The Powder River Basin (PRB) unconventional source rock play in Upper Cretaceous Niobrara Formation is exploits up to four different landing zones, although well spacing and field development are still being optimized. Wine rack development can be improved with an understanding of produced fluids as a function of wellbore placement as well as regional geology. Within the context of thermal maturity, our study addresses whether Niobrara oils exhibit unique geochemical signatures stratigraphically among landing zones and regionally across the basin.
We use high-resolution gas chromatography (HRGC) to identify inter-paraffin peaks, which are highly diagnostic of subzones to geochemically fingerprint and differentiate among oils produced from different sub-zones within the same formation. Those HRGC parameters were coupled with standard oil geochemistry (whole oil GC) and API gravity to scrutinize oils produced from 16 horizontal wells completed in the Niobrara Formation. Each well represents one of four sub-zones of the Niobrara Formation that are defined by petrophysical properties and lithofacies variations: Upper B Marl, Lower B Marl, B Chalk, and C Chalk. Observations were tied back to source rock thermal maturity indicator data, producing gas-oil ratios, geosteering interpretations, and performance metrics.
Niobrara oil compositions cluster regionally, as seen from a set of key molecular ratios (e.g., pristane/phytane, pristane/nC17, phytane/nC18), indicating subtle regional changes in kerogen type and thermal maturity. Inter-paraffin peak ratios plotted using Principal Component Analysis (PCA) show geochemical composition separation among landing zones and highlight anomalous wells. Further, most Niobrara oils tie closely to predicted fluid phase based on thermal maturity, though some oil compositions point to an overprint of higher maturity fluid that is associated with increased GOR and stronger well-performance. These wells are interpreted to lie along a path of preferred migration from basin locations of higher source rock maturity to locations of lower rock maturity.
Produced oils from standalone wells in each landing zone are shown to be representative geochemical end members, which could be used for production allocation to determine the vertical drainage overlap of wells in different layers. The strong relationship between bulk fluid phase properties (API gravity and GOR) and geochemical source and maturity parameters (e.g., expulsion temperature) illustrates that the Niobrara generally behaves as a self-sourcing system with important exceptions (i.e., evidence of regional migration). Further, this work demonstrates the utilization of oil geochemistry and thermal maturity measurements as predictive tools for modeling produced fluid phase in this formation.
