Subsurface description and modeling of geologic heterogeneity in large subsurface datasets : using temporal and scalar hierarchies, Powder River Basin, WY and MT, U.S.A.
Melick, Jesse John
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Three-dimensional fluid-flow simulation models provide attractive tools for understanding the potential behavior of the subsurface. Retention of high-resolution geologic heterogeneity in the characterization of large volumes presents significant challenges to this modeling. A 2D dataset donated by Industry constrains a hierarchical stratigraphic framework based on 30,000 wells with log curves, 60 surfaces crossing the 70,000 cubickilometer Powder River Basin from Precambrian basement to top of the Cretaceous Lewis Shale. Five sedimentary systems subdivided into 25 stratigraphic intervals make up the 3D representation of 70 discrete modeling areas. These sedimentation regions group distinct sedimentary attributes (e.g., porosity, thickness, sedimentary architecture). These attributes relate to suites of rock properties, such as porosity, percentage of thickness with porosity and well log shape, which were compiled from 4000 wells with donated/purchased log ascii files, 15 cores, 300 wells with public core plug data, 115 published oil field reports, and basin rimming outcrops. Sedimentary system analysis considered regional controls on the depositional setting from the craton-scale to the pore-scale and it employed techniques to group information and replicate the effect of fine-scale geologic heterogeneity in a static reservoir model. This process highlights the importance of understanding the role of tectonic anisotropy on the preservation of stratigraphic sequences when interpreting the depositional environment. Subdivision into the 70 sedimentation regions permitted calculation of the gross pore volume in each sedimentary system, using total porosity and a percentage of the vertical thickness for each modeling volume. The total volume calculated depended on the method; stratigraphic layering and sedimentation regions provided 600 cubic kilometers and equating to storage capability of over 250 gigatons of supercritical carbon dioxide, whereas using factors and no stratigraphy, the total volume was calculated at 460 cubic kilometers. Pore volume distribution in the subsurface is more accurately characterized with high-resolution stratigraphic and sedimentation region analysis. Integrated tectonic analysis provides context that better constrains the application of outcrop analogs and depositional models, which guide sedimentation region analysis. This dissertation addresses the impact of geologic heterogeneity from crustal anisotropy to distributions of porosity and permeability and provides a tool to assess feasibility of gigaton-scale carbon dioxide sequestration.