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Observations of Core UV Fluorescence of Middle Bakken and Three Forks Reservoirs, Williston Basin, North Dakota, Part 2. Integrating with Fluid Geochemistry to Identify Producible Oil Distribution and Estimate Reserves


Core UV fluorescence in the Middle Bakken (MB) and Three Forks (TF) formations results from oil present in the reservoir parts of the core and/or mineral fluorescence. Examination of core UV fluorescence in a large number of cored wells from North Dakota has identified different types of UV fluorescence that can be described as “blue-white”, “gold” or absence of it. The objective of the present study is to use a petroleum system approach and geochemistry methods to characterize and provide evidence for the meaning of the observed different types of core UV fluorescence with respect to producible oil. Identifying the distribution of producible oil in MB and TF reservoirs has a major impact on lateral well targeting, oil-in-place and reserves estimates in unconventional tight light oil plays. Targeted sampling of different types of core fluorescing intervals was performed on ten wells located at different maturity levels in Williston Basin. Over 200 samples were collected from both MB and TF reservoirs. The oil extracted from the core was analyzed for basic oil composition and biomarkers. Fluid inclusion analyses were performed on selected samples to identify elements of the charge history. The results were integrated with core petrophysical measurements and the sequence stratigraphic framework. The results show that there is a significant heterogeneity, often on less than a foot scale, in both reservoir rock properties and oil saturations. The core intervals with “blue-white” (BW) fluorescence have oil extracts exceeding 10 to 90 times the amount of oil from the core intervals with “gold” or no fluorescence. Moreover, the BW fluorescence is characterized by high hydrocarbon fractions content, which relates to higher API gravity oils that can be producible and higher porosity of the reservoir rock. MB and TF intervals with “gold” or no UV fluorescence are characterized by no or trace amounts of extractable oil (less than 0.01-0.04 wt % of the rock). It is possible the “gold” or no UV fluorescing intervals (with trace oil), especially in TF, have elevated gas saturations. The “gold” core UV fluorescence is attributed predominately to mineral fluorescence. In conclusion, the study presents robust evidence that the distribution of producible oil in MB and TF reservoirs is highly variable, with a strong control by both maturity and thickness of the source rocks, and can be recognized by the “blue-white” UV fluorescing intervals of the rock.