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Reservoir trends and anomalies in deep-water submarine canyon environments: 21st century progress and limitations derived from outcrop studies of the Upper Cretaceous to Paleogene Great Valley forearc, western California, U.S.A


Submarine canyons rank among the primary sediment capacitors along continental margins, yet the distribution of petroleum reservoir facies in canyons remains one of the main challenges for determining rock volume, net-to-gross and porosity. This study revisits classic outcrops of submarine canyon-fill and provides a reinterpretation of sediment transport and depositional processes, stacking patterns, stratigraphic architecture and implications for reservoir presence and quality through stages of canyon evolution. Coastal and mountainous outcrops of the Upper Cretaceous to Paleogene Great Valley forearc (Peterson 1965; Ingersoll 1981; 1990; Williams 1997; Lowe 2004) including the Carmelo, Pigeon Point, Panoche, Cortina and ‘Merle’ formations (Fig. 1) are well suited to evaluate the range and variability of conglomerate- and mudstone-prone lithofacies in three major zones of a submarine canyon environment: i) upper canyon; ii) middle canyon, and; iii) lower canyon to canyon-to-incised-channel transition zone. These zones of the submarine canyon exhibit a wide spectrum of architectural elements interpreted to represent generic canyon-fill as well as intra-canyon channels, levees, splays, and overbank sub-environments constructed of five primary lithofacies (Fig. 2): i) matrix-supported conglomerate interpreted chiefly as the product of cohesive debris flows; ii) clast-supported conglomerate interpreted as incomplete Lowe (1982) R123S123 sequences; iii) thick-bedded sandstone interpreted as partial or well developed Lowe S123 and Bouma (1962) Tabcde sequences; iv) medium- to thin-bedded sandstone and mudstone interpreted as partial Bouma Tabcde sequences, and, in part, M (sensu Lowe and Guy 2000) and H (sensu Haughton et al. 2009) divisions and; v) mudstone. Overall, these deposits represent the products of high- and low-density turbidity currents, slurry flows to transitional flows (Lowe and Guy 2000; Haughton et al. 2009; Kane et al. 2012; Power et al. 2013; Rotzien 2017), cohesive debris flows and hemipelagic to pelagic sedimentation. The highest variability in scale occurs in the matrix-supported debris-flow deposits; individual sedimentation units can range from centimeters to tens of meters in thickness. The fill of the canyons in this study area typically reveal five evolutionary stages: i) basal incision or mass evacuation of sediment or rock; ii) bypass and deposition of either mudstone or poorly sorted matrix-supported debrite-rich intervals; iii) inception of high-energy, highly erosional conglomerate- to sandstone-rich flows depositing generally lenticular turbidite bedding; iv) deposition of laterally extensive turbidites indicating late-stage canyon and channel filling, and; v) mudstone deposition and abandonment creating the potential for a canyon-wide seal (Rotzien 2018 and references therein). Stacking patterns and spatial arrangement of lithofacies, large-scale bedforms, erosional surfaces, and abundant disturbed strata indicate canyons host the highest energy sediment gravity flows in a deep-water depositional system. Conversely, laterally equivalent frontal splay and overbank packages demonstrate lower energy depositional environments with statistically lower degrees of heterogeneity based on bed type, bed thickness, bed length, matrix type and grain size distributions (Schwalbach et al. 2009). These somewhat underemphasized observations on the nature of submarine canyon environments call into question the efficacy of targeting canyon-fill successions in modern deep-water exploration and development drilling programs, and may lend additional credibility to exploration toward potentially lower net:gross, yet also lower uncertainty depositional environments such as frontal splay complexes located at the distal terminus of the depositional system. While this study provides a remarkable window into submarine canyon environments in a forearc depositional system, these data also have analog value to petroleum reservoir type and architecture in passive to active margins including foreland, strike-slip, hybrid and intracratonic basins more generally.