An Early Paleocene Cold Seep System in the Panoche and Tumey Hills, Central California (United States)
Published:January 01, 2007
Daniel Minisini, Hilde Schwartz, 2007. "An Early Paleocene Cold Seep System in the Panoche and Tumey Hills, Central California (United States)", Sand Injectites: Implications for Hydrocarbon Exploration and Production, Andrew Hurst, Joseph Cartwright
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Apaleoseep system consisting of hundreds of sand injectites and authigenic carbonate structures crops out in the Panoche and Tumey Hills, central California. This paleoseep system developed on the western margin of the Great Valley forearc basin and is contained within the uppermost, early Paleocene part of the dominantly siliciclastic Moreno Formation. It is 20 km (12 mi) long and is distributed over more than 700 m (2296 ft) of stratigraphic section. Injectites appear in the lower 600 m (1968 ft), thinning upward from 3 m (9.8 ft) to less than 1 cm (0.4 in.), and cooccur with the paleoseep carbonate structures in the uppermost 200 m (660 ft) of section. The paleoseep slab, mound, and concretionary carbonates are 13C depleted (to −46‰ Vienna Peedee belemnite) and commonly contain pipelike structures and the remains of chemosynthetic macroinvertebrates, including tube worms and lucinid bivalves. Their diverse morphologies likely reflect different rates and styles of fluid flow, but most show a similar paragenesis beginning with biologic colonization and pervasive micrite authigenesis and concluding with sparite precipitation in vugs and conduits. The close stratigraphic and compositional associations of paleoseep carbonate structures with injectites suggest that they were contemporaneous, and that injectites controlled the location of the seeps. Variations in abundance, morphology, and geochemistry of the authigenic carbonates, fossils, and injectites across the outcrop area indicate considerable variability in seep venting rates locally, regionally, and throughout the nearly 2-Ma duration of the seep system. Thus, the Panoche and Tumey Hills locality offers a four-dimensional view of the nature and evolution of a large, injectite- driven, cold seep in a forearc setting.
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Sand Injectites: Implications for Hydrocarbon Exploration and Production
Sand injectites are described in scientific literature as an increasingly common occurrence in hydrocarbon reservoirs, in particular in deep-water clastic systems, where they are known to influence reserves distribution and recovery. Seismically-detectable injected sand bodies constitute targets for exploration and development wells and, subseismic sand bodies provide excellent intra-reservoir flow units that create field-wide vertical communication through depositionally extensive, low-permeability units. As sand injectites form permeable conduits in otherwise low-permeability units they facilitate the expulsion of basinal fluids; hence they act both as a seal risk and mitigate timing and rate of hydrocarbon migration. Injected sand bodies form intrusive traps, which are distinct from structural or stratigraphic traps. Included in this publication are 10 chapters on subsurface examination of sand injectites, 1 chapter on theoretical considerations, and 13 outcrop analogs in reservoirs across the world. Captured in this volume is at least a taste of the global and stratigraphic distribution of sand injectites, and an attempt to introduce readers to sand injectites and their significance in the context of hydrocarbon exploration and production. The book is not intended as a complete review of the field-based literature, but emphasizes high quality case studies from the surface and subsurface. The geographic scope of the book is large, and illustrates the diversity of geological settings in which these fascinating and economically significant features are found.