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Bypass-Dominated Mud-Rich Channel-Fill Deposits of the Paleogene Scripps Formation at Tourmaline Beach, San Diego, CA: An Analog for Low Net-to-Gross Slope Canyon and Channel Systems

Abstract

Over the past two decades, hydrocarbon exploration for turbidite channel reservoirs has been mostly focused on sediments deposited within mid-slope to lower slope environments of passive margin systems. In recent years, exploration has also been focused on the hydrocarbon prospectivity of channelized systems in more proximal slope settings. The Paleogene Ardath and Scripps Formations are exposed in a series of well-exposed coastal cliff outcrops in the vicinity of San Diego, CA, USA. These outcrops are composed of mud- and sand-rich submarine canyon and channel-fill deposits, which are interpreted to have been deposited in proximal submarine slope environments. As such, they are potentially a useful analog for comparing to proximal canyon systems viewed on seismic, and for reservoir modeling purposes. At Tourmaline Beach, the Scripps Formation is composed of a series of stacked channel elements within a larger-scale slope canyon or valley. These elements are stacked in an organized architectural relationship, with successively younger channel elements offlapping southward. The channel fill deposits are dominated by low net-to-gross (<20%), thin-bedded, heterolithic deposits and erosionally-based intervals of highly deformed, interbedded, mud-rich sediments. Thick-bedded sandstone deposits are rare. These channel deposits are interpreted to be dominated by a combination of thin-bedded, mud-rich turbidites and mass transport deposits. The thin-bedded turbidites are interpreted to reflect downslope bypass of turbidity currents. The stacked, organized pattern of the channel architecture reflects the underfilled nature of each erosional channel element, in which channel abandonment relief attracts subsequent turbidity currents and localizes successive channel elements in proximity. The low net-to-gross of the mud-rich bypass sediments and mass transport deposits highlights the fact that reservoir presence and quality is a major risk factor in exploration for, and development of hydrocarbons in proximal slope channel and canyon systems. Thus, interpretation of these systems on seismic data should focus on the lithological nature of the channel fill deposits, leveraging channel patterns and seismic attributes, rather than just the presence of channel systems.