New stratigraphic interpretations, geochemistry, and petrophysics of the lower Mancos group, Douglas Creek Arch, northwestern Colorado, U.S.A.
Abstract
A study of regional stratigraphy, geochemistry, and petrophysics of the interval between the Lower Mancos Shale and the Dakota Formations on the Douglas Creek Arch was conducted to investigate its resource potential as an unconventional gas accumulation.The purpose of this study was to test the hypothesis that the Lower Mancos Shale is a potential exploration target on the Douglas Creek Arch and evaluate various organic facies within this interval.Additionally, a comparison of the lower Mancos group of northwestern Colorado to previous studies of this unit across the state was performed.To correlate separate, continuous, and mappable intervals in the Mancos B Shale to the Dakota formations, a study of well logs in this area was conducted.Within the stratigraphic interval thirteen key surfaces were identified: 1) Dakota Sandstone; 2) lower Tununk Shale; 3) Tununk Shale; 4) Frontier Sandstone; 5) Carlile Shale; 6) Niobrara 1; 7) Niobrara 2; 8) Niobrara 3; 9) Niobrara 4; 10) Niobrara 5; 11) Niobrara 6; 12) Niobrara 7; 13) Mancos Shale.Reservoir and source characteristics of each interval were investigated.The calcareous Niobrara Formation was identified on top of the Carlile Shale.To support the stratigraphic correlations and calcareous nature of the Niobrara Formation, an outcrop located in New Castle, Colorado was examined.Samples of the selected units were collected from the field.The petrology of the samples was analyzed.The identified stratigraphic units were tied to the results of geochemical analysis performed on drill cuttings from existing boreholes.This allowed for the identification of intervals with moderate to high organic content.Formations in the study interval that may have source rock generative potential are: lower Tununk Shale, Tununk Shale, all the intervals within the Niobrara Formation, and part of the Mancos Shale.Additionally, a basic petrophysical study of the identified intervals within the lower Mancos group was conducted including the calculation of water saturations and porosities.In the final stage of the project, structure and isopach maps of the separate, continuous intervals of lower Mancos group were created.Based on the results, potential source rocks and reservoirs were identified.This study provides an opportunity for additional exploration targets in
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