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Mapping the Extent and Distribution of Oil Formation in the Upper Bakken Formation, Williston Basin

Lewan, M. D.; Marra, Kristen; Lillis, Paul G.; Higley, Debra K.; Gaswirth, Stephanie

The extent and distribution of source rock that has generated and expelled oil are important elements in defining a petroleum system and in determining whether an accumulation consists of migrated or in situ (i.e., locally sourced) oil. This study uses hydrogen indices (HI) from Rock-Eval analyses on 813 samples from the upper part of the Bakken Formation from 633 wells to map the extent and distribution of its oil formation in the Williston Basin. With calibration, HI has been shown to be a good proxy for kerogen atomic H/C ratios, which are directly related to the amount of generated oil expelled from a source rock based on hydrous pyrolysis. Previous studies have shown that the relationship between HI and atomic H/C ratio is not correlative at HI values greater than ~400 to 450 mg/g TOC. At values higher than these, the atomic H/C ratios remain constant, which indicates no oil formation is occurring despite differences in the HI values. The upper HI limit for the onset of oil formation from the upper Bakken Formation was determined by comparing atomic H/C ratios on kerogen isolated from 35 samples that represent a full range of HI values from 113 to 544 mg/g TOC. The narrow range of stable carbon isotopes (δ13C = -29.8 ± 0.4 ‰) of the kerogens indicate that they all have similar precursors and therefore, are comparable. The relationship between kerogen atomic H/C ratios and HI indicate that the onset of oil formation occurred at an HI of 450 mg/g TOC. A hydrous pyrolysis relationship between atomic H/C ratios and transformation ratios (TR) for generation of expelled oil indicates a TR of 0.10 at an HI of 400, 0.25 at an HI of 300, 0.47 at an HI of 200, and 0.77 at an HI of 100. The resulting TR-contour map shows that the Bakken Formation in the Parshall field occurs updip from the onset of oil formation (450 HI contour), which indicates a migrated oil accumulation. Conversely, Bakken Formation in the Elm Coulee field occurs primarily between the 0.10 and 0.62 TR map contours, which suggest an in situ oil accumulation. The TR-contour map also provides a means of evaluating oil-formation kinetic parameters derived by various pyrolysis methods and calibrating thermal-history models with respect to oil formation when vitrinite is scarce or its reflectance is suppressed.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013