Statistical Model for Source Rock Maturity and Organic Richness Using Well-Log Data, Bakken Formation, Williston Basin, United States: GEOLOGIC NOTES

Abstract

A study of the Bakken Formation, the proposed source rock for much of the hydrocarbons generated in the Williston basin, was done using bulk density, neutron porosity, and resistivity logs, and formation temperatures. Principal components, cluster, and discriminant analyses indicate that the present-day distribution of organic matter controls much of the variability in the log values. Present-day total organic carbon values are high in the central part of the basin near northeastern Montana and along the east edge of the basin, and low in the area of the Nesson anticline and along the southwest edge of the basin. Using a regression of density on temperature and the analysis of residuals from this regression, hydrocarbon maturity effects were partially separated from depos tional effects. These analyses suggest that original concentrations of organic matter were low near the limits of the Bakken and increased to a high in northeastern Montana. The pre-maturation distribution of total organic carbon and the present-day total organic carbon distribution, as determined by statistical analyses of well-log data, agree with the results of geochemical analyses. The distributions can be explained by a relatively simple depositional pattern and thermal history for the Bakken.