This study, which focused on the origin and movement of oils in the Three Forks, was successful in spite of the scarcity of significant stains within the Three Forks. The key to our success was oversampling and screening to select the best stain samples. This study also showed that an accurate appraisal of maturity of extracts from both source rocks and stains could be achieved using some new interpretational techniques. Of particular value was our development and use of a “basket” of several molecular maturity indicators, which helped improve the signal-to-noise ratio.
We produced a map of relative maturity in the Bakken source rock across the entire study area. In general, Bakken maturity correlates well with present-day burial depth, but some anomalies exist. These anomalies suggest that present-day burial depth is an imperfect proxy for thermal maturity. Other factors, such as variations in regional heat flow, thermal events related to volcanism or convective events, structural lineaments, and differential amounts of uplift probably play a role.
Comparison of maturities of migrated and unmigrated fluids allowed us to determine with confidence the maturity range within which the main generation and expulsion windows fall in both the Bakken and Three Forks.
Stains from the Three Forks were partly correlated with Bakken source rocks, but other organofacies also appear to have contributed. Various possible explanations are considered. Migration pathways are discussed in some detail.
Our map of maturity of the Three Forks stains is extremely interesting, and has important implications for possible lateral migration within the Three Forks. Some data suggest a possibly important role for regional fracture networks associated with tectonic lineaments. This concept might lead to new play concepts within or beyond the conventional play limits. Unusual characteristics of the Parshall Field are also discussed.
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