Sequence and mechanical stratigraphy: An integrated reservoir characterization of the Weber sandstone, western Colorado.

This doctoral dissertation combines rock physics with the spatial geometries of sediment bodies to model the elastic properties of the reservoirs and presents three contributions. First, it uses rock physics to extrapolate rock bulk modulus parameters, and demonstrate diagenetic control on mechanical stratigraphy in reservoir sandstones. Second, it uses rock physics to link sequence stratigraphic sedimentology with the wavelet-dependent seismic attributes that describe a seismic trace. Wavelet dependent seismic attributes are then extracted to relate seismic profile to geologic features. Third, in the absence of a pervasive joint-network architecture, the shear modulus was used to determine the overall rigidity contrast between different facies, showing how differential diagenesis influences mechanical stratigraphy. The dissertation consists of three studies focused on an area where dune and stream deposits intertongue. The first chapter describes the architecture of the facies sequences in the Weber Sandstone. This is important because fluid migration in basins reflect the control on sedimentology and stratigraphic framework. Previous studies concentrated mostly on the anisotropy of eolian dunes, showing compartmentalization based on six eolian reservoir zones interleaved with five laterally correlatable fluvial horizons. A core and multi-well gamma ray logs generated eolian and fluvial facies attribute, and sinuosity maps. When these are combined with stacking pattern of the lithofacies sequences over flattened fluvial event reveals the geometric relationship of sediment bodies, allowing interpretation of controls on sedimentation. The second chapter uses well-logs and rock physics to guide the interpretation of the seismic data. Gamma ray log relative stratigraphic eolian thickness attribute maps, and rock elastic data uses interpreted stratigraphy at well locations to predict the porosity, velocity and Acoustic Impedance. The rock thickness affects impedance and influences the reflection coefficient, thus enabling prediction of seismic amplitude and skewness of the phase, guiding analysis of the seismic profile. The third chapter examines the diagenetic controls on the mechanical stratigraphy. Influence of diagenesis on porosity and Vclay for local stratigraphic sections establishes diagenetic control on rock elastic velocity. Shear modulus enables interpretation of the relative rigidity using the overall rigidity contrast between less rigid eolian facies and the more rigid carbonate cemented and bioturbated eolian sandstone facies. The average rigidity contrast is higher for the carbonate cemented sandstones relative to the eolian dune facies when both are compared against the texturally immature fluvial horizons. An application of this approach is that faults are predicted to easily propagate through the eolian facies, important as pathways for fluid migration. Reworked eolian sandstone shows the lowest rigidity contrast and may have poorest reservoir properties