3D GPR characterization of sandy mouth bars in an outcrop reservoir analog: Cretaceous Ferron Sandstone, south-east Utah

Outcrop analog studies have periodically been used to understand the complex sedimentology and stratigraphy of subsurface reservoir architecture for accurate well placement and enhanced oil recovery (EOR) planning during field development. Ground-penetrating radar (GPR) has greatly facilitated analog outcrop study progress as it is relatively high resolution, inexpensive, quick, and has a several meter depth of penetration. A 3D GPR survey was conducted to visualize architectural elements of friction-dominated distributary mouth bars within proximal delta front deposits in Cretaceous Ferron Sandstone at the top of the Notom Delta in south-east Utah. Sensors and Software’s Noggin SmartCart 250 MHz was used over a 25 m x 15 m grid. We employed an orthogonal acquisition geometry and a spatial sampling of 0.5 m for the inline (dip direction) and 1.5 m for the crossline (strike direction). Standard processing flows including time-zero correction, dewow, gain, background subtraction and 2D migration were used to increase the signal-to-noise ratio. Formation velocity estimates from the hyperbola matching yielded 0.131 m/ns which is comparable to the literature velocity of about 0.125 m/ns. The calculated average dielectric constant (directly related to volumetric water content) is 5.2 matches unsaturated sandstone. The depth of GPR penetration is limited to approximately 3 m - likely due to the compaction/carbonate cementation in the rock and interbedded layers of finer-grained material contributing to higher attenuation of the GPR signal. The vertical resolution is about 0.125 m, enabling the imaging of the dune-scale cross sets (15-20 cm thickness). Calculation of the medium porosity via an adapted Wyllie Time Average equation yields 7.8 % which is consistent with the average porosity (5-10%) obtained from the literature. Bedding
diagrams from local cliff exposures show gently NE dipping accretion of single large foresets that were interpreted as small scale unit bars, which are the building blocks of the large mouth bars. The GPR radargrams are not only capable to image that, but also reveals their 3D shape. A closer look into GPR images also reveals the distinction between various proximal mouth bar facies: upper friction-dominated dunescale cross beds and bar scale large foresets from lower inertia-dominated basal planar beds.