Amplitude Anomalies in a Sequence Stratigraphic Framework: Exploration Successes and Pitfalls in a Subgorge Play, Sacramento Basin, California
Jeffrey A. May, Mark S. Przywara, Thomas A. Mazza, Ruble Clark, John Dlouhy, Roger Hettenhausen, 2002. "Amplitude Anomalies in a Sequence Stratigraphic Framework: Exploration Successes and Pitfalls in a Subgorge Play, Sacramento Basin, California", Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories, John M. Armentrout, Norman C. Rosen
Download citation file:
The Sacramento Basin is part of the Great Valley, a prolific hydrocarbon province that is the remnant of a Late Mesozoic-Early Cenozoic forearc basin in California. A series of buried submarine canyons extend seaward from the eastern margin of the forearc. These “gorges” have formed during multiple episodes of relative sea-level fall during the Tertiary, truncating Late Cretaceous through Eocene marine and nonmarine sandstones. Mudstones dominate the canyon fill, creating lateral and top seals for numerous gas reservoirs.
The late Paleocene Meganos Gorge crosses a proprietary three-dimensional (3-D) seismic survey where DDD Energy and OXY U.S.A. jointly have drilled numerous gas discoveries. Five discoveries occur in fluvial-deltaic sandstones of the Maestrichtian Mokelumne River Formation from traps beneath the Meganos Gorge, a configuration with which we have had 100% success. The key to this success is understanding the associated amplitude anomalies within their sequence stratigraphic and lithologic context.
Initially, we (1) identified all amplitude anomalies, (2) mapped the base Meganos Gorge sequence boundary, and (3) mapped regional flooding surfaces within the highstand Mokelumne River section, paying particular attention to truncations beneath the sequence boundary. Two gas fields in the area are analogs for subcrop production from the Mokelumne River: McDonald Island Field (now used for gas storage), which has an estimated ultimate recovery (EUR) of ~184 billion cubic feet (bcf) of gas, and King Island Field, which has a EUR of ~11 bcf.
We next conducted amplitude-versus-offset (AVO) analyses for all lithologies that yield anomalously high amplitude signatures. We built a database for lignites, low-velocity mudstones, carbonate-cemented sandstones, and conglomerates, in addition to gas-charged sandstones. Finally, we risked our subgorge prospects based on AVO response, structural position relative to the canyon-base sequence boundary, and juxtaposition of lithologies across the sequence boundary. The analytical steps used here can be applied to the continued discovery of subcrop reservoirs associated with other gorges in the Sacramento Basin, as well as the search for hydrocarbons trapped beneath submarine canyons in deep-water basins worldwide.