Bluebell Field oil production is unconventional in that fractures provide the preponderance of the storage capacity and the fluid pathways in this tight sand/carbonate reservoir. Identifying fracture density away from existing well control would allow a drainage area to be mapped and would aid in determining the well spacing needed to effectively drain remaining hydrocarbon reserves in Bluebell Field. An added benefit of identifying natural fractures is the possibility of uncovering additional new drilling opportunities in zones not previously considered as targets and thus adding new reserves.
A 35 square mile wide azimuth 3D seismic program was conducted in 2010–2011 over a portion of Bluebell Field. One of the goals, based on the seismic data, was to locate areas of high fracture intensity in the productive Eocene Green River Formation to Paleocene Wasatch Formation interval and to match that against cumulative fluid production to find under-drained areas for future well locations. Post-stack multi-attribute statistical analysis was conducted over three reservoir intervals in the Wasatch interval since the Wasatch had produced over 80 percent of the cumulative production. The analysis consisted of seismic attribute selection and optimization by step-wise multivariate linear regression using cross-validation techniques which included hidden well prediction. After extensive analysis of over 300 attribute maps, a combination of seven seismic attributes was eventually chosen for the statistical modelling and comparison with production data. The resultant map of predicted fracture network was then correlated with cumulative fluid production from the first Wasatch well in each section. Best results (0.842 correlation coefficient) were obtained over one of the three modeled reservoir intervals. This reservoir interval, approximately 700 feet thick, correlates with the ‘most prolific oil-producing zone’ (geologic marker W180-WAS200) that had been identified from previous geologic work.
Practical application of this technique along with offsetting well drainage patterns has enabled Devon Energy to high grade future drilling locations.