Detailed internal architecture of ancient distributary channel reservoirs using ground-penetrating radar, outcrop and borehole data-case studies: Cretaceous Ferron Sandstone, Utah

The main task in reservoir characterization research for increasing primary and secondary recovery, and reducing uncertainty in oil production forecasts is describing reservoirs in sufficient detail. 3-D reconstruction of reservoir architecture using outcrop analogs is hampered by limited exposure of mostly 2-D outcrops. Integrating ground-penetrating radar (GPR) and outcrop data can provide realistic 3-D models of reservoir analogs with explicit description and quantification of heterogeneities at different scales of investigation, from flow simulator voxel details to inter-well distances.

GPR is a high resolution geophysical technique that can provide indirect information on lithologic and petrophysical properties of shallow subsurface rock units at vertical resolutions down to a quarter of a meter and depth of penetration of tens of meters.

The procedure for, and utility of, applying 3-D GPR data to construct accurate 3-D reservoir analog models in ancient siliciclastic rocks, suitable for subsequent hydrocarbon flow simulation, is demonstrated through two case studies of distributary channel deposits from the Cretaceous Ferron Sandstone Member in east-central Utah.

Detailed architectural elements and bounding surfaces are identified and separated in outcrop and well-cores and then extended to GPR data. Radar facies characteristic to each element were interpreted based on the internal configuration and continuity of reflections as well as reflection termination patterns against higher-order bounding surfaces.

Point bars deposits in marine-influenced, lower delta plain channels show complex facies and geometries that resemble both fluvial, with upward fining grain size distribution and lateral migration of inclined surfaces, and tidally influenced point bars with extensive mud drapes on the inclined bedset surfaces, and upstream migration of the bedsets. The paleochannel bankfull width and depth are minimum 150 m and 4 m, respectively, but the upper part of the inclined beds are completely removed by erosion and we do not know the exact dimensions of the inclined beds.

Quantification of the heterogeneities at different scales inside the reservoir analogs is obtained through geostatistical analysis of different CPR attributes: relative amplitude, instantaneous frequency and instantaneous amplitude. In the alluvial channel sandstone at Coyote Basin the maximum correlation length ofthe dominant internal features ranges between 4 and 6 meters, and the anisotropy factor ranges between 0.6 and 0.95. The main heterogeneities within the point bar deposits at Corbula Gulch are shale drapes on third-order accretion surfaces, and mudstone intraclast conglomerate along third-order erosional surfaces of the inclined beds. The mudstones are longer in direction parallel to the flow (15 m) but are discontinuous (only 5 m continuity) because of deep erosion by the overlying inclined bed. In direction perpendicular to the flow (down-dip the inclined beds) the mudstone drapes are more continuous (10 m length).