ABSTRACT

Well-production data are utilized here to quantify the heterogeneity of oil and gas reservoirs and plays in the American portion of the Williston Basin and surrounding region. Variation coefficients (defined as VC= (F5-F95)/F50, where F5, F95, and F50 are the 5th, 95th, and 50th [median] fractiles of a probability distribution) are calculated for cumulative production in older wells and are examined with respect to geologic characteristics of reservoirs, conventional versus unconventional accumulations and plays. Cumulative production in older wells (as an estimate of ultimate recovery) measures the cumulative effect of complex geologic, engineering and economic processes.

Our analysis shows that Bakken oil plays of the Williston Basin and Sunburst/Bow Island and Bowdoin gas reservoirs of northern and central Montana are, in terms of cumulative production, the most homogeneous plays and reservoirs and are thus the most predictable. The Bakken oil reservoirs and Bowdoin Dome Gas Field were each identified as portions of large continuous-type (unconventional) plays in the United States Geological Survey 1995 National Petroleum Assessment.

Within the Williston Basin and central Montana, Winnipeg, Nisku, and Tyler oil reservoirs are the most heterogeneous and the least predictable with respect to cumulative production. Carbonate reservoirs of north-central Montana and the Williston Basin tend to exhibit low to intermediate cumulative production heterogeneity, whereas sandstones exhibit intermediate to high heterogeneity.

These results indicate that quantitative measures of production variability, expressed as dimensionless coefficients of cumulative production, are potentially valuable tools for documenting geologic reservoir heterogeneity in older fields.