Importance of Structural and Tectonic Inheritance for Unconventional Basin Play Prospectivity: An Example From the Powder River Basin

Summary In basins with unconventional plays, the importance of structural and tectonic inheritance on explorability and producability is oftentimes neglected. While a structural evaluation of the area of interest is routinely achieved locally (e.g., to predict natural fractures in the target zone), integration of this information into a regional framework is hampered by several factors – these can include a lack of appropriate company expertise, time and acreage constraints and the prevailing notion that structural information is less important for a particular unconventional play. The importance of an adequate regional structural evaluation is particularly relevant for the basin modeler, as basin models are used to predict maturity and fluid type by determining burial and thermal histories. In many basins, however, burial history alone does not adequately describe the observed maturity-controlling temperature gradient (e.g., Williston Basin). In these cases, an integration of structural and tectonic basin histories will commonly improve the accuracy and predictability of the basin model and basin maturity, respectively. This can be achieved in most cases by integration of available well information with regional datasets such as gravity and magnetics. In this study, we detail the integration of publicly available basin-wide structure data into a basin model for the Powder River Basin. Located in northeast Wyoming and southeast Montana, the Powder River Basin represents a foreland basin that formed during the Laramide orogenic event (Late Cretaceous – Early Cenozoic) when basement rock was thrust eastward loading the underlying crust and creating the Bighorn Mountains. As a result, sediment thickness and depth to basement typically increase towards the west and reach a maximum close to the major basin bounding fault system at the western edge of the basin. One would assume that any temperature distribution will follow that trend. However, analysis of available maturity and temperature data shows two major southwest-tonortheast trending thermal anomalies crossing the basin – a trend that is clearly at odds with the existing sediment thickness pattern. Hence, the temperature trend cannot be explained solely by burial history. We propose that temperature and temperature gradient in the basin is controlled at least partially by the makeup of the underlying basement. Comparison with magnetic anomaly data shows that the temperature trends can be correlated with magnetic highs that have been interpreted to represent major tectonic boundaries between several basement domains. We speculate that these domain boundaries represent pathways for heat to migrate upward through the basement into the sediment section. The resulting basin maturity distribution can significantly alter exploration and production strategies. The control of basement structures on heat flow and basin temperature distribution has been investigated previously. However, we argue that it has been underutilized when it comes to characterizing the impact of these datasets on unconventional play assessment. The method discussed herein represents an economic time-saving tool to improve the accuracy of play maturity and predictability of hydrocarbon commodities. This is especially important in unconventional settings where source presence is not a major controlling factor, but rather the goal is to delineate oil versus gas recovery. In these settings, mapping local or sub-regional maturity patterns are imperative for a successful exploration strategy.