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Downslope Variability in Deep-Water Slope Channel Fill and Stacking Patterns: Insights From Outcrop and Shallow Seismic Analogs

Abstract

Lithologic variations in deep-water slope channel systems are commonly examined using outcrop and seismic data to elucidate reservoir heterogeneity in analogous petroliferous systems. Though many studies have described these variations along depositional strike (across-channel), few have focused on deciphering down-depositional-dip changes due to: (1) the paucity of along-slope perspectives afforded by most outcrops; or (2) the limited resolution of seismic data. To inform hydrocarbon exploration in deep-water slope systems, we report sandstone proportion trends for slope channel systems of the Cretaceous Tres Pasos Formation (Chile) along a dip-parallel 40-km-long outcrop belt. Results are compared with data from a 46 km-long dip-parallel segment of a seismically imaged channel system from the Pleistocene Benin-major Canyon (offshore Nigeria). We use these data to: (1) elucidate along-slope sandstone proportion changes for both systems at various scales; and (2) assess how channel stacking patterns affect common reservoir prediction calculations (i.e., net reservoir thickness to gross interval thickness ratio; NTG).

For our analysis, we examined data for slope channel elements (up to 30 m thick; 400 m wide), and for channel complex sets (>100 m thick; >1000 m), which are composed of packages of stacked channel elements. Outcrop data were derived from 150 measured sections (total thickness: 4587 m) and 9 depositional-strike-oriented cross-sections from along the dip-oriented transect. Measured section results show that mean channel element NTG increases from 67% to 88% downdip. Cross-sections from the outcrop data set show that channel complex set NTG does not vary systematically downdip. NTG values at this scale are markedly lower than for channel elements, commonly spanning 10-40%. Seismic data were derived from 24 depositional-strike-oriented cross-sections from along the Benin-major Canyon; due to the limited resolution of the data set, only channel-complex-set-scale trends could be assessed. NTG values for the seismic data set also do not vary systematically along dip, and often span 15-35%. Overall, these findings imply that sandstone proportion increases downdip in channel elements, and that NTG values at larger scales are tied to stacking patterns. The results provide insight into depositional trends in deep-water systems, and can inform reservoir prediction in analogous petroleum systems globally, including the nature of updip stratigraphic traps.