3-D Gpr Imaging Of Complex Stratigraphy Within The Ferron Sandstone, Castle Valley, Utah

Abstract

Two detailed 3-D Ground Penetrating Radar surveys were recorded on outcrops of the Ferron Sandstone in Castle Valley, Utah in an effort to image internal sedimentary structures. The Ferron Sandstone is a fluvial to proximal marine sandstone which is considered to be a good analogue for some Gulf Coast oil and gas reservoirs. Because of its excellent exposure in Castle Valley, the stratigraphy of the Ferron has been, and continues to be very extensively studied to determine the relationships of bedforms to the distribution of reservoir properties. Our goal in imaging these outcrops with 3-D GPR is to interpret reservoir heterogeneities at interwell scales. Correlation of these heterogeneities to known porosity and permeability distributions from outcrop studies will be used to establish three dimensional scales of reservoir flow behaviour. These data will subsequently be used to improve flow simulations of analogous oil and gas reservoirs. Results of this study indicate that 3D GPR is indeed suitable for this application. The first 3-D survey was recorded on a 15m by 15m grid with traces recorded every 25cm in both directions. Adjacent outcrop showed the near-surface sandstones to be of a tidally reworked deltaic lithofacies, overlying lower coastal plain carbonaceous siltstones. More than 4m of penetration and finer than 25cm resolution were achieved using a PulseEKKO IV GPR system with 200 MHz antennae. The second 3-D survey was recorded about 250m away from the first, over a stacked distributary channel to nearshore sandstone lithofacies. 100 MHz antennae were used over a 25m by 25m grid, with 5Ocm spacing in both directions. More than 8m of penetration and finer than 50cm resolution were observed. A 2-D line with 50 MHz antennae and 1 .Om trace spacing was recorded to connect the two 3-D surveys and to provide a more regional picture of stratigraphy. Reflections were recovered from as deep as 20m. The data were processed both at Chevron, using the Promax seismic processing system, and at UT Dallas, using in-house software. Data were flattened on the air arrival, gained, spatially smoothed and then migrated using a fixed-offset prestack migration. Results were visualized with VoxelGeo, a software package which allows the user to selectively render the volume with transparency to see the interior of the data cube in 3-D perspective. Connectivity analysis within the cubes highlights discrete reflectors, and by implication, the main flow units of the facies. Work is underway to correlate the vertical and horizontal scales of heterogeneity from the GPR data, with the facies’ permeability distribution from outcrop studies.