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The Variability of Reservoir Quality in Submarine Slope Channel Complexes: Insights From an Outcrop Analogue, Tres Pasos Formation, Chilean Patagonia


Submarine slope channel complexes are important, but heterogeneous exploration targets. Owing to the sub-seismic nature of most heterogeneities, reservoir quality and connectivity prediction can be challenging. Outcrop-based architectural studies augmented with detailed bed-scale observations can be used to bridge the gap between seismic data and detailed reservoir models. This study investigates the stratigraphic evolution of two seismic-scale slope channel complexes from the Tres Pasos Formation, southern Chile. Architectural differences between an early phase of lateral channel migration and a late phase dominated by aggradation of vertically aligned channels are investigated. Sedimentary logging, petrographic analysis and quantification of sedimentological characteristics were used to understand reservoir quality at different stratigraphic hierarchical orders (i.e., channel elements, complexes). From channel element axis to margin, bedding changes from amalgamated to non-amalgamated, laminated and ripple dominated sandstones and thin bedded turbidites. Abundant mudstone intraclast lags overlie erosion surfaces, which are commonly mantled by mudstone drapes. The lower channel complex (min. 500 m wide and 40 m thick) is composed of channel elements deposited during several lateral cut and fill cycles, which makes a distinction of each element complicated. Towards the top, a thinner element (10-15 m thick; up to 400 m wide) has an asymmetrical fill and a thick mudstone drape (up to 1.4 m) overlying its basal surface. The overlying vertically aggrading channel complex is 80 m thick and separated from the underlying channel complex by MTDs. Channel elements are 15 to 20 m thick, up to 400 m wide, and characterized by symmetrical fill with mudstone-draped basal erosion surfaces. The early stage slope channel complex was subject to multiple cut and fill phases instead of constant lateral migration, resulting in a high degree of facies heterogeneity due to prevalent mudstone drapes and intraclast lags (i.e., potential flow barriers and baffles). Channel element morphologies tend to be more variable with apparently thinner, asymmetrical fill at the top, recording development of high sinuosity. This variability leads to a more complicated distribution of reservoir facies compared to the overlying, vertically aggraded channel complex. Our study allows refinement of reservoir models for submarine slope channel systems, reducing risk and uncertainty in these challenging targets.