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Deducing Reservoir Connectivity via Analysis of Slope Channel Fills Along a 35-km-long Longitudinal Transect of a Shelf Margin System, Tres Pasos Formation, Chile

Abstract

Thick (>100 m) successions of sandstone-prone slope channel deposits comprise important hydrocarbon reservoir targets worldwide. Key perspectives into reservoir connectivity in petroliferous slope channel deposits have been gleaned from analysis of seismic and well data. However, because many seismic volumes offer limited resolution (10s-100s of m), and well data only provides a 1-D perspective into lithologic patterns, understanding reservoir connectivity and the nature of petroleum traps in upslope and downslope positions is challenging. To elucidate reservoir connectivity in slope successions, we examine slope channel fills that crop out along a 35 km long depositional dip oriented transect in the Cretaceous Tres Pasos Formation in Chile. The objective of this study is to document slope channel fill and stacking patterns along the transect, highlighting application to reservoir connectivity and updip stratigraphic trap prediction. Along the entire transect, the most recognizable channel fill is 10-20 m thick, 200-300 m wide, and consists of thick-to-thin bedded sandstone. However, this channel fill style is much more prevalent downdip. In updip zones, channel fills are commonly composed of siltstone; thin-bedded coarse-grained units, such as conglomeratic lags, are sporadically preserved in the axial regions of these fills. Channel fill stacking patterns, which inform inter-channel reservoir connectivity, are assessed via examination of lateral (LO) and vertical offset (VO) values; LO records the distance between successive channel fill axes, and VO documents the elevation difference between successive channel fill bases. Overall, stacking patterns shift from dominantly lateral (LO: up to 528 m; VO: up to 36 m) to mainly vertical in nature (LO: up to 224 m; VO: up to 11 m) downdip, which suggests that vertical connectivity between sandstone-prone fills is greater downslope. The ratio of cumulative sandstone-prone channel fill thickness to gross interval thickness (FT:GT) provides another measure of reservoir connectivity. FT:GT increases downdip from 0.14 to 0.54 in the investigated channel belt, which indicates a decline in the proportion of intra-channel siltstone downslope. These results suggest that 3-D connectivity between sandstone-prone channel fills is likely lower in upslope zones, and becomes progressively higher downslope.