Lithofacies Prediction for “Linked-Debrites” in a Mud-Rich Deep-Water Setting, Onshore Eel River Basin, Northwestern California

T. J. Greene¹ and G. Gordon^2
1California State University, Chico, CA, [email protected]
²California State University, Bakersfield, [email protected]

The onshore Eel River basin in northwestern California contains over 3,000 feet of mud-rich bathyal deposits within the Pliocene lower Rio Dell Formation. The lower Rio Dell Formation contains discrete deep-water sand packages up to 150 feet thick (20-40% net sand) that correlate from outcrops (Eel River and Price Creek) to wells in the Grizzly Bluff Field approximately 5 miles away. Detailed sedimentologic descriptions of correlative units in lower Rio Dell Formation core (distal position) and outcrop (proximal position) reveal many packages of “linked debrites” demonstrating decreasing turbulent energy in the distal direction. Individual flow units commonly have three definitive zones: Zone 1 is a structureless, dewatered, clay-bearing sandstone with a sharp basal contact; Zone 2 is a chaotic transition zone with irregular contacts containing variable mixtures of sandstone and mudstone showing evidence of liquefaction and upward sand injection; and Zone 3 is a highly sheared, convolute mudstone with interspersed mud-clasts, silts, injected sands, and organic debris. The more distal flow units exhibit no grading, little evidence of sorting, no traction structures, and a larger percentage of Zone 3. The more proximal flow units show more traction structures, slightly better sorting and grading, less clay in Zone 1, and have a smaller percentage of Zone 3.

Scanning Electron Microscopy, X-ray Diffraction, and Laser Particle Size Analysis data confirm the textural observations in outcrop and core. For the distal deposits, Zone 1 portions contain abundant clays (25-30%) with high proportions of detrital clays, as well as characteristic bi-modal grain size distributions. For the proximal deposits, Zone 1 portions also have up to 23% clays, bi-modal grain size distributions, but less detrital clays.

These observations can be useful in predicting reservoir quality in linked debrites as well as serve as an excellent analog for the Late Jurassic North Sea deep-water fan deposits.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009