Supplementing Subsurface Characterization with Insights from Outcrop: Sandstone Distribution in Slope Deposits of the Tres Pasos Formation, Chile

Extensive outcrop belts of deep-water deposits (e.g., Delaware Basin, West Texas; Karoo Basin, South Africa) provide tremendous insight into analogous, hydrocarbon-bearing units in the subsurface. Outcropping Cretaceous strata (Tres Pasos and Dorotea formations) of the Magallanes Basin, Chile represent a complete continental slope-scale profile >1500 m thick. Mappable geomorphic features include toe-of-slope sandstone bodies, mass-transport deposits, slope readjustment surfaces, intraslope sediment accumulations, slope clinoforms up to 650 m thick, the shelf-slope break, and shelf to non-marine sandstone bodies. The complex stratigraphic architecture is similar to that of the seismically mapped Brookian succession of the Alaskan north slope.

The lower slope to toe-of-slope region represents an important locus for coarse-grained sediment deposition in the depositional profile. Two basinward-stepping toe-of-slope composite sandstone packages are the focus of detailed investigation. Each of these packages is 10's of meters thick and at least 10 km in length along depositional dip and shows a repeated downstream change in stratigraphic architecture. Sediment bypass surfaces characterized by intra- and extra-basinal clast conglomerate lag deposits correlate down slope to lenticular bodies interpreted as channel complexes (~400 m wide and <20 m thick). More distally these units transition into high aspect ratio depositional elements interpreted as channelized sheets. Toe-of-slope sandstone bodies are overlain by concordant mudstone and siltstone or widespread, fine-grained mass-transport deposits. Detailed reconstruction of the stratigraphic architecture provides invaluable information that can be used to supplement data in hydrocarbon-prone basins characterized by sparse 1-D wellbore information and widespread, but relatively low resolution seismic datasets. Stacking patterns and mapped distribution of strata provide information at exploration to reservoir modeling scales.