Amplitude Anomalies in a Sequence Stratigraphic Framework: Exploration Successes and Pitfalls in a Subgorge Play, Sacramento Basin, California

Jeffrey A. May: EOG Resources; Mark Przywara, Thomas A. Mazza: Ruble Clark: DDD Energy, Inc.; John Dlouhy: Yuma Petroleum; and Roger Hettenhausen: Oxy U.S.A., Inc.

Abstract

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 submarine canyons extend seaward from the eastern margin of the forearc. These "gorges" 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 3-D seismic survey where DDD Energy and Occidental Petroleum jointly drilled numerous gas discoveries. Five occur in fluvial-deltaic sandstones of the Maestrichtian Mokelumne River Formation from subgorge traps, with which we had 100% success. The key to this achievement 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 A.M.I. are analogs for subcrop production from the Mokelumne River: McDonald Island Field, with an EUR of ~184 BCF (now used for gas storage), and King Island Field, with an EUR of ~11 BCF

We next conducted 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.