Repeated Cycles of Progradation and Incision of Upper Cretaceous Shelf-Margin Deltas, Northern San Joaquin Basin, California

Harrison, Denise H.¹, Steven D. Jones², Paul J. Harrison¹, John M. Armentrout³, David H. Suek⁴, David B. Codding⁵ (1) Fall-Line Exploration, Dillon, CO (2) Lariat Exploration, Centennial, CO (3) Cascade Stratigraphics, Inc, Damascus, OR (4) Black Coral LLC, Centennial, CO (5) Yates Petroleum, Artesia, NM

Detailed analysis of 2D and 3D seismic data and log facies demonstrates that the shelf margin on the east side of the Northern San Joaquin Basin was incised repeatedly during the Santonian and Campanian. It appears that sand-prone progradations or shelf margin deltas filled these canyon incisions.

Some of the canyons are aligned with basement faults on a relatively steep slope influenced by ongoing tectonic activity. The first cycle of progradation downlaps within the basement canyons and the adjacent upper slope. This initial cycle of progradation is deeply incised by subsequent phases of erosion associated with relative falls in sea level. It appears that the initial laterally extensive prograding system was dissected into a series of locally preserved shelf-margin deltas separated by younger canyons. The presence of sand in the preserved deltas is supported both by differential compaction across the preserved prograding facies and coarsening-upward sands in a well penetrating the edge of one delta. A shoreward well with fining upward sequences suggests that as sea level continued to rise, shore-face sediments were deposited over the preserved deltas.

The progradational and canyon erosion cycles are followed by a regional transgression that deposited the predominantly mudstone facies of the Sacramento Shale which provides an updip seal for potential hydrocarbon traps. The sealing potential is supported by overpressure in the sandstones in an updip canyon-head well, and by the overlying regional high-amplitude drape seismic facies associated with the shale.