern Moffat County, Colorado

Abstract

The Niobrara Formation is a self-sourced resource play and unlike in conventional reservoirs, key production can occur in "sweet spots" that may extend down-dip off structure. Mapping the location and orientations of high-density fracture zones is key to identifying these productive areas in tight petroleum systems. This study integrates a 3D seismic interpretation with well log analysis and forward modeling in order to determine how high density fracture and fault zones in the Niobrara relate to regional structures and paleo-tectonic controls. Seismic curvature, variance, and ant-tracking attribute maps show 5 major thrust faults, several smaller reverse faults, a few normal faults, and a number of linear features below seismic resolution. The features identified on these attribute maps suggest transpressional tectonics with an overall WSW-ENE direction of shortening from the Laramide orogeny. During compression, the rigid Colorado Plateau to the southwest reinforced the orientation of pre-existing NW-SE trending faults in the study area resulting in strain partitioning. This formed a complex transition zone between two major NW-trending thrust faults. Production distribution suggests the existence of fracture swarms associated with this transition zone with the best production occuring along linear features that align with expected orientations for conjugate fracture sets in a left lateral strike-slip fault system. Isochrons of key time intervals highlight three major tectonic periods: Precambrian extension, Pennsylvanian uplift and erosion, and Late Cretaceous to Middle Tertiary compression. These tectonic periods are separated by intervals of quiescence and widespread, horizontal deposition. Forward modeling of the study area tests the validity of the structural history interpretation and highlights areas of high strain that may have resulted in fracture formation. The strain analysis suggests that medium amounts of strain help improve production but extremely high strains result in dry holes. One possible explanation is that overly intense fracturing allows water to flow easily through the area resulting in cementation whereas too little fracturing prevents production and thus a "medium" amount of strain is necessary to allow for production but avoid cementation. This integrated approach using seismic and well log analyses may be applied to explore other trends within the Sand Wash basin and nearby regions particularly where pre-existing structures affected stress and strain distributions during the Laramide orogeny. The relationships identified in this study between current stress regimes and paleo-tectonic controls allow for better identification and prediction of "sweet spots" when pre-existing structures affect the distribution of stress and strain. In areas where good seismic and well log data are available, the same procedures conducted in this study may be applied to produce potential high-density fracture and fault zone maps that may then be analyzed to determine the most prospective locations. Even in areas where seismic and well log data may be limited, these relationships permit exploration play trends to be high-graded for further data acquisition and analyses.