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Quantitative Outcrop Characterization of Incised Valley Fill Combining UAV-Based Photogrammetry and Traditional Geologic Field Methods


Tidal regions represent an important zone of transition from predominantly fluvial to marine processes. The character of these tidal environments and geologic record they contain are highly sensitive to changes in relative sea-level. Within the geologic record, transgressive deposits have received less attention than their regressive counterparts. Of these, estuarine incised valley systems have received the greatest attention. Though estuarine incised valley fills (IVF) possess significant potential as hydrocarbon reservoirs, the dynamic nature of paralic systems make them especially heterogeneous, introducing substantial uncertainty to reservoir volume/performance prediction. Outcrop analogues provide a unique opportunity for the quantitative characterization of geologic heterogeneity inherent in these systems, providing spatial statistics required for improved reservoir forecasting. This research presents a quantitative characterization and geologic model of IVF from the Cretaceous Straight Cliffs Formation of southern Utah.

A 3D exposure 1 km long and 300 m wide, oriented slightly oblique to depositional dip, consists of two shoreface successions (15-30 m total thickness) unconformably overlain by 5-30 m of IVF. A digital terrain model (DTM) was created using Structure from Motion (SfM). Decimeter-scale stratigraphic sections and paleoflow measurements were coupled with the DTM, with the model serving as a 3D canvas for geologic interpretation. Here, IVF deposits consist of fine-medium grained sandstone, similar in texture/composition to the underlying shoreface succession. Thick-bedded sandstone deposits form discrete geobodies 120-300 m-wide and 1-5 m-thick separated by laterally continuous 0.1 to 0.5 m-thick silt/mudstone. Smaller lenticular sandstone bodies and contorted siltstone beds are also observed. Depositional facies are grouped into 4 facies associations: 1) shoreface, 2) mid-estuarine bar, 3) tidal channel, and 4) debris flow. Bed- to geobody-scale dimensional data quantify the thickness, aerial extent, stacking and internal architecture of facies and facies associations. Quantitative results are synthesized in a digital outcrop model (DOM) capturing the 3D framework and spatial facies relationships. Sandstone connectivity is limited, occurring where silt/mudstones have been eroded by subsequent high-energy deposition. Sandstone bedding/textures are commonly indistinguishable between the channel and bar-related facies associations.